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CLOSE THIS BOOKSmall-Scale Production of Portland Cement (HABITAT, 1993, 92 p.)
IX. ENVIRONMENTAL CONTROL IN SMALL CEMENT PLANTS BASED ON VSK TECHNOLOGY
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VIEW THE DOCUMENT9.1 Dust-generation sources and characteristics

Small-Scale Production of Portland Cement (HABITAT, 1993, 92 p.)

IX. ENVIRONMENTAL CONTROL IN SMALL CEMENT PLANTS BASED ON VSK TECHNOLOGY

Of late, environmental control aspects have been receiving increasing attention. Of the two aspects of pollution which are of immediate concern, viz., air and water pollution, the mini-cement plants based on the VSK technology are virtually free of water pollution as no effluents arc involved. Even as regards air pollution, which can basically be caused by both particulates and undesirable gases, the VSK cement plant operation ensures negligible emission of the latter, such as carbon monoxide, NOx etc., as the optimized operating conditions of the kiln generally prevent the emission of such gases. The generation of particulates, however, is inherent in the manufacturing process and the associated unit operations, which is controlled by the application of high-efficiency dust collectors wherever required. The operation of a vertical-shaft kiln, which is the heart of the process, is such that negligible emission takes place as the shaft kiln itself acts as an effective filter.

The dust-generation sources, their characteristics and present practices to ensure lower dust emissions are discussed in the following paragraphs.

9.1 Dust-generation sources and characteristics

The various dust-generation sources and their characteristics are considered below.

Mining

Mining is generally carried out manually for mini-cement plants. The level of dust generation during drilling, blasting and loading of limestone is thus relatively low. Dust-suppression techniques, such as water spray, are found to be adequate in most situations.

Crushing and raw materials proportioning

In the crushing section, generally a combination of primary and secondary crushers are used for size reduction. A single cyclone/fabric filter is mostly used to collect dust from both the crushing and the raw materials proportioning section. Sometimes, depending upon the specific situation, separate dust collectors for these two sections are also recommended.

Raw grinding

Depending upon the raw-material characteristics, air is swept of raw materials and fuel. In both the cases, cyclones and fabric filters are used to keep the level of emissions low.

Raw-material blending and homogenizing

The ground raw meal is blended and homogenized pneumatically in blending silos. The dust generated is collected in fabric filters to ensure low emission.

Kiln section

Raw meal is extracted from the nodulizer feed-hopper and is fed to the nodulizer where water is also sprayed to form nodules. In order to collect dust from the nodulizer a fabric filter is used.

The system design and the technology used in the modern vertical-shaft kiln (CRI-MVSK) have adequate inbuilt provisions to restrict dust emission from the chimney within the limits prescribed by the statutory authorities. This is a unique feature performed by the MVSK, which, in addition to clinkerization, also reduces dust generation significantly. The green bed of moist nodules which are constantly distributed on the top by the rotary feeder, would trap most of the dust particles and only allow gases into the stack. In fact, when CRI-MVSKs are operated according to standard procedures, the emission levels are very much within the stipulated norms. In addition, the principle of natural draft, which is employed in the system, further ensures very little entrainment of dust particles and even the small solid particles which may get into the gas stream would settle down on the bed due to gravity as the velocity is very much less in the stack.

Whilst, in general, operational experiences have proved that the kilns based on CRI-MVSK technology do not pose any problem with regard to dust emission, a dust collector of simple design has been developed by the National Council for Cement and Building Materials (NCB), New Delhi, which has been successfully installed in CRI-MVSK cement plants. The gases from the VSK are made to pass through the dust-settling chambers, where the dust particles become settled and the dust so collected is recirculated into the process.

Clinker handling

Clinker discharged from the VSK through the triple air-lock discharge gate/material block tube is crushed in a jaw crusher. Clinker and gypsum stored in separate hoppers are extracted to be fed to the grinding mill. A fabric filter is used to collect dust from the discharge gate/material block tube, clinker crusher and clinker and gypsum extraction points beneath their respective hoppers.

Cement grinding

In small cement plants based on the VSK technology, open-circuit tube mills arc generally employed for cement grinding. The mill works on the overflow principle and the product is directly conveyed to the storage silo. A fabric filter is used to facilitate dust collection to ensure a very low level of emissions.

The various types of dust collection equipment adopted for the different unit operations in an MVSK plant as described above, are summarized in table 8.

Table 8. Types of dust-collection equipment used in a CRI-MVSK cement plant

Unit operation

Equipment used

Crushing section

Multicyclone/air fabric filter

Raw materials proportioning section

Multicyclone/air fabric filter

Raw mill

Cyclone and fabric filter

Blending

Air fabric filter

VSK

Dust-settling chamber

Clinker extraction and handling

Multicyclone

Cement mill and packing

Air fabric filter

Control of fugitive dust

Specific instances of fugitive dust generation may include dust blown by wind from the raw-materials stockpile, dust caused by vehicular traffic within the factory, dust leakage from conveyors, conveyor transport points, silos, storage hoppers and packers etc. Good housekeeping, proper maintenance, wetting of dusty areas, use of enclosed storage wherever feasible etc., would considerably reduce fugitive dust.

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