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CLOSE THIS BOOKTools for Mining: Techniques and Processes for Small Scale Mining (GTZ, 1993, 538 p.)
Technical Chapter 16: 0ther Sorting and Separating Techniques
VIEW THE DOCUMENT16.1 Roasting oven, calcining furnace
VIEW THE DOCUMENT16.2 Salt gardens, salt works, salterns
VIEW THE DOCUMENT16.3 Sulfur production in heap smelting or chamber ovens
VIEW THE DOCUMENT16.4 Autoclaves for extracting sulfur
VIEW THE DOCUMENT16.5 Copper sulfate plant
VIEW THE DOCUMENT16.6 Electrostatic sorting
VIEW THE DOCUMENT16.7 Flotation

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

Technical Chapter 16: 0ther Sorting and Separating Techniques

16.1 Roasting oven, calcining furnace

Metal Mining
Beneficiation, Special Techniques of Separation

germ.:

Abrostofen

span.:

horno de calcinacion

TECHNICAL DATA:

Dimensions:

oven deck plate approx. 2 × 1 × 0.1 m

Weight:

approx. 50 kg

Form of Driving Energy:

thermal energy from heat of combustion

Operating Materials:


Type:

Fuels: oil, coal, gas or wood, possibly NaCI for chloridized roasting

ECONOMIC DATA:

Investment Costs:

when locally produced < 200 DM

Operating Costs:

high energy and fuel costs

Related Costs:

cleaning of exhaust gases

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

large quantities of fuel have to be made available or be transported

Special Feed Characteristics:

ores may contain sulfides but no! arsenic, selenium or mercury

Recovery:

after a sufficient retention period the feed oxidizes quantitatively (i.e. nearly 100%)

Regional Distribution:

Bolivia

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high


during roasting of sulphidic ores large quantities of sulfur dioxide are released; additionally there is a high risk of possible release of very volatile and highly toxic metals, such as mercury, arsenic and selenium amongst others, either as elements or oxidized

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

simple metal construction on top a brick oven foundation

Lifespan:

verylong |————|————| very short


depends on the aggressiveness of the roasted products

Bibliography, Source: Ullmann

OPERATING PRINCIPLE:

Roasting is a thermal process for eliminating sulfides, whereby the sulfide and other sulfide-compounds are oxidized, e.g:

2FeS + 3½O2 ® Fe2O3 + 2SO2

This reaction begins at approx. 105° C if there is sufficient oxygen partial pressure. The products to be roasted are spread out on a flat pan-shaped furnace plate and heated to over 105° C.

SPECIAL AREAS OF APPLICATION:

Roasting is performed when it is necessary to free oxidic concentrates of sulfide, for example tin-ore and wolframite concentrates. In addition, sulfide-containing gold ores are roasted prior to being leached with cyanide.

AREAS OF APPLICATION:

For purifying gold concentrates. This involves the roasting of high-grade pre-concentrates, whereby hematite and other iron minerals are broken down and, following a brief grinding, are pulverized into powder. The product is then freed of Fe-mineral fines by means of air (wind) classification or simple manual blowing.

REMARKS:

During the roasting process, large quantities of volatile, gaseous sulfur dioxides are generated. When, besides sulfides, arsenic, selenium or mercury compounds are also present in the products, volatile compounds of these elements also develop. The vapors or gases of these compounds are all highly toxic, and therefore it is not advisable to operate small roasting plants without cleaning of gas emissions.

For special requirements, such as to acquire easily-soluble halide for the leaching process or to eliminate high-metting-point minerals, chlorinated roasting can be performed under addition of common salt (sodium chloride).

SUITABILITY FOR SMALL-SCALE MINING:

The use of roasting ovens is appropriate for small-scale mining needs only in special cases since they are highly detrimental to the environment.

16.2 Salt gardens, salt works, salterns

Salt Mining
Beneficiation, Special Techniques, of Separation

germ.:

Salzgarten, Salinen

span.:

salines

TECHNICAL DATA:

Depositional surface area:

crystallization area: depends on seasonal climate fluctuations; ratio of evaporation to crystallization area = 1: 7

Dimensions:

storage ponds: approx. 1 m in depth; large ground basins, evaporating ponds: approx. 20 cm in depth; crystallisation ponds: approx. 20 cm in depth, thickness of brine layer in crystallisation ponds can be as shallow as a few millimeters

Driving Energy:

seasonally changing solar energy between approx. 5000 and 30.000 kJ/ha/year

Form of Driving Energy:

direct use of solar energy

Throughput/Capacity:

evaporation: approx. 0.2 - 0.5 (1.5) cm/day, approx. 100 - 300 t/ha/year

Technical Efficiency:

approx. 43 m³/t NaCI produced

ECONOMIC DATA:

Operating Costs:

mainly labor and pumping costs

Related Costs:

possibly secondary cleaning facility, rakes and shovels for harvesting salt; possibly tractors, shovel loaders, etc.

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

location should be characterized by extensive sunshine and low atmospheric humidity (high evaporation potential)

Special Feed Requirements:

ocean saltwater or water (brine) from salt lakes of saltwater composition: NaCI 2.723 %; MgCI2 0.334 %; MgSOA 0.225 %; CaSO4 0.120 %; KCI 0.076 %; NaBr 0.010 %; CaCO3 0.011 %; H2O 96.495 %

Regional Distribution:

technique is distributed worldwide; approx. 30 % of the world production of NaCI is produced by evaporation and crystallization of saltwater.

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————————| very high


space requirements, possibly damage to sea-coast areas

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

simple, but involves very large surface-area excavations, possibly excavation machinery needed

Lifespan:

very long |————|————| very short

Bibliography, Source: Ullmann, Muller

OPERATING PRINCIPLE:

The winning of NaCI in salt gardens is achieved by evaporating saltwater (from oceans, seas, or salt lakes) according to the separation procedure listed below. The crystallization ponds are fed in such a way that carbonates and gypsum are precipitated separately, as the following processing steps indicate:

1. Pumping of saltwater into the initial evaporation ponds.

2. Concentration of the brine in these ponds to a density of approx. 6.5° Be (Baume-Scale: 6.5 mass-% salt solution).

3. Transfer of this pre-concentrated brine to a second evaporation zone, where the evaporation continues up to 17° Be; at this density the bulk of the gypsum precipitates out

4. Transfer to a third evaporation zone, where the brine reaches the saturation point for NaCI. At 20° C the corresponding density is 25.6° Be.

5. Transfer of the saturated brine into crystallization ponds, where density may not exceed an upper limit of 29 - 30° Be. In this way approx. 23 kg NaCI can be won from 1 m of saltwater, while the manganese and potassium salts remain in the mother liquor (spent solution).

6. Removal of the mother liquor from the cristallization pond.

7. Harvesting of the salt.

AREAS OF APPLICATION:

Used for production of common table salt and NaCI for other purposes (e.g. electrolysis) from ocean saltwater and water from saltwater lakes. Bromine and magnesium compounds, as well as potassium salts, could also be separated and won as by-products. Chloridic mother liquors from salt lakes are then further evaporated and subsequently processed in a subsequent flotation facility where sylvinite for use in KCI production is floated out.

REMARKS:

Micro-organisms, plankton, algae and halophillic bacteria have an important influence through their ability to accelerate evaporation due to their radiation-absorbent coloring. The growth of micro-organisms can be enhanced by the addition of nutrients rich in nitrogen or phosphorus.

In situations where the salt is too greatly contaminated by magnesium and gypsum, it can be purified by a washing process in which the salt is washed in a screw conveyor with a saturated counter-current NaCI solution. Gypsum is removed in the float, and Mg-salts dissolve into the solution. The remaining product is dewatered in a hydro-cyclone and then centrifuged.

The average composition of common table salt is:

NaCI:

99.50 % (dry matter)

CaSO4:

0.25 %

MgCI2:

0.15 %

MgSO4:

0.01 %

KCI:

0.02 %

Insoluble residue:

0.02 %

Residual moisture:

3.00 %

Separation sequence of evaporation of ocean saltwater at 27° C:

Remaining saltwater

Initial separation of


100.00

calcite

CaCO3

32.22

gypsum

CaSO4

12.13

halite

NaCl

2.45

astrakanite

Na2SO4 × MgSO4 × 4H2O

2.18

epsomite

MgSO4 × 7H2O

1.96

kainite

MgSO4 × KCl3 H2O

1.63

hexahydrite

MgSO4 × 6 HgO

1.22

kieselite

MgSO4 × H2O

1.18

carnallite

KCl × MgCl2 × 6H2O

0.93

bischofite

MgCl2 × 6 H2O

In the production of potassium salts, it must be taken into account that the demand for agricultural fertilizer is subject to vast seasonal fluctuations, requiring large-capacity covered (weather-tight) storage facilities.

Small amounts of common salt can also be won by collecting precipitated salt crystals developing from salt water sprayed along the sea coast, for example on the cliffs along the shores of the Sinai Peninsula, at Cape Verde, etc.

In polar climates, smaller amounts of salt freeze when water sprays onto pack-ice and freezes, causing salt to crystallize out (Rassol). The freezing procedure is also utilized at an industrial scale for concentrating salt brines in cold climatic regions.

Besides the production of salt from ocean saltwater, production from dry, or former, saltwater lakes in the steppe, desert and high-altitude desert areas of Latin America is of great regional importance. Here, the actual winning of salt occurs by cutting the salt with huge axes into "Quader" or rectangular blocks (panes) weighing approx. 10-15 kg. To remove the most recent, or uppermost, crystallized epsom (bitter) or waste salts, the top layer is split off about 3 cm down. The further processing takes place in small milling plants in which the salts are coarsely crushed and then ground to the desired final grain-size.

Mined salts are, in some cases, heavily contaminated with bituminous substances or other minerals such as gypsum, quartz or a number of others. In order to clean them, the salts are dissolved, possibly boiled, and then enriched into pre-concentrates in cooling towers before being crystallized either under the influence of natural evaporation or by heating.

Rainfall hinders the operation of salterns or salt gardens. The diluting influence of rain water may be countered either by covering the crystallisation basins with plastic film or cement roofs, by diverting the highly-concentrated saltwater into deep, covered rain ditches, or by draining off the lighter (lower specific-density) rain water over an overflow weir.

Salt gardens are always built so that the initial solution, for example sea water, is conveyed by means of a pump into the highest-situated initial evaporation ponds. From here, the saltwater is discharged over weirs without pumps to the lower-lying subsequent evaporation steps and crystallization ponds.

SUITABILITY FOR SMALL-SCALE MINING:

The production of salt in salterns is, in terms of investment requirements, a typical small-scale mining technique which, given a suitable location, yields high specific production quantities and high-quality products.

16.3 Sulfur production in heap smelting or chamber ovens

Mining on Industrial, Minerals and Rocks
Beneficiation, Special Techniques of Separation

germ.:

Schwefelgewinnung im Schmelzmeiler, Kammerofen

span.:

recuperacion de azufre en carboneras de fusion, horno de cameras

ital.:

calcarone, calcarelli

TECHNICAL DATA:

Dimensions:

diameter up to 30 m, height up to 6 m, with inclined bottom (approx. 15° - 20°)

Weight:

brick masonry

Extent of Mechanization:

not mechanized

Form of Driving Energy:

energy of combustion

Mode of Operation:

intermittent

Technical Efficiency:

30 - 60 % recovery, in chamber oven up to 78 %

Operating Materials:


Type:

sulfur as fuel

ECONOMIC DATA:

Investment Costs:

depends on possibilities for purchasing construction material, starting at approx. 1000 DM

Operating Costs:

mainly labor costs

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

atmospheric partial pressure of oxygen must be sufficient to burn sulfur; for this reason the maximum topographic elevation is limited.

Recovery:

comparably low in heap smelting due to losses caused by capillary forces and burning of sulfur

Replaces other Equipment:

autoclaves

Regional Distribution:

Sicily


chamber oven

Experience of Operators:

very good |————|————| bad


heap


due to the low degree of melting

Environmental Impact:

low |————|————| very high


due to emission of H2SO3 (sulfurous acid)

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

masonry work

Lifespan:

very long |————|————| very short

Bibliography, Source: Ullmann, Buch der Erfindungen "Book of Inventions" (in German) 8 edtn..lV. Bd.

OPERATING PRINCIPLE:

The simple method of winning sulfur by smelting is performed in mounded ore piles, in which sulfur is openly piled against a brick wall (with drainage outlet) where it partially burns and partially melts. The losses of sulfur due to the combustion reaction to sulfur dioxide can exceed 60 %. In order to extract sulfur in a smelting pile, small-lumped feed material is piled into a round brick-walled construction 10 to 30 m in diameter and up to 7 m in height, and covered with spent melt, gypsum and clay. The pile is ignited on the surface, opposite the drain outlet at the deepest point in the pile, in order to generate heat to melt the sulfur. The burning is controlled through the exhaust outlet and openings in the cover. The duration of the smelting process in such a pile lasts about 3 weeks. The sulfur is extracted from the lower end and allowed to crystallize in flat basins or moistened wooden molds, such as is described for autoclaves.

Better smelting results can be obtained with the chamber oven, in which already-melted residues with residual sulfur content are burned, generating hot gases which are then directed through a duct into a second chamber where they serve as the heat source for smelting sulfur out of fresh raw ore. A multi-chamber design avoids the need to re-handle the ore, since the melted ore is burned in a second processing step, thus using the smelting chamber as a combustion chamber.

AREAS OF APPLICATION:

To extract sulfur from raw ore of volcanic or sedimentary deposits containing elementary sulfur.

REMARKS:

The burning of sulfur to produce hot gases for smelting purposes requires high partial pressure of oxygen. At the high altitude of the Andes, where many volcanic deposits of sulfur are being mined at elevations of up to 6000 meters above sea level, the air pressure is not sufficient for burning sulfur. In this situation, the mine operators resort to the autoclave technique which uses steam for smelting.

Chamber ovens can also process feed containing < 15 % S, and are therefore often used for reprocessing residual material from autoclave processing.

Due to its non-dependence on external fuel and its non-mechanized technology devoid of high investment costs, smelting In chamber ovens is very economical.

SUITABILITY FOR SMALL SCALE MINING:

Due to the high environmental Impact associated with the winning of sulfur in smelting-piles, the use of this method is only appropriate in situations where other fuels are not available. At high altitudes the partial pressure of oxygen is not sufficient for combustion, so that the use of chamber ovens or smelting piles is not possible.

16.4 Autoclaves for extracting sulfur

Mining of Industrial Minerals and Rocks
Beneficiation, Special Techniques of Separation

germ.:

Autoklaven zur Schwefelgewinnung

span.:

autoclave pare la recuperacion de azufre, recuperacion de azufre en autoclaves

TECHNICAL DATA:

Dimensions:

horizontal, slightly inclined cylindrical boiler approx. 1 m in diameter, 4.75 m in length; or vertical, of cylindrical dimensions

Form of Driving Energy:

thermal steam-energy from combustion

Alternative Forms:

preheating of water with flat-plate or concentrating solar collectors

Technical Efficiency:

depends on feed content of ore and absorptive capacity of host rock

Operating Materials:


Type:

water fuel, e.g. in Bolivia plant material llareta (Lat. azorella compacta)

Quantity:

approx. 5 m³/t S

ECONOMIC DATA:

Operating Costs:

very high fuel costs

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Location Requirements:

sufficient fuel must be available, or possibilities must exist to transport fuel in large quantities. Grain size of feed: the ore is fed as lumps, but sizes > 20 cm should be avoided due to the low thermal conductivity of sulfur.

Special Feed Requirements:

S content of feed should be as high as possible, which is achievable by selective mining, hand picking of rocks or preliminary flotation.

Output:

In contrast to the extraction of sulfur by heap melting or in chamber ovens, the autoclave process is highly independent of altitude. In Bolivia there are autoclaves in use at altitudes exceeding 4500 m above sea level.

Replaces other Equipment:

heap smelting, chamber oven

Regional Distribution:

in small-scale mining in Latin America, especially in the high-altitude cordillera region

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high


through waste deposition, gas emissions from steam-generating oven, and destruction of vegetation through the use of biogenic fuels

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

local construction companies

Lifespan:

very long |————|————| very short

Bibliography, Source: Ullmann

OPERATING PRINCIPLE:

To smelt sulfur in autoclaves, the lumpy feed is charged into vertical or horizontal cylindrical boilers in layers. The boilers are pressure-sealed following charging, and injected with hot steam at 4 - 5 bar, during which the thermal energy of the steam is transmitted to the ore. At temperatures exceeding 114°C, the surphur begins to melt. It is important that the steam continues to flow through the heaped feed material, since, on the one hand, sulfur is a poor heat conductor, and on the other hand, there is a sharp increase in dynamic viscosity of the melt above 158°C, which has a viscid (semifluid) effect. The melted sulfur flows downward and is discharged periodically through the bottom outlets, from which it then flows into large flat basins where the dark-brown sulfur melt cools down during crystallization and turns into a solid yellow mass. When this mass has reached a thickness of 30 - 50 cm it is manually broken up by means of crowbars and carried off as blocks. The spent melt is removed from the autoclave following depressurization of the melting chamber and discarded as waste. To simplify material transport, the feed material for horizontal autoclaves is brought into the melting chamber in small wagons. Depending upon the composition of the feed, this material still contains significant amounts of sulfur which either could not flow out during melting, or remained in capillary bonds. The recovery from technical-scale plants ranges between 47 % and 55%.

AREAS OF APPLICATION:

Used to extract sulfur from raw ore feed preferably containing > 25 % S. When the S-content is lower, the feed can be pre-concentrated via flotation and then dewatered prior to smelting in autoclaves.

REMARKS:

The energy costs for generating steam are extremely high. In small scale mining in the western cordillera region in Bolivia, a resinous plant is used as fuel. In any event, it should be investigated whether fuel consumption could be reduced by empoying solar energy collectors to preheat the water to just below the boiling point.

Alternatively, the melted sulfur can be poured into moistened wooden boxes, with inclined walls, and then recovered in the form of uniform castings.

SUITABILITY FOR SMALL-SCALE MINING:

Autoclaves are suitable even at high altitudes for the winning of sulfur from high-grade ore feed; however, the high energy consumption creates logistical, economical and ecological problems.

16.5 Copper sulfate plant

Metal Mining
Beneficiation, Special Techniques of Separation

germ.:

Kupfersulfatfabrik

span.:

fabrica de sulfato de cobre

TECHNICAL DATA:

Extent of Mechanization:

not mechanized/semi-mechanized

Operating Materials:


Type:

H2SO4, possibly fuel

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Replaces other Equipment:

production of sulfide copper concentrates

Regional Distribution:

Bolivia

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high


minimal environmental impact due to acidic residues in the tailings. With oven drying, very high impact due to destruction of vegetation and/or gas emissions.

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

masonry construction: tanks, drying basins, crystallization basins

Lifespan:

very long |————|————| very short

Bibliography, Source: Mina Azurita/Bolivia

OPERATING PRINCIPLE:

To produce a copper sulfate, the feed material is placed in a large reaction-tank (basin), soaked with a diluted sulfuric acid solution, and left to react for approx. 2 weeks. The resulting copper sulfate solution is then concentrated either by solar thermal evaporation of the water, or by heating over a fire in drying-calottes made of lead vessels. When the solution reaches the point of saturation, it is pumped into crystallization tanks where copper sulfate crystals form along the bottom and walls, and on iron-wire spirals hung from above.

AREAS OF APPLICATION:

The production of copper sulfate from raw ore, tailings piles, or beneficiation wastes is suitable for feed which is porous, rich in weathered copper minerals, rich in sulfides or sulfates, and of low iron content. During leaching of such material, sulfide is transformed through partial oxidation (supported by bio-catalyzation) into H2SO4 over H2SO3.

REMARKS:

In the vegetation-poor highlands of Bolivia, a three-man copper sulfate operation for processing old tailings piles has proven economical despite high energy and transportation costs.

SUITABILITY FOR SMALL-SCALE MINING:

Suitable technique for the processing of low-grade sulfidic and oxidic copper ores with relatively low investment costs.

16.6 Electrostatic sorting

Ore Mining, Mining of Precious Stones, Gold Mining, Salt Mining
Beneficiation, Special Techniques of Separation

germ.:

Elektrostatische Sortierung

span.:

concentracion electrostatica

TECHNICAL DATA:

Dimensions:

1 - 3 m in height

Weight:

large units are several thousand kg

Power Consumption:

electrical separation efficiency, voltages lie within the range of 5 - 90 kV at field strengths of 3 - 9 kV/cm several 100 W plus roller drive

Form of Driving Energy:

electrical

Alternative Forms:

none

Throughput/Capacity:

up to 5 t/h, in potash up to 25 t/h

Technical Efficiency:

comparatively low degree of separation in one processing step, therefore electrostratic separation always involves a multiple-step procedure

Operating Materials:


Type:

surface-active or chemosorptive substance, tensides for selective separation of various non conductors

ECONOMIC DATA:

Related Costs:

drying, preparation, conditioning

CONDITIONS OF APPLICATION:

Operating Expenditures

low |————|————| high

Personnel Requirements:

working with very high voltages demands strict compliance with safety rules by personnel

Grain Size of Feed:

minimum oF 150 ,um; extremely poor separation when higher proportions of < 40 ym are present due to dust adhesion on larger grains, adhesion on electrodes, etc.; maximum grain size 2 - 6 mm

Special Feed Requirements:

as mentioned above, the dust particles must be removed from the feed; additionally, a narrow-band classification is necessary, since electrostatic separation is characterized by a sharp, precise classification. This is attributable to the comparably high gravitational forces.

Replaces other Equipment:

is employed as an inexpensive sorting process in potash, diamond, and metal mining.

Regional Distribution:

worldwide, but has only seldom been applied as a beneficiation technique in small-scale mining

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very high

Suitability for Local Production:

very good |————————| bad

Under What Conditions:

qualified manufacturers experienced in high-voltage technology, capable of working with various materials, might possibly be in a position to build electrostatic separators

Lifespan:

very long |————|————| very short

Bibliography, Source: Schubert, EP 0231441, DE 3035589 C2, DP 2134298, Singewald, Bock, DP 2614146, DP 2125286, DP 2213370, DP 3146295, DP 3216735, DP 2609048, DP 3233780, DP 3233528, DP 3603165, and 166, DP 3825434

OPERATING PRINCIPLE:

Electrostatic separation utilizes the differences in conductibility of the various feed components. The feed material, which has been pretreated or conditioned with surface-activating reagents, is placed in an electrical field which, depending upon the type of construction of the separator, has various effects on the mineral grains:

- in a roller separator, the material is fed onto a roller electrode and directed past a counter-electrode. The non-conductive material is not charged by the counter electrode, but rather only polarised, creating weak adhesive forces which draw this material to the roller electrode. Conducting material transmits its charge to the roller electrode, then becomes recharged and is repelled by the roller electrode and drawn to the counterelectrode. Separating blades divide the sorted material into a conducting and a non-conducting product.

- In a plate separator, the feed must be triboelectrically charged prior to separation. These electrostatically charged particles fall through the gap of a capacitor field between two electrode plates and are differentially deflected by the electric field, depending on the particle material.

- In a high-tension separator, the feed is charged by a corona electrode. Through the rotation of the roller out of the sphere of influence of the corona discharge, the moving conductor material immediately loses its charge; due to the centrifugal force of the roller and the gravitation, these particles are propelled away from the roller, while the nonconductors continue to adhere to the roller and fall off later.

Significant parameters for the separation in electrostatic separators are the inner and surface resistances, the electricity contents, etc. as well as the moisture and grain-size distribution. In order to separate non-conducting materials, the feed must be specially prepared, depending upon the characteristics of the non-conductors, through, for example, the following measures

- thermic pretreatment to dry or alter the surface characteristics of the mineral particles

- triboelectric charging, for example in vibrating screens, rotating drums, fluidized-bed apparatuses

- conditioning with surface-active tensides which affect the surface conductibility of the minerals and their hydration sheaths.

AREAS OF APPLICATION:

- for the separation of heavy-mineral pre-concentrates, for example zircon, monacite, rutile, columbite, tontalite, scheelite, cassiterite, etc.

- for the separation of quartz from hematite concentrates, gold pre-concentrates.

- sorting of phosphate raw materials

- production of pre-concentrates during winning of diamonds

- processing of potassium salts, for example with sylvanite, carnellite, kieserite, etc.


REMARKS:

Specifically for the electrostatic separation of raw potassium (potash) salts, many reagents have been tested and patented which allow a selective separation of potassium-salt mixtures, such as aromatic carboxylic acids such as O-cresotic acid, phthalic acid, cinnamic acid, atropic acid, vanillic acid, salicylic acid, benzoic acid, fatty acids with 6 to 15 C atoms (e.g. hydroxyphenyl butyric acid), etc., which are added in amounts of 50 - 200 g/t, for example supported by auxiliary conditioning agents such as HCI-gas, NH3, acetic acid, silicic acid, etc. These reagents are, for instance, vaporized onto the surface, whereby temperature variations play an important role. Small-scale mining operations can only be advised against employing tints highly sensitive, difficult-to-control technique. All of the other above-mentioned substances can be electrostatically sorted in small-scale operations.

The separation in the electrical field is greatly affected by moisture in both the air and the material. Films of moisture within the molecular density range can already negatively affect the surface conductibility of the feed. The relative air humidity for successful sorting ranges between approx. 2 and 25 %.

SUITABILITY FOR SMALL SCALE MINING:

Electrostatic separation for secondary cleaning of concentrates and winning of by-products in ore-mining operations is an economic technique which is also of interest for small-scale mining purposes. The electrostatic separation of salt to produce potassium (potash) salt concentrates is only suitable for small-scale mining if the mineralogical composition of the feed is not too complex. However, this is not the case for most of the deposits being mined by small-scale mines. Additionally, the unsuitability for local construction generally requires the use of imported equipment.


Fig.: Principle of electrostatic sorting. Source: Otero.

Table: Behavior of minerals in the high-voltage electrical field

Minerals attracted to the rotor

Minerals repelled by the rotor

baryte

cassiterite

beryl

chromite

calcite

galena

quartz

gold

diamond

magnetite

feldspar

graphite

monacite

haematite

scheelite

ilmenite

sillimanite

pyrite

zircon

rutile


wolframite

16.7 Flotation

Mining General (Gold, Ore, Coal,Industrial Minerals)
Beneficiation, Sorting

germ.:

Flotation

span.:

flotacion

Manufacturers:

Aker, Booth, Denver, Galigher Comp., KHD, Krupp, Machinoexport, Minemet Ind., Hoechst (reagents), Outokumpu, Sala, Wemco, Maxwell, INCOMEC, Volcan, Eg. Ind. Astecnia, IAA, COMESA, FAHENA, FINA, Famia, Fund Callao, MAGENSA, MAEPSA, Met. Mec. Soriano, PROPER, IMPROCON, MILAG

TECHNICAL DATA:

Dimensions:

flotation cells approx. 1 × 1 × 0.8 m up to 5 × 5 × 2.5 m and larger

Weight:

approx. 1 - 20 t

Extent of Mechanization:

fully mechanized

Power:

2.2 kW to 100 kW, approx. 1.5 - 5 kW/m³ volume of flotation cells

Form of Driving Energy:

electromechanical

Mode of Operation:

continuous

Operating Materials:


Type:

compressed air reagents bubbles < 2 mm in diameter

Quantity:

0.3 - 2 m³ /min m³ of slurry

ECONOMIC DATA:

Operating Costs:

high grinding costs

Related Costs:

dosing mechanism for reagents, grinding facility, classifying facility, settling pond for tailings

CONDITIONS OF APPLICATION:

Operating Expenditures:

low |————|————| high

Maintenance Expenditures:

low |————|————| high

Personnel Requirements:

for good separation results, precise control of slurry density, alimentary quantities and concentration of reagents is necessary

Grain Size of Feed:

50 · 200 ym

Recovery:

with preliminary flotation and subsequent cleaning of sulfidic ores, considerably higher than with gravimetric methods

Replaces other Equipment:

wet-mechanical sorting processes

Regional Distribution:

worldwide, flotation is the most widely used sorting process for mineral raw materials, partially also used in small~scale mining; approx. 2 billion tons of raw material is floated annually

Operating Experience:

very good |————|————| bad

Environmental Impact:

low |————|————| very bigh


high environmental impact through discharge of reagents with the tailings. The use of tailing ponds, neutralization basins, etc. and precise dosages of reagents are absolutely necessary.

Suitability for Local Production:

very good |————|————| bad

Under What Conditions:

flotation cells can be locally produced, e.g. from wood, iron, ferro-concrete or plastic materials; remaining components from imports

Lifespan:

very long |————|————| very short


when components subject to wear are made of elastomers

Bibliography, Source: Stewart, Priester, Taggert, Schubert, Gerth, Manufacturers information

OPERATING PRINCIPLE:

The flotation process utilizes the differences in surface wettability of various minerals, which can be artifically influenced, to achieve a separation. The completely-liberated feed material is suspended in a slurry containing approx. 30 % solids (by volume) and the valuable mineral selectively hydrophobed through the addition of collector reagents, which are mostly long-chained hydrocarbons of specifically regulated pH-values. This conditioned slurry then flows into the flotation cell' where it is brought into contact with injected, dispersed air bubbles; the electively-hydrophobed valuable-mineral particles adhere to the bubbles and travel upwards as a foam-mineral mixture (possibly stabilized through the addition of foam reagents or 'frothers') to the slurry surface where this "float" is then skimmed off. To suppress the unwanted hydrophobing of accompanying minerals and to enhance their removal with the "non-float", depressant reagents are added to the slurry. In the indirect flotation process, the valuable mineral is concentrated in the hydropilic non-float.

AREAS OF APPLICATION:

For the selective extraction of valuable minerals from raw ore feed:


- sulfide minerals
non-ferrous metal minerals (sometimes following sulfidizing)
precious metals

- fluorite, apatite, phosphorite, sulfur
- wolframite, scheelite, cassiterite, industrial minerals (sand and gravel)
- coal, graphite
- potassium (potash) salts
- quartz, keolin, feldspar, mica

SPECIAL AREAS OF APPLICATION:

To separate impurities and accompanying minerals from mineral-material mixtures

e.g.

reversed iron-ore flotation


reversed magnesite or calcite flotation


cleaning of glass sands

REMARKS:

For small-scale mining needs, flotation cells with external air supply can be recommended. This process requires more equipment and therefore higher investment costs, however permits regulation of the air supply to accommodate fluctuations in feed-quantity, feed contents, slurry density, etc. Self-aspirating cells allow a narrow range of variation only by changing the rpm of agitation.

Of importance for successful flotation is freshly-exposed surfaces. Especially sulfidic ores, which are easily subject to surface corrosion, require wet grinding prior to flotation.

Oil-flotation: W. Haynes/England/1860

Flotation with reagents for the separation of graphite, 1877 by Gebr. (brothers) Bessel/Germany

Foam-flotation: since the mid-twenties, important for very fine feed: agglomeration flotation (not economically significant)

REAGENTS:

Collectors
For sulfide minerals, anionic sulfhydryl collectors such as xanthate and dialkyldithiophosphate (for example, aerofloat, phosokresol) at concentrations of 10 - 200 g/t feed are applied,for non-sulfidic minerals: use of anionic oxhydryl or cationic collector, for example, long-chained, non-saturated (as much as possible) fatty acids or their soaps, which have previously been dissolved in hot oil, in concentrations of between 100 and 1000 g/t feed; by these quantities, the cost of reagents substantially affects operating costs. Silicates, halides and oxidic zinc ores are floated with organic amines as collector. To strengthen natural hydrophobia, for example in sulfur and coal or through the addition of an artificial hydrophobia, saturated hydrocarbons such as petroleum and oils are suitable.

Foaming agents/Frothers
Terpene and cresol or synthetic foaming agents added in quantities of around 5 - 50 g/t during sulfide flotation reduces the size of the bubbles and stabilizes the foam by lowering the surface tension.

Depressing agents/Depressants
Examples: zinc sulfate to depress zinc blende (sphalerite) in Pb-Zn-ores, cyanide to depress gold and silver, copper minerals, etc. by complexing.

Activators
Examples: addition of small quantities (1 - 10 g/t) of cyanide to clean mineral surfaces; sodium sulfide to convert oxide layers in sulfides; copper sulfate to activate zinc blende.

pH-reagents
to establish basic conditions: hydrated (slaked) lime, soda or caustic soda; to establish acidic conditions: sulfuric acids.

For small-scale mining, of special interest are individual components such as stator/impeller units from Aker which can be installed into existing, or possibly locally-manufactured, cells. In addition, these parts, being highly subject to wear, are normally made today of elastomers (for example polyurethane) which are extremely wear-resistant.

In order to assure the quality of the end products of flotation, precise control of the process is crucial. It is essential that the quantities of reagents added during flotation remain constant. Whereas this is performed today in large mechanized plants via dosing pumps, in small-scale mining, bucket-wheel proportioners have proven to be extremely effective. By altering the volume and/or number of buckets, or by modifiying the rpm of the bucket-wheel disk, they can be adjusted to cover a wide range of dosages. Furthermore, they are very sturdy, simple, accurate and suitable for local manufacture.

In addition to the process control, flotation also requires continuous monitoring of product quality. A simple periodic product sampling with the batea or gold pan assists many plants in quickly detecting possible deviations from the standard values. Small pan-shaped or inverted roof-shaped wooden troughs are used for this purpose.

Local products are sometimes used as reagents for the flotation, for example, natural oils, wastes from wood processing and from paper plants, etc. In this way the costs for imported reagents can be decreased substantially.

Tailings from flotation also provide a good aggregate or filler for lean mixed concrete backfill consisting of approx. 10 % cement, 60 % mine waste and 30 % flotation tailings.

EXPERIENCE FROM APPLICATION IN DEVELOPING COUNTRIES:

Representing the simplest forms of foam flotation, pipe flotation, in addition to flotation in sluices and settling basins (buddies see 14.10), is also being used.

The slurry, preconditioned with reagents, is allowed to fall into an open vertical standpipe, whereby air is drawn down along with it (after the principle of the water-jet vacuum pump). The aerated slurry is directed through the pipe into the flotation cell; perforations in the pipe allow the bubbles to escape and the flotation to take place. The float is subsequently scooped or skimmed off.

The quality of the flotation can be assessed simply by visual inspection of the bubbles on the slurry surface. A thick, fine-bubbled, and especially dark-colored foam indicate a correct reagent dosage and good mineral loading on the bubbles.

A foam with big bubbles and a transparent appearance removes only low quantities of "float" minerals and indicates an insufficient addition of reagents or an incorrect pH-value.

SUITABILITY FOR SMALL-SCALE MINING:

Flotation of sulfides is a suitable technique for small-scale mining, particularly if local manufacturers build the flotation cells and are dependent only on a few imported components. The selective sulfide flotation can also be considered appropriate for supplementing gravimetric beneficiation in small-scale processing operations.


Fig.: Designs of standard commercially-sold flotation cells. Source: Young.


Fig.: Types of impellers for standard, commercially-sold flotation cells. Source: Young.


Fig.: Operating principle of a flotation cell. Source: Otero.

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