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CLOSE THIS BOOKTools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)
D. Beneficiation
VIEW THE DOCUMENTD.1. Definition
VIEW THE DOCUMENTD.2. Initial conditions and problem areas
VIEW THE DOCUMENTD.3. Proposals for procedural and organizational solutions
VIEW THE DOCUMENTD.4. Environmental and health aspects
VIEW THE DOCUMENTD.5. Processing of diamonds
VIEW THE DOCUMENTD.6. Gold beneficiation processing
VIEW THE DOCUMENTD.7 The processing of phosphate-containing raw-minerals into p-fertilizers

Tools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)

D. Beneficiation

D.1. Definition

The term beneficiation includes all procedures related to the enrichment of raw ores to produce marketable concentrates. These include not only mechanical procedures (e.g. wet mechanical processing) which leaves the material composition of the mineral unchanged, but also chemical procedures (e.g. leaching), which transforms the valuable mineral into other chemical compounds. In addition to the separation of valuable minerals from the non-desired material for purposes of concentration, or the so-called sorting process, the pre- and post-preparatory activities such as crushing, classification, drying, etc. are also included under beneficiation.

Significant values which define the success of beneficiation operations are the concentration factor, total weight recovery and valuable mineral recovery as well as the contents of the concentrate.

D.2. Initial conditions and problem areas

The processing of raw ores into marketable products is a problem of major concern for small-scale mining operations. At present, small-scale mining in developing countries is characterized by a distinct dualism. On the one hand, small operations exist which process their products using modern techniques. Problems with energy supply, the acquisition of spare parts, the availability of operating funds, or simply an inadequate knowledge of the equipment frequently drive these operations to the brink of economical efficiency. On the other hand, there are a number of small-scale mining operations which use more primitive labor-intensive methods and simple machine technology to process the raw ore. These operations are also confronted with substantial technical, organizational and economic difficulties. The major problems associated with these traditional processing methods are:

- minimal throughput, or low specific recovery

Traditional beneficiation facilities produce a throughput value for raw ore which is, in some cases, clearly below 1 t/MS. As a result, small-scale mining processing operations in developing countries are labor-intensive. The problem is intensified by the fact that, as a rule, the beneficiation is performed as a noncontinuous operation with frequent breaks and repetition of feeding, drawing, and deposition activites, resulting in high proportions of idle time. In some cases, the idle periods can total up to 50 % of the total work time in small-scale mining beneficiation plants.

- low recovery

Desired-mineral recovery of only 50 % or even less occurs frequently . Half of the valuable minerals, which are expensive to extract underground, end up in the tailings piles. As a result, the recovered concentrate represents high production costs. The reasons for this minimal recovery are predominantly attributable to poor organization and planning of the work steps: too coarse or too fine grinding, insufficient classifying, inappropriate equipment selection, interrupted noncontinuous work operation, careless processing of fine-grained material, etc.

The causes for the low concentrate contents which sometimes occur are:

too wide a range of classification of the feed material for the wet mechanical sorting,

too coarse comminution (liberation not yet attained) or

very finely intergrown ores, for which the separation cut-off size in the fine-grain beneficiation is insufficient.

A modification of the entire beneficiation method is not possible for small-scale mining operations; however, there are a number of technical processes available which, when combined, can significantly increase the recovery in modern facilities to values typically in the range of 70 % and even up to 80 - 90 %, depending upon the degree of intergrowth in the minerals. These processes are:

Physical processes

*

wet and dry mechanical

*

optical

**

magnetic electrostatic processing

Surface-physical processes

*

flotation

Chemical processes

*

amalgamation and leaching, and finally

Biochemical processes


microbial leaching

Of these, only those marked with a * are of relevance for small-scale mining. Those marked with ** are generally only applied for secondary cleaning of the concentrate (re-concentration) and are farmed out to subcontractors (commission beneficiation). All remaining processes are not suitable alternatives for use in small-scale mining beneficiation due to high investment costs, high degree of complexity, local-market restrictions, or the absence of tradition surrounding the particular process in small-scale mining.

A mechanization and modernization of existing processing plants in small-scale mining operations is hindered by a chronic lack of capital or available funds. Credits for financing mechanization are also not available due to insufficient knowledge of the mineral reserves and the lack of feasibility studies necessary to allow the deposit to serve as collateral.

It should also be emphasized that mechanization and modernization cannot always be regarded as positive entities. This is clearly indicated through numerous examples where partially malfunctioning or already abandoned modern beneficiation facilities have been rejected in favor of the simple, traditional small-scale mining techniques. The dependence on energy, spare parts and operating materials, combined with the loss of flexibility as a result of high investment costs, often led to more severe problems than those associated with traditional techniques.

Therefore, the possibilities of technical improvements are limited to the most economic inexpensive investments through the purchase and step-by-step introduction of locally-manufactured equipment. Furthermore, organizational improvements in the processing procedure can provide substantial economic advantages.

As an example, the cooperatively-run lead-silver mine in Pulacayo, Bolivia illustrates the necessity for changes in the work organization of the processing procedures. The miners, organized into cuadrillas (four-man mining team) operate about 30 parallel, traditional beneficiation plants, some of which are only able to process coarse-grained feed exceeding 1 mm in size. Fine-grained material with very high silver contents are discharged as tailings without being separated. The follow a rotating schedule between production and processing in which about one third of a monthly work phase is spent on processing. The ore quantity being mined by the cuadrillas for processing is, however, too small to support a beneficiation which includes fine-grain separation. Only the combined raw-ore output of several cuadrillas would provide a volume sufficient to justify fine-grain separation steps.

Beneficiation facilities exhibiting fewer, less critical problems, can also benefit from the incorporation of organizational or procedural improvements. Even minor optimization can achieve lasting improvements in operating efficiency. The critical role played by beneficiation in small-scale mining has long been overlooked.

Other concepts presented as solutions to small-scale mining beneficiation problems have often failed, as illustrated below:

Mobile processing plants which periodically concentrate the raw ore from an entire small-scale mining area, have failed due to problems with infrastructure and technology. According to a study done by the KfW on the possibility of introducing a mobile processing plant for lead-silver ores in the deposit-rich highlands of Bolivia, no appropriate location or mining region could be found. The lack of homogeneity of raw ores and deposits, poor road connections, and the subsistence existence of small-scale mining operations which demand a rapid return on capital, prevent the successful introduction of mobile processing plants.

Equally problematic is introduction of central raw-ore processing plants. High transportation costs and low ore values limit the profitability of central processing. Furthermore, the experience in small-scale mining in Bolivia has shown that central processing plants can only be successful when operated as non-profit enterprises, possibly sponsored by mining-related governmental agencies, and must be run at high capacity. Such an endeavor requires realistic analysis and weighing of facts prior to implementation. Concepts which deliver ore in the form of preconcentrates, whether as hand-sorted products or as pre-concentrates from simple, traditional separation facilities, expand the area of economic influence (marketing base) and offer more efficient solutions than those involving the sale of raw ores or poorly-processed final concentrates.

Almost without exception, mines located in isolated remote areas have to process their ore in their own beneficiation plants due to the high cost of transportation. The following chapter offers suggestions regarding planning, construction and operation of processing plants appropriate for the needs of small-scale mining.

D.3. Proposals for procedural and organizational solutions



D.3.1 VARIOUS BENEFICIATION PROCEDURES

A number of different processing methods are available for separating raw ores into marketable mineral-ore concentrates, by-products and waste. The composition of the raw ore, the chemical and physical characteristics of the minerals contained in the raw ore, the grain-size distribution, etc. determine which of these methods are most appropriate for separating the desired mineral from the non-desired host material.

The primary processing methods available are:

THE GRAVIMETRIC BENEFICIATION PROCESS:

For the sorting of raw ore feed in which heavy minerals are the valuable mineral source, either dry or wet mechanical methods are employed, depending on the location, which utilize the difference in density between minerals to achieve the separation. In gravimetric processes, variations in density-specific phenomena (e.g. falling speed, radial acceleration), which appear in a sorting medium of air (dry sorting) or water (wet mechanical sorting), to produce a separation of the feed into two or more components (streams), one chiefly containing ore minerals and the other host-rock particles.
Equipment used in gravimetric sorting includes sluices jigs, sink-float (heavy-medium) separators, buddies, spiral separators, cyclones, pneumatic classifiers (sifters), etc.

The gravimetric separation is the processing method typically used in small-scale mining.

THE FLOTATION PROCESS:

In flotation, the different electro-chemical surface characteristics of minerals are utilized in the separation process, in that some minerals in a fine-grained slurry are made hydrophobic through the addition of reagents (collectors, activators). Air injected into a tank (flotation cell) containing the slurry carries the hydrophobic particles to the surface where they collect as foam which is then subsequently scooped off. This form of separation, where the mineral concentrate is removed in the foam is known as direct flotation; when the mineral remains in the heavy liquid component, the process is known as indirect flotation. By varying the pH-values of the slurry and the reagent additives, different minerals can be selectively recovered.

In mechanized ore mining, flotation is the most widely used processing method.

THE AMALGAMATION PROCESS:

This process is applied on precious-metal ores. Gold, silver and some of their compounds have the characteristic that they can alloy with mercury. These alloys are referred to as amalgams. To separate the precious metal, the raw ore is processed together with mercury, the amalgam removed, and the compound then dissociated into the precious metal and mercury by distillation. The amalgamation is performed in washing pans (bateas), sluices, vessels, barrels, amalgamating drums, Chilean (edge) mills, stamp mills, amalgamating bottles or tables.

THE MAGNETIC SEPARATION PROCESS:

Magnetic separation makes use of the varying magnetic susceptability of the minerals contained in the ore being processed. This physical characteristic (magnetism) enables individual, magnetic minerals to be separated from non-magnetic, or less magnetic ones through the use of a magnet.

THE LEACHING PROCESS:

Separation by leaching utilizes chemical solutions, transport and precipitating phenomena Here, under specific Eh-pH conditions, minerals are dissolved by certain acids, leaches or solutions. The presence of bacteria can have a catalytic effect on the reactions. In a separate facility, the metals are dissociated from the solution and concentrated. Leaching is performed in tanks, on ore stockpiles, or in-situ.

Cyanide leaching is gaining in importance in gold mining.

THE ELECTROSTATIC SEPARATION PROCESS:

Electrostatic separation is based on the varying ionization characteristics between minerals subjected to an electric field. This procedure is, however, seldom used.

In addition, there are methods of optical sorting which, however, are only of marginal importance.

Fundamentally, the design of a beneficiation facility should limit, as much as possible, the number of different sorting procedures employed; the greater the number of processes, the more expensive the machinery, and the more complicated, sensitive and unmanageable the beneficiation operation becomes in general.

D.3.2 PREVENTING IDLE PERIODS AND ACHIEVING A CONTINUOUS MODE OF OPERATION

Ore beneficiation in the traditional small-scale mining industry in Bolivia is usually performed as a discontinuous process. Substantial time is lost through intermediary storage of products, restocking the supply of feed or in preparing the equipment. Observations by the authors in the processing facilities indicate that this idle, wasted time accounts for up to 50% of the total time worked. Consequently, attempts to improve processing-plant throughput should be directed toward achieving a continuous mode of operation in which the entire raw-ore feed quantity undergoes comminution, sorting and classifying steps.

D.3.3 CAREFULLY-PERFORMED COMMINUTION

The output in processing plants drops with decreasing grain-size of the feed. Even in modern mechanized plants, the finest grains present difficulties for the operation. Hence, it is absolutely necessary to ensure that comminution is performed so as to produce the smallest possible quantity of fines. This is especially important for those valuable minerals which exhibit a brittle to very brittle tenacity, such as cassiterite, sphalerite and the tungsten minerals scheelite and wolframite. The tenacity describes the fracturing behavior (as opposed to cleavage or scratch (abrasive) hardness) of the mineral, and decisively governs the behavior of a mineral during comminution. Brittle minerals tend to be comminuted quicker, frequently resulting in over-milling (and associated higher proportion of fine material). To prevent the valuable mineral from being reduced to such fine fractions that they can be separated only with great effort, a carefully-controlled crushing is required. When grinding is necessary, it should be limited to a short period, after which a classification can be performed with subsequent regrinding of the over-sized grains.

It is often possible to omit grinding to a large extent.


Figure

D.3.4 PREVENTION OF OVER-GRINDING

Grinding until liberation, or the dissociation of valuable minerals from host rock or waste material, is of fundamental importance to beneficiation technology. In so doing, the occurrence of intermediate products (middlings), i.e. intergrowths of host material with the mineral ores within individual grains, is prevented. On the other hand, however, such a technically-appropriate grinding (from the beneficiation point of view) creates other problems.

High concentrations of fines resulting from excessive grinding not only adversely affect the separation by reducing recovery but also raise energy consumption during grinding, which comprises up to 50 % of the processing costs in modern plants in some cases. In traditional manually-operated processing plants, this energy is produced by hand, for example through the use of simple "see-saw" (or rocker) crushers.

To eliminate the above-mentioned problems, grinding should be entirely omitted when possible and replaced by the processing of coarser-grained material to produce pre-concentrates. A regrinding of the middlings from the coarse-material sorting considerably reduces the feed quantity for the grinding process. This results in lower energy costs (also of importance in terms of energy-supply investment costs), relatively high recovery values, but non-optimal concentrate contents, however.

When the optimal grain size is exceeded, increased losses of the valuable mineral occur during the pre-concentrate separation process as a result of intergrowths which inhibit the complete liberation of the mineral contained in the large grain sizes.

Raw ore characteristics such as degree of intergrowth, grain-size distribution of the valuable minerals, etc. determine whether a grinding of the feed material is absolutely necessary. Whenever possible, the separation steps should receive only a crushed or broken feed material, such as can be produced by a roll crusher, which yields a final grain size of up to 1 mm. The roll-crusher produces a product which is homogeneous in granulation and therefore exhibits a relatively low proportion of fine fractions.

D.3.5 PROCESSING OF NARROW GRAINSIZE RANGES

In all of the sorting processes, and particularly the gravimentric processes, a classifying effect occurs in addition to a separating effect. Many of the gravity-separation processes are based on sedimentation within a water media, whereby the settling velocity plays a major role. Particle behavior of large, light grains (of low specific density) and small, heavy grains (of high specific density) is similar, which is indicated by their almost identical speed of falling. In order to minimize the classifying effects during classification separation, it is necessary to achieve a sufficiently thorough pare-classifying of the feed material to permit further separation to occur only on narrow grain-size ranges. Many of the traditional small-scale mining plants in Bolivia classify their feed, with grain-sizes between less than 30 mm and the finest fraction, into only five or even fewer grain-size fractions. This results in a relatively low total recovery, and low valuable-mineral content in the concentrate, since the concentrate contains impurities of waste-material particles which exhibited the same hydraulic behavior as the valuable-mineral particles during gravity separation.

The density and solids content of the slurry feed are essential parameters for achieving good separation results from classifying, sorting, and clarifying steps. The maximum allowable solids-contents of the feed for the various separation equipment are presented in the following table (from Trawinski, Priester):

Table: Standard Values for Solid Contents of Slurry Feed for Classifying, Sorting and Clarifying Processes


Solids Content of Feed

Conical hydrocyclone

max. 20 % by vol


in extreme cases up to


40 % by vol

CBC cyclone

max. 15 % by vol


in extreme cases up to


25 % by vol

Countercurrent hydro-classifier

25 - 40 % by vol

Rake classifier

30 - 50 % by vol

Spiral classifier

30 - 50 % by vol

Tables

max. 15 - 20 % by vol

Spiral cleaner

max. 15 - 20 % by vol

Conical separator

max. 20 % by vol

Jigs

max. 10 % by vol

Dense medium cyclone

max. 10 % by vol

Countercurrent separator

max. 25 % by vol

Buddles

max. 10 - 15 % by vol

Circular buddies

max. 10 % by vol

Sedimentation barrel

approx. 30 - 50 % by vol

Filter press

15 - 40 % by vol

Disk-type vacuum filter

10 - 20 % by vol

Rotary drum vacuum filter

10 - 30 % by vol

Rotary-drum belt-type


vacuum filter

20 - 40 % by vol

Thickener

max. 5 - 10 % by vol


Fig.: Schematic diagram of a classifying separation with wide-range (1) and two narrow-range (B1 and B2) classified feed fraction; black circles; valuable mineral (heavy component), white circles: vaste mineral (light component).

Table: Gran-size ranges (in ym) of feed material for various beneficiation equipment and techniques: upper and lower limit, respectively; extreme values in parentheses.

Conical hydrocyclone

(5) 10 - 200

Rake classifier

200 - 5000

Hydraulic classifier

(20) 50 - 1000 (2000)

Wet screen classifier

(50) 75 - 5000

Dry screen classifier

(40) 100 - 10000

Pneumatic cyclone

(10) 50 - 150

Sizing drum

250 - 50000

Hand sorting

5000 - (500 mm)

CBC cyclone

20 - 500

Shaking table

(20) 50 - 1000 (3000)

Spiral separator

(30) 50 - 1000 (3000)

Cone separator/ fanned sluice

(30) 50 - 1000 (3000)

Sink-float separator

(400) 500 - 5000

Dense-medium cyclone

200 - 5000

Jig

(80) 100 - 5000 (10000)

Sluices

(60) 100 - 1500 (3000)

Bartle's-Mozley table

(2) 5 - 100 (200)

Low-intensity wet magnetic separator

(40) 50 - 2000 (5000)

High-intensity wet magnetic separator

(10) 20 - 500 (2000)

Flotation

(5) 15 - 500

Foam Flotation

(100) 150 - 1500 (2000)

Selective agglomeration

2 - 50

Thickener

0 - 50

Bartle's belt table

(5) 10 - 100

Amalgamation

(20) 50 - 2000

Gold leaching

0 - 750

Fluidized bed concentrators or centrifuge

20 - 2000

Flotation in sluices

200 - 3000

Flotation in buddies

20 - 250

Washing gulley

100 - 2000

Sludge pond, buddle

(20) 50 - 1500

Mechanized buddle

10 - 500

Dolly tub (Schanz process)

20 - 2000

Air Jigs

30 (200) 500 - 2000

Air tables

50 - 600 (50 mm)

Dry magnetic separation


- low intensity

100 - 5000

- high intensity

80 - 1000

Electrostatic sorting

(75) 100 - 1000 (1500)

Electrodynamic sorting

(40) 70 - 2000 (5000)

Magnetic induction

500 - 10000

Dry Sluice

75 - 1500

Dry vibrating Sluice

200 - 1500


D.3.6 INCREASING SPECIFIC THROUGHPUTS

The use of certain equipment or processes in a traditional beneficiation procedure limits the throughput of the entire operation. An example of this is the employment of sludge ponds to recover the fine-grained solids in a slurry, which perform with such a low throughput so that some processing plants have entirely eliminated any separation of fines. Considering that the finest grains contain significant amounts of the valuable mineral, this decision is detrimental in terms of total plant recovery values. As an alternative, solutions such as those applied, for example, in San Cristobal, Porco, Bolivia should be emphasised. In this region, the parallel operation of multiple sludge ponds has been implemented in order to increase the specific throughput in the fine-grain separation steps. The simultaneous, parallel running of other processing steps in a continuous or semi" continuous operation is likewise possible, such as pinched sluices, spiral separators, funnel furnaces, etc.

D.3.7 HYDROCLASSIFYING VS. SCREENING

Especially in wet mechanical beneficiation processing, the type of classifying employed determines the precision of separation in the sorting process. Hydroclassifying is clearly more appropriate than screening for feed-preparation of sorting equipment such as furnaces, buddies and sluices. The reason for this is that a hydroclassified material is separated according to equal settling rates, which means that larger, lighter particles and smaller, heavier particles end up in the same fraction. When one of the above-named pieces of equipment is charged with this feed, a better spatial separation between heavy and light materials is achieved than with screened feed due to the drifting resulting from the flow forces applied on the grain surfaces. A further advantage for small-scale mining is the continuous operating mode which hydroclassifying provides in non-mechanized plants.

At present, screening is the primary feed separation method used in traditional small-scale mining in developing countries. It has, however, the following disadvantages:

- low throughputs
- low separation precision
- higher operating costs, and
- discontinuous mode of operation,

which are eliminated with hydroclassifying.

D.3.8 PRODUCTION OF PRE-CONCENTRATES

In order to avoid handling large quantities of material in the beneficiation facility, one of the initial steps should be the production of pre-concentrates, particularly where feeds of low valuable-mineral content are involved, such as tin ore with around 2 % Sn, but also for higher-content feeds as well. These pre-concentrates can be produced by one of two methods, or a combination of both. The simplest method involves manual hand sorting, whose importance and application to "selective semi-mechanized mining" in solving beneficiation problems specific to small-scale mining has already been described in Horvay (1983). Through manual sorting or hand picking, a marketable hand-sorted concentrate, as well as a pre-concentrate for further beneficiation, is obtained. The alternative method for achieving pre-concentrates employs sorting equipment with high specific throughput.

Whereas hand sorting of pre-concentrates is realistically limited to material of grain-sizes larger than 10 mm, a wide assortment of pre-concentrating sorting equipment is available for feed material of grain-sizes ranging from 30 to 100 mm. These are:

piston jig

Sluices, sludge ponds, spiral separators,

pinched sluices

The employment of any of the above-named sorting equipment requires a presizing of the feed material in order to achieve a sufficiently high recovery from the pre-concentrating process.

- The primary advantage of pre-concentrating the raw ore is that the quantity of feed entering successive sorting steps is reduced. Pre-concentration of feed material, for example tin ore, from 2 to 4 % Sn (in this case), eliminates 50% of the waste material (assuming 100% recovery), and the throughput quantity in the succeeding steps is reduced to one half.

- For all of the marketable hand-sorted pre-concentrates, further beneficiation efforts (comminution, classifying, sorting) are not necessary. Losses in recovery can also be avoided for this portion of the ore.



D.3.9 HOMOGENIZATION OF FEED MATERIAL

In small-scale mining in Bolivia, the authors could repeatedly observe that totally inhomogeneous feed material served as charge for the various separation equipment. A sludge pond for fine-grain separation serves as an example in which first the middlings from a previous separation step and then raw ore comprised the feed, whereby the second feed input (raw ore) was deposited directly onto the sedimentation cone of the first (middlings). This procedure can lead to spatial variations in concentrations within the subsequent sedimentation cone due to extreme periodic variations in the granulation and heavy-metal content of the feed. This problem can be solved by mixing the separate feed constituents in order to attain a homogenous feed prior to further processing. Homogeneity of the feed is essential for semi-continuous or continuous processes whose operating parameters such as feed quantity and rate, inclination of separating tables, reagent additives, etc. are determined by the slurry-feed characteristics.

Sufficient homogenisation can frequently be achieved through very simple methods, such as the dumping of different feeds, one on top of the other, onto the cone-shaped discharge pile. While the cone as a whole is inhomogeneous, removing the material from the side near the bottom of the cone induces a certain degree of homogeneity through the resulting sliding and resettling of material.

D.3.10 SECONDARY PROCESSING OF THE MIDDLINGS

In all traditional beneficiation procedures, middlings are produced as a by-product of the processes. These products occur as two different forms, namely:

- middlings produced as a result of low separation accuracy in the sorting facility. Although the components are liberated, i.e., the valuable mineral occurs as free grains and is no longer intergrown with host rock, waste material, or secondary minerals; however, the grains are not separated according to whether they do or do not contain the valuable mineral. This type of middlings frequently occurs in mechanized gravity-separation processing, especially when the specific characteristics, such as density, do not vary greatly between the valuable mineral and the host rock particles.

- middlings which emerge from prior comminution steps but still exist in an unliberated form where individual grains still contain intergrowths of the valuable mineral with host rock material. This type of middling product occurs even in the most precise separation processes, and valuable mineral so contained cannot be separated by further processing without additional comminution.

The two forms of middlings can also occur as a combination. In any event, the nature of the middlings must be determined prior to any successive processing to prevent any unnecessary expensive regrinding of already-liberated material and resulting reduced recovery due to the increased proportion of fines. The washing pan (common for panning for gold) offers a simple, fast and reasonably priced apparatus for quickly determining the characteristics of the middlings and the further processing steps required:


- liberated middlings require secondary separation
- non-liberated middlings require recrushing or regrinding prior to secondary separation.

In small-scale mining in developing countries, middlings frequently receive only very incomplete further processing, or are simply discarded as waste. This practice, however, cannot be economically justified; no additional mining costs, and only minimal grinding and separating costs, are incurred by secondary processing of the middlings, costs which generally can be recovered through marketing of the resulting products. Only in cases of very fine-grained material should the sale of middlings to an operation with mechanised beneficiation be considered.

D.3.11 CLASSIFYING OF FEED PRIOR TO SEPARATION PROCESSING

Technical journals of the last century record the debate among engineers over the English versus German classifying methods used in beneficiation processes.

The "Harzer" or German method involves first sizing the feed material and then separating the narrow grain-size fractions, while the English method makes use of the sizing effect of the wet mechanized separation operation and then classifies only the products. For example, in the case of sludge-pond separation, the feed material, which includes a wide range of grain-sizes, is sorted and the concentrate extracted and then classified. The coarse-sized material thus obtained comprises the end concentrate, and the undersized material constitutes the pre-concentrate which is then further separated.

Mining of base metals (non-ferrous metals) in Latin America, particularly tin mining, was primarily influenced by Anglo-Saxon engineering during the 19th century. As a result, the English method of wet-mechanized beneficiation has been dominated even in small-scale mining. The advantage of this method is that only the concentrates and the middlings are classified, with waste material remaining unclassified. In so doing, the cost of sizing is minimised. I However, the several disadvantages associated with this method justify a reconsideration of the use of the English method in small-scale mining. On one hand, the sorting of feed material which exhibits a wide-range granulation occurs at such high slurry flow velocities that the coarse grain fractions are also separated out. These high slurry velocities also frequently cause the fine and very fine-grained particles to be carried off due to their large specific surface area and be removed as part of the tailings. A sizing of the material before sorting would have the advantage that the fine and finest grain fractions could be separated and sorted at low slurry flow velocity. On the other hand, the disadvantages of subsequent classifying via settling are substantial. Materials exhibiting a wide range of grain sizes are more difficult to sort by settling processes than materials of narrow-range granulation. This is explained by the fact that the bonds in narrow-range grain-size fractions are loosened more easily during the pulsating settling process, therefore requiring less energy. This is of significance particularly in the manually-run jig separation process in small-scale mining in developing countries.

D.3.12 BENEFICIATION OF FINE-GRAINED MATERIAL

A significant shortcoming of beneficiation facilities, both in large-scale operations as well as in small-scale mining processes in developing countries, is the insufficient attention devoted to fine grain sorting. Beneficiation plants with low valuable-mineral recovery (for example, less than 50 % recovery from modern mechanized beneficiation of tin ores at the Bolivian state-owned COMIBOL mines), lose, as a rule, a large quantity of the valuable-mineral source in the fines. Additionally, plants are encountered where fine-grain sorting is non-existent. The rich silver ores of Pulacayo (Bolivia) are beneficiated using traditional small-scale mining methods whereby, in some cases, all material smaller than 1 mm lands on the waste pile. In view of this problem, the significance of fine-grain beneficiation cannot be emphasized enough. Especially for two raw-ore types, the concentration of valuable mineral in the fine and finest fractions is of great importance:

- raw ores which exhibit a fine intergrowth and therefore require fine grinding to liberate the valuable mineral. Deposits of non-iron metallic ores, for example those of sub-Volcanic or submarine emanative genesis, or sulfide veins with oxidized valuable-mineral sources, or stratified tin deposits, such as frequently occur in the Latin American Andes, exhibit this fine intergrowth and must be handled during beneficiation with special attention regarding the fine grain distribution. Similarly, alluvial gold deposits where the gold occurs as fine grains also belong to this category of raw ores. In traditional mining operations without any separate fine-grain separation processes, the recovery can lie well below 10% in some instances.

- raw ores where the valuable-mineral sources are brittle minerals which are quite easily subject to overgrinding during comminution. Sphalerite, cassiterite (tin ore) and scheelite are only a few examples of this type of brittle mineral ore (see Table). A pre-concentrate can already be achieved by performing selective classifying, since the fine" grain fraction already represents a pre-concentrate following the initial comminution step. In any event, beneficiation of these raw materials requires that particular consideration be given to the fine-grain fractions.

One solution is demonstrated by the unfortunately already historic example of a wet mechanized silver-ore beneficiation performed in Pulacayo, Boliva up until 1952, where even separating table wastes were resorted using mechanized sludge ponds to achieve very fine concentrates. In mining of gold alluvial deposits, gold centrifuges and cyclones, for example, could be employed; for extracting non-iron metallic-ore concentrates, the above-mentioned mechanized sludge pond, Bartles-Mozley-separating table, or blanket sluice (corduroy table), etc. could be used.

Difficulties are encountered in the wet mechanical separation of fine and very fine-grained material due to the fact the final falling velocities of minerals of varying density approach similar values with decreasing grain size. Thus, beneficiation of the finest grains involves greater expense and achieves lower recovery at a lower degree of concentration compared to those separation facilities for coarsergrained material. Nevertheless, in the majority of cases, a fine or finest-grain sorting stage is of advantage.

D.3.13 MANUAL SORTING BY HAND

Hand sorting of raw ores, feed materials and concentrates during beneficiation is of special significance in small-scale mining operations, as mentioned by Horvay (1983) amongst others. Examples from fluorite mining in the Upper Pfalz region of Germany (until 1988) show that even under conditions of extremely high labor costs, as found in German mechanized mining, hand picking plays an important role. Pre-concentrates or concentrates can be won by manual sorting, performed either as negative selection (sorting out of waste-rock material) or as positive selection (selection of chunks of pure ore). In both cases, the material load on the beneficiation plant is obviously reduced. Moreover, for positively-selected material, losses due to low valuable-mineral recovery are minimised. One difficulty is the relatively low efficiency, or performance rate, which characterizes the hand-sorting operation in Andean small-scale mining, as a result of very poor working and sorting conditions. Improving the hand-sorting operation by providing cleaner material (through wet classifying of coarse material, for example), better lighting, etc. at the separation tables or sorting belts could increase efficiency.

D.4. Environmental and health aspects

Beneficiation operations exert substantial impact on the natural environment.

Noise pollution from comminution (crushing or grinding) processes and the operation of power equipment.

Air pollution from multiple sources:

- Dust pollution, especially as a result of dry sorting of feed material, for example from air classifiers and dry classifying.

- Air pollution resulting from mercury vapors produced during open-circuit distillation of amalgam occurs frequently in gold mining and leads to a variety of health risks due to mercury contamination: loss of hair, decay of fingernails and bones, and can also result in death (see also Section D.6.5.1).

One solution to this problem is the implementation of closed-circuit amalgamation through the use of distillation retorts.

Water pollution. Contamination of the drainage system from beneficiation processes is an especially serious and dangerous problem, particularly considering the multiple utilization of surface water. In semi-arid regions with defined dry periods, rivers represent the ultimate source of water. The use of water can basically be categorized according to three primary purposes:

1. as drinking water for the population, which usually uses untreated water directly from rivers.

2. for irrigation purposes in agriculture, and

3. as process water for small industries, whereby mining with its widespread geographical distribution and large water requirements for raw-ore beneficiation is the primary industrial consumer of water.

Legal restrictions regarding water rights, environmental regulations, etc. in the Andes region are generally either non-existent, or their compliance not controlled, so that competitive surface water consumption needs pose serious problems.

The situation is worsened by the fact that the natural biochemical decomposition of toxic pollutants in the water occurs at a substantially slower rate in the Andes region due to the high elevation with its low oxygen partial pressure and very low water temperatures.

Specifically, the following pollutants may be released by beneficiation processing:

- Sludge/Silt: occurs in all wet-mechanized and flotation beneficiation processes.

- Toxic flotation reagents: those reagents most commonly used in small-scale mining in Latin America are sulfuric acid, diesel oil and long-chained carbon-hydroxides (frothers and collectors) such as xanthate. In small-scale mining, the manner in which these substances reach the drainage system is not known. Especially problematic is the regulation of dosages of reagent additives used for the noncontinuous, low-recovery flotation processes. Compared to large industrial-scale flotation operations, the concentrations of reagents used in small-scale mining are extremely high.

- Highly toxic cyanides: used as activator substances in selective sulfide flotation and in leaching. The decomposition of cyanide normally takes two years; however, in the Andes region the process is accelerated as a result of more intensive ultra-violet (UV) radiation at higher elevations. Demarcated sludge ponds (sedimentation basins) for receiving waste products are urgently necessary in small-scale mining as well.

- Amalgam and mercury: These two substances are released in water during amalgamation of gold in stamp mills, sluices, tables, and Chilean (edge) mills. More detailed information on this subject is presented in Section D.6.5.

The mining industry's most serious environmental problems stem from the beneficiation operations. In this area, changes and remedial measures designed specifically for small-scale mining are particularly important, a fact which justifies the large amount of attention given to beneficiation in this handbook.

D.5. Processing of diamonds

Diamonds constitute an important branch of precious-stone extraction. Small-scale mining accounts for about 10 - 15 % of the entire global diamond production. The methods employed for mining and processing diamonds differ according to the geology of the deposit:

1. Primary deposits, diamond-containing volvcanic breccia (tuff) of basic and ultra-basic rocks such as Volcanic chimney vents, the socalled Kimberlite or blue ground; additionally, Precambrian olivine rock.

2. Secondary deposits which consist of weathered, decomposed products of diamond-containing rocks. These may develop either as hardened sediments, for example in the form of conglomerates, as loose sediments in river beds, or above blue-ground deposits as yellow ground.

In the beneficiation of diamonds originating from primary deposits or hardened sediments, care must be taken to prevent the precious raw stones from being crushed or ground along with the waste rock. The high cleavability (splitting tendency) of diamonds requires a very careful grinding of the parent rock. In the South African small- and medium-scale diamond mining operations, a special form of grinding has been developed. Following extraction, the feed material is thinly distributed over the ground surface where it is exposed to natural weathering. On the so-called 'floors' the weathering process is enhanced by adding water. The sorting of the comminuted material can proceed in different ways:

OPTIC-MECHANICAL :

Performed by hand sorting, whereby the feed material is spread out in a thin layer on a sorting table and the diamonds, being highly visible due to their light refracting characteristic, are manually sorted. The lower profitable grain-size limit lies at a weight of about 5 mg (1/40 karat).

Modern optic-mechanical separation procedures involve photometric or radiometric sorting which utilizes artificially-induced luminiscence of the diamond. A thin stream of material flows through an optical detector which responds to the special optical characteristics of the diamonds and steers a pneumatic nozzle which blows the diamond grain out of the material stream.

GRAVIMETRIC :

Gravimetric utilizing the density of the diamond (3.52 g/cm³ )

In the processing of diamonds, small-scale mining in developing countries frequently uses jigs, mostly in the form of manual separators. In this procedure, the lower profitable grain size is about 150 ym due to the minimal density variations between the quartz, the main sediment component, and the diamonds, the valuable mineral (q = 1.48 for separation in water). To increase the precision of separation, jigs with beds are used; for diamond processing, glass balls serve as the bed material (glass bed).

Grease tables utilize the strong hydrophobia exhibited by diamonds; the feed material is suspended in a slurry which flows over the greased surface of a stationary sorting table. The diamonds are drawn to the grease, from which they are later individually recovered.

Additional methods of gravimetric separation include the sink-float separation processes with FeSi-slurries in normal sink-float separators or in heavy-media cyclones. The FeSi, a weighting material for increasing the slurry density which appears 65 to 90 % in the <0.05 mm fraction, is recovered after sorting by means of magnetic separation.

ELECTROMECHANICAL:

Electrostatic-separators can successfully separate diamond-containing feed up to maximum grain size of 6 mm, whereby the semi-conducting characteristic of diamonds are utilized as the basis of separation.

D.6. Gold beneficiation processing



D.6.1 GENERAL INFORMATION

The processing of precious-metal-containing raw minerals places special demands on the separation method employed due to the physical and geochemical characteristics of the gold and the economic geology of the deposits. Gold generally occurs in deposits where the content of raw ore, in primary deposits, lies at 100 - 200 g/t (maximum) and 1 - 2 g/t (minimum). The lower limit serves as the grade cut-off for marginally economically mineable deposits. Sedimentary deposits generally contain between 0.2 and approx. 20 · 50 g/t raw precious-metal ore. The beneficiation of these ores must, correspondingly, concentrate to a factor of up to 100,000. At the same time, comparatively larger amounts of raw ores must be mined and processed in order to cover the production and processing costs. Due to the low wages in most developing countries, coupled with the predominance of manual labor, a gold recovery of about 0.3 9 gold per man-shift is still considered an acceptable production level.

A further problem confronting gold processing, specific to precious-metal alluvial (placer) deposits, is that many operations can only mine seasonally. During the rainy season, the rivers often carry such large quantities of water that mining activities in the riverbed or along the banks are hindered. Correspondingly, the beneficiation facilities must either be located above the high-water level or be semi-mobile to allow its transfer to higher ground at the start of the rainy season.

Additionally, the grain shape of gold, both from sedimentary deposits as well as after grinding, is in many ways unfavourable for hydromechanic gravity processing. Flat, flour-type gold grains of the smaller particle range can only be gravimetrically separated with major difficulty, despite the high specific density of gold.

D.6.2 ALTERNATIVE PROCEDURES

Among the various techniques for general mineral processing, many can be applied specifically for the processing of gold ores. The methods can be differentiated according to those methods which produce pre-concentrates that are not marketable as final concentrates, and those methods which lead to marketable final concentrates containing between 90 and > 99 % Au. These processing alternatives are presented in the following tables, in which those techniques appropriate to small-scale mining appear in bold face.

Table: Methods for the processing of gold ores into pre-concentrates.

Type of separation

Process

Steps

Equipment

Reeagents

mechanical

GRAVIMETRIC BENEFICIATION

sorting

gold pan, jigs, sluices (wet + aero), tables (wet + aero), animal skins centrifugal classifiers, spiral classifiers, CBC cyclone separators, heavy material traps



DENSE MEDIUM SEPARATION

sink-float separation

sink-float separator, glass flask

dihydrogendodecawolfra. mate =3.1g/cm³

electrical

ELECTRO-STATIC BENEFICIATION


esta-separator


surface mechanical

FLOTATION





- indirect

conditioning, sorting, washing

conditioning tank flotation cell

frothing agents, collectors, depressants, activators


- direct

conditioning, sorting, washing


pH-reagents

magnetic

MAGNETIC SEPARATION

sorting

magnetic separator


* analytical method only

Table: Methods for Beneficiation of Gold Pre-concentrates into high-quality marketable Gold Concentrates

Clase of separation

Process

Steps

Equipment

Reagents

mechanical

HAND PICKING (SORTING)





COAL-GOLD AGGLOMERATION

agglomeration, separation, stripping

reaction vessel/tank

oil, activated carbon

thermal

ROASTING, AIR CLASSIFYING

roasting, air classifying

roasting furnace



GOLD VOLATILIZATION

chloridized roasting, voiatilization


sodium chloride, chlorine gas


SEPARATION by SMELTING

smelting, separation of gold

furnace, caucible

borax, soda, potash


FIRE ASSAY*

smelting with gold collector, separation, cupellation

muffle or retort furnace, crucible, cupel

taste (assay) lead, borax, soda, potash

chemical

AMALGAMATION with open or closed Hg cycle

alloying, separation, distillation

in stamp mill, chilean mill, amalgamating barrel, gold trap, sluice, amalgamating table, gold pan, centrifuge, amalgam press distillation retort

mercury, possibly caustic soda, sodium amalgam ammonium chloride, cyanide or nitric acid for joining finest Hg beads surface act. agts., tensides.


CYANIDE LEACHING as heap leaching, vat leaching, agitation leaching with Merill-Crowe, CIP, CIC-, CIL process or zinc precipitation

chemical solution as complex, adsorption, stripping

leaching tanks, adsorbers

Na cyanide CaO for adjusting pH, Zn
(possibly + PbNO3) or activated carbon


THIOUREA LEACHING

chemical solution as complex, adsorption, stripping

leaching tanks, adsorbers

thiourea, pH-reagents, Al or Fe powder SO2


CHLORINATION BIO-D- LEACHANT PROCESS

formation of halogen- complexes (e.g. tetra- chlorine- bromine com plex)

reaction tanks, leaching tanks

chlorine gas, organic bromodimethylhydrates


BRINE LEACHING



salt solutions, manganese dioxide, sulfuric acid


LEACHING with solutions, containing thiosulfate, rhodonate, polysulfides or nitrite




biological

BACTERIAL LEACHING



bacteria, air as oxidizing agent

* as an analytical method only. Techniques printed in bold type are wholly or partly applicable to small-scale mining.

D.6.3 PROCESSING PROCEDURES

Depending upon the nature of the raw ore, and the investment possibilities for the beneficiation plant to acquire equipment, processes emerge which include production of final concentrates. A few selected procedures for gold-ore beneficiation of various raw ores and grain size magnitudes are presented as flow sheets on the following pages.


Flow sheet of a small-scale manually-run gold-ore beneficiation plant in the Philippines.


Flow sheet of a small-scale tank leaching of tailing containing free (liberated) gold and adsorption using locally produced activated carbons, Brazil.


Flow sheet of a small gravimetric beneficiation plant for primary gold-ores in the Andean region of Narino, Colombia.


Flow sheet of a large mechanized beneficiation plant for primary gold-ores with a combined gravitational and leaching process. The hydromechanically-extracted pre-concentrates are amalgamated, the fine material processed by a tank leaching and the Merrill-Crowe process and then refined by a smelting separation.


Flow sheet of a beneficiation plant for primary gold ores with amalgamation and leaching, Mina Los Guavos, Narino, Colombia.


Flow sheet of a mechanized beneficiation plant for primary gold-ores from Bolivia (Mina Luchusa/Dept. La Paz), wich is however also typical for small and medium-scale operations in Chilean gold mining.


Flow sheet of a small gold beneficiation facility in vein-ore gold mining in Ecuador.


Flow sheet of a small manual alluvial-gold beneficiation operation in Brazil.


Flow sheet of a mechanized alluvial-gold beneficiation facility near Barbacoas, Narino, Colombia.

In small-scale gold mining in developing countries, gravity separation for pre-concentrating as well as amalgamation for production of marketable final concentrates are of major importance. The technical optimization of these processes in order to increase recovery and pre-concentrate contents are critical aspects in improving beneficiation operations. Suggestions for attaining these goals are offered in the respective technical sections of the handbook.

D.6.4 PRODUCTION OF BY-PRODUCTS

A further strategy for improving the economic success of gold processing plants includes the marketing of by-products. In primary gold deposits, the deposit genesis of the associated (paragenetic) mineral composition determine the economic feasibility of mining or processing by-products, which, amongst others, include:

- Antimonite

- Scheelite

- Copper pyrite

- Bismuth minerals

- Uranium minerals

- Silver and silver minerals

- Galena (PbS)

- Sphalerite

ALLUVIAL GOLD DEPOSITS

Fluvial heavy-mineral deposits, which include important gold deposits, represent the physical deposition of heavy, weather-resistant minerals, meaning that the sought-after gold can be associated with numerous other minerals. Some of these by-product minerals are easy to extract and market separately (the names of which appear marked with a * in the following table).

Table: Potential Composition of Alluvial Gold Deposits and their Primary Characteristics

Mineral

Chemical Formula

Color

Density (g/cm³)

Hardness according to Mohs

Remarks

Free gold*

Au(+Ag)

golden-yellow

15.6 - 19.3

2.5


Magnetite

Fe3O4

black

5.2

5.5 - 6.5

shiny, strongly magnetic

Ilemenite

(Mg,Fe)TiO3

black

4.5 - 5.0

5.6

weakly magnetic

Garnet

+3M³+2(SiO4)3²

red, brown

3.8

6.5 - 7.5

vitreous luster

Zirkon *

ZrSiO4

brown, light yellow, colourless

4.7

7.5

diamond luster

Hematite

Fe2O3

dark steel grey, black

4.9 - 5.3

5.5 - 6.5

rounded grains

Chromite

FeCr2O4

iron black to brown black

4.1- 4.9

5.5

possibly weakly magnetic

Olivine

(Mg,Fe)2SiO4

olive green

3.3 - 3.4

6.5 - 7.0

vitreous luster, transparent to translucent, good cleavability

Epidote

HCa2(Al,Fe)3SiSO1V3

pistachio green

3.2 - 3.5

6.7


Pyrite

FeS2

bronze yellow

4.9 - 5.1

6.0 - 6.5

angular grains, metallic luster

Monazite *

(Ce,La,Di)PO4+ThO2

yellow

4.9 - 5.3

5.0 - 5.5

resinous or greasy luster, rounded grains

Limonite

2Fe2O33HO

dark brown

3.6 - 4.0

5.0 - 5.5


Rutile

TiO2

red brown, red

4.2

6.0 - 6.5

metallic diamond luster

Platinum *

Pt (possibly also Ir)

steel white

16.5 - 18.0

4.0 - 4.5

malleable, flakes and grains

Iridium *

Ir (also Pt, etc.)

silver white greyish

22.6 - 22.8

6.7

angular grains

Iridosmium*

Ir Os

tin white to light steel-gray

19.3 - 21.1

6.7

flat grains, tenacious, good cleaveability

Cinnabar

HgS

red

8.0 - 8.2

2.0 - 2.5


Wolframite*

(Fe,Mn)WO4

black, dark grey

7.2 - 7.5

5.0 - 5.5

semi-metallic luster, good cleavability (axotomous)

Scheelite *

CaWO4

white, light yellow, brown or grey

5.9 - 6.1

4.5 - 5.0

diamond to greasy luster, translucent

Cassiterite *

SnO2

brown or black

6.8 - 7.1

6.0 - 7.0

friable, rounded grains

Corundum *

Al2O3

brown, yellow

3.9 - 4.1

9.0

diamond to vitreous luster

Saphire

Al2O3

blue

3.9 - 4.1

9.0

diamond to vitreous luster

Ruby

Al2O3

red

3.9 - 4.1

9.0

diamond to vitreous luster

Diamond *

C

white, colourless, pale

3.5

10.0

diamond to greasy luster

Native Mercury

Hg

tin white

13.6


small opaque, fluid beads

Amalgam

Hg, Ag, Au

silver white

13.0 - 14.0


friable to tenacious

Galena

PbS

lead grey

7.4 - 7.6

2.5 - 2.7

metallic luster,very good cubic cleavage, brittle

Silver *

Ag

silver white

10.1 - 11.1

2.5 - 3.0

tenacious,malleable, black tarnished

Copper

Cu

red brown

8.8 - 8.9

2.5 - 3.0

tenacious, flexible

Bismuth

Bi

tin white

9.8

2.5

friable, metallic luster

Cerussite

PbCO3

colourless, white

6.5

3.0 - 3.5

diamond luster

Columbite * Tantalite

(Fe,Mn)(Nb,Ta)2O6

iron-black, grey, brown-black

5.3 - 7.3

6.0

iridescent, semi- metallic luster, good cleavability

Quartz

SiO2

colourless

2.6

7.0

vitreous or greasy luster, no cleavage

Felspar

Silicates with K, Na, Ca, Al, etc.

colourless, white, light yellow, cream, pink

2.5 - 2.7

6.0 - 6.5

good cleavage vitreous luster,


D.6.5 TECHNICAL AND ECOLOGICAL PROBLEMS IN AMALGAMATION

Amalgamation is one of the most important processes in gold production in small-scale mining in developing countries. Gold in ore slime is alloyed with mercury into an amalgam and then this is separated by heating into mercury steam and gold.

The simplicity of the technique and its gold yield has favoured amalgamation in the eyes of small-scale miners. Health risks and ecological dangers are however not considered. As a result of the faulty application of the procedure, reports of massive mercury poisoning in developing coutries are encountered now and again, Unfortunately, such incidents are not to be viewed as isolated accidents. The same is true for Latin America where, according to estimates, a million people work directly in gold mining. In Brazil alone 650.000 people are active in this branch. Other important gold producing problematic countries of South America are Bolivia, Chile, Ecuador, Colombia, Venezuela and Surinam. But also in countries of other continents, for instance, the Philippines, New Guinea and Ghana, environmental problems as a result of the application of mercury are increasing.

The sensibilization of people to ecological issues in Brazil in connection with the destruction of tropical rain forests has lead to intensified investigations into the mercury problems.

Exact figures about the extent of Hg-emssion in the tropical ecosystem are hardly available especially since figures about mercury purchase do not exist and the real gold production - which eventually allow inferences - lies very evidently above the official output. According to Brazilian reports, mercury consumption lies between 35 and 200 t Hg/y. The most recently promulgated mercury prohibition in Brazil has largely remained ineffective.

D.6.5.1 TOXICOLOGY OF THE HG AND ITS COMPOUNDS

The toxicity of mercury depends greatly upon the nature of the compound and the state of oxidation of the mercury.

Some 75 - 80 % of steam-forming mercury, such as that released in amalgam distillation in an open cycle or circulation, is reabsorbed pulmonarily by humans. It reaches the kidneys through the bloodstream and with a half-life period of about two months, it is excreted again from the body as Hg-protein compound. The toxic effect results from the Hg2+-lons. The manifestation of an acute Hg poisoning through inhaling of mercury steam proceeds in stages as follows:

1. Colic, vomitting and intestinal inflammation

2. Kidney and urinary tract complaints

3. Acute intestinal inflammation, and eventually

4. Formation of cysts on the gums (stomatitis mercurialis) accompanied by a heightened light sensibility (Photophobia).

If the mercury steam is inhaled for a longer period, chronic poisoning develops (Merkurialismus). The symptoms are:

- formation of cysts
- HgS-deposits in the body
- nervousness, and trembling
- speech impairments, concentration difficulties, among others

Many "garimpeiros" ("gold diggers" in Brazilian Portuguese) suffer from acute and chronic mercury poisoning.

Inorganic Hg2+-compounds exhibit analogous toxicity consequences.

Organic mercury compounds, particularly methyl mercury (CH3Hg+) are highly toxic for men. When taken in through contaminated food, this is assimilated in the blood due to its stability and solubility in fat, and leads to damages to nerve ends.

Metallic mercury cannot be absorbed by the human body and is not toxic in this form.

D.6.5.2 MERCURY CYCLE

Mercury that is released in the atmosphere by evaporation is oxidized in the course of time under the influence of ozone, air humidity and ultra-violet radiation. Precipitation transport these ions to earth. Further details of the Hg-cycle in the biosphere and the incorporation of metallic mercury through mining. Microbial anaerobic conversions especially in aquatic ecosystems and on land can lead to a methylizing of the mercury.

The above mentioned inorganic and organic Hg-compounds reach the human body through drinking water, food and through respiration. An especially critical point is the Hg-concentration through the food chain of fishes which can accumulate this element at a multiple rate compared to the concentration in the environment. This effect is even increased by the fact that the Hg-transformation in methyl mercury and its intake in warm, tropical bodies of water (depending upon the temperature and Eh-pH conditions) is especially high.

New researches in Brazilian ecosystems show clear violations of the effective "Maximale Arbeitsplatz Konzentration" (MAK) and "World Health Organization" (WHO) threshold value for Hg and its compounds (see box).

WHO- and MAK-limits for mercury

drinking water

maximum 4 yg/1


fish

maximum 0.5 yg/g weight;



or

maximum 0.2 mg methyl-Hg


per person and week



or

maximum 0.3 mg inorganic Hg

fruits

maximum 0.03 mg/kg weight


air one breathes

for Hg°

maximum 0.1 mg/m³ (MAK)


for organic



Hg-compounds

maximum. 0.01 mg/m³ (MAK)

The following mercury contents were found in:

drinking water, up to

11/µ/ltr

in fishes, up to

2.7 µg/g peso fresco

in spawns, up to

3.8 µg/g

in the air, up to

0.3 mg/m³

The last figure was obtained in the surroundings of an amalgam distillation plant The extreme values measured exceed the corresponding reference measurement in drinking water by a factor of 250, in fishes by about 300 times and in the air by a factor of 14,000. Unfortunately, it was found that the mercury concentrations were not restricted to the particular area. Rather, significant mercury levels were found within a radius of about 200 km around the mining area.

This manifests itself in the mercury levels in human blood, in the urine and in the hair particularly in comparison to the normal values contained in parentheses below:

in the blood

in urine

in the hair


ppb Hg

ppb Hg

yg/g dry weight


up to 175

up to 225

up to 40.0


(10-13)


(0.5-8)

(0.7-3)

The indian natives are affected in a particular way by the mercury contamination of the environment since they nourish themselves almost exclusively from their immediate surroundings. River fishes are, for example the most important source of animal protein for this ethnic group. To worsen their situation, migrating gold diggers are competing in their living space.

D.6.5.3 AMALGAMATION IN SMALL-SCALE MINING FOR GOLD

The first use of amalgamation for gold production presumably dates back to mining in Bosnia in the reign of Emperor Nero (54- 68 A.D.). Until now, small-scale mining uses this technique very intensively.

For amalgamation, liberated, basic, not incrusted Au- for instance through fine iron oxides - in grain sizes of between 2 mm and 20 - 50 m is appropriate. The lower grain size is essentially determined by the interfacial tension of the mercury and that of the water as well as the shape of the grain.

Amalgamation may be applied in sedimentary ores as well as the primary intergrown gold ores. In alluvial deposit mining, the already liberated gold is made to combine with Hg. Riffle troughs whose riffle spaces are filled with mercury mainly serves this purpose. The entire suspended feed is made to flow through the trough. During this process, the slurry extracts about 5 - 30 % of the mercury from the troughs for which generally there is no available catchment mechanism. The addition of bigger amounts of soap or similar tensides should increase the beneficiation output while Hg losses decreases.

Primary ores require the exploration of valuable minerals. The miner amalgamates either directly during the grinding or crushing or in a separate procedural stage after grinding. In simultaneous amalgamation and crushing, small-scale mining utilizes pan grinders, stamping mills, ball mills or manual weight crusher. For post-activated amalgamation, amalgamating barrels, amalgamating furnaces (see respective technical discussions) and manual amalgamation in washing pans are used.

On the average, losses on metallic mercury from the sorting and amalgamation installations account for about 40 - 50 % of total losses.

The resulting amalgam-Hg mixture is separated into the highly viscous amalgam (Au2Hg and Au3Hg) and liquid mercury by squeezing-through a piece of leather or a towel (usually through the shirt of the miner).

The production of gold takes place through heating of the amalgam lumps with about 50 - 60 wt. % Hg, 40 - 50 % Au which are wrapped in paper. The procedure is done in an open ceramic bowl using a blow lamp and in temperatures of 350-600%. The steamed mercury goes directly uinto the atmosphere; it accounts for about 50 - 60 % of the entire Hg emissions.

Researches of the author in Ecuador and Colombia have shown that retorts (see technical discussion 15.7) for Hg distillation in closed cycles are known and usually available but are rarely applied. The main reason behind this is the discoloration of the gold after distillation in the retorts- presumably as a result of Fe-compounds-, and consequently a lesser valuation of the product by small-scale miners. It is to be determined to what extent technical improvements could encourage acceptance of distillation retorts.

In some cases, especially when mercury supply is short, a fresh banana leaf placed on top of a vaporising dish or basin can serve to the partial recovery of mercury. Mercury condenses on the surface of the leaf.

D.6.5.4 WAYS OF MINIMIZING RELEASE OF MERCURY DURING AMALGAMATION

As described above, the emission of mercury takes place in metallic form during amalgamation as well as in the form of steam through the separation of the amalgam in mercury and gold. This section discusses ways of avoiding both of these sources of contamination.

Metallic mercury is produced in the amalgamation devices almost exclusively as crushed mercury in the form of tiny pearls, referred to as "floured mercury". The surface of these tiny globules are usually inactive due to impurities (among others, very fine mineral particles, fat and oil sediments from the water), other chemical transformations (for example, incrustations from antimony amalgam and the globules' inherent surface tension. This means that the globules can neither serve amalgamation nor fusion of gold and are discharged during high slurry speeds.

In vein ore mining, it is possible to avoid such loss of mercury. This is done through the separation of the simultaneous processes of amalgamation and grinding or comminution in pan grinders (as in Chile) or stamping mills (as in Ecuador and Colombia) using two procedural steps. The effect of high slurry speeds during amalgamation is avoided.

In alluvial gold deposit mining the use of mercury in trough washing, during which huge amounts of mercury is released in the environment, should be avoided. Instead, it is recommended in the two branches of mining to first produce the highest possible quality of pre-concentrate. Among the devices appropriate to wet mechanized gravity sorting using specifically high throughput are: fluid bed centrifugals, spiral separator, cone separator and fine grain separator with bedding, tables or improved sluices. Through post-concentration by means of precipitation of mineral by-products, for instance through magnetic separation, the pre-concentrates can be further improved.

Afterwards, the relatively small amounts of concentrate can be amalgamated in appropriate amalgamation installations such as in closed amalgamation barrels (referred to in Ecuador as "chancho") or in quick grinding mill (for example, Berdan pan). These devices also allow the addition of reagents for the improvement of the surface activity of mercury, for example (NaOH, nitrate amalgam, ammonium chloride, cyanide and nitric acid or tenside.

The least that can be done in plants in which a change in the course of the process is not possible would be planning post-activated sink angles to catch the "floured mercury".

To prevent the release of mercury steam, the "queimador" (the one which distills the amalgam) must distill only in close mercury cycle. Amalgam pressing and distillation retorts sink the mercury loss up to below 0.1% per distillation. Distillation retort must be user-friendly and hence, in their construction, care must be taken especially in the choice of materials and the cooling systems.

Alternatively, the amalgam can be chemically separated. In the process, the hot thinned or diluted nitric acid dissolves the mercury and gold slime comes off. From the resulting mercury nitrate solution mercury is again deposited with base metals. Ores containing silver, lead to a successive Ag-concentration in mercury which then necessitates distillation to purify.

Generally, it appears that the centralisation of amalgamation and/or distillation is to be advised. Miners can then further process pre-concentrates or amalgam in these central plants.

D.6.5.5 TECHNICAL ALTERNATIVES TO THE SUBSTITUTION OF AMALGAMATION

As alternative to amalgamation, there are available a number of methods of gold production or extraction. Most of these however have not gained currency in mining until today. Among these is the "gold-coal agglomeration" which was developed about 70 years ago but has not found application in beneficiation. Others such as cyanide leaching has extensively dispensed with amalgamation in large-scale mining. Due to complicated procedures, difficult control and the dependence on huge amounts of reagents etc., these alternatives are not rendered less appropriate for small-scale mining. Further development, technically, of the gravimetric beneficiation such as the sorting in centrifugal area, the increased utilization of vibration devices or equipment or the combination of gravity beneficiation with other physical processes correspond more to the requirement of small-scale mining. Such devices, for example, the Knelson centrifugal, allow the production of pre-concentrates with very high quality and whose final conversion into marketable products takes place in one last procedural step. This is the melting separation of gold pre-concentrates in gold and clinker using borax as fluxing agent, a process which the "mineros" usually do themselves.

D.7 The processing of phosphate-containing raw-minerals into p-fertilizers

Around 85% of phosphate fertilizers produced today come from phosphorites and sedimentary phosphate mineralizations which are comprised of fine mineral crystallises. For application in developing countries, the simplest fertilizer processing methods are most appropriate for meeting the needs of small-scale mining. A direct feed of raw mineral is frequently not practical due to the relatively low solubility of untreated phosphorites. This can be alleviated by performing a short activation grinding, which dissociates the mineral bonds and consequently increases the specific surface area, and also fractures the crystallite structure which significantly increases the weatherability and therefore the solubility of these minerals.

The other important raw material from which phosphate fertilizers are produced is apatite. This mineral is even less soluble than phosphorite, so that a direct application of apatite onto cultivated fields exhibits a fertilizing effect only after a period of 10- 15 years. In small-scale mining in Zambia, the apatite raw-ore with about 10 - 20 % POx content, is currently being processed by having it react with sulfuric acid which produces a readily soluble mineral fertilizer. Small processing plants, with capacities of up to 2t/d, employ cement mixers as reaction tanks for the treatment with sulfuric acid.


Fig.: Impact and circulation of mercury into the ecosystem by small-scale mining.

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