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In this article we will discuss about the methods and techniques used to treat waste water from cement and ceramic industry.
Portland cement is a powder that, when mixed with water, will bind sand and stone into a hardened mass called concrete. Portland cement concrete is an attractive construction product due to its low cost, high compressive strength and durability.
Concrete is an important ingredient in economic development, especially the development of infrastructure and large public projects for the development of natural or human resources, such as dams, bridges, railroads, schools, airports and the like.
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure or may be amorphous (e.g. a glass). Because most common ceramics are crystalline, the definition of ceramic is often restricted to inorganic crystalline materials, as opposed to the non-crystalline glasses.
The earliest ceramics were pottery objects made from clay, either by itself or mixed with other materials, hardened in fire. Later ceramics were glazed and fired to create a coloured, smooth surface. Ceramics now include domestic, industrial and building products and art objects.
Cement Industry:
The basic raw material for the production of cement is lime. Lime is obtained from a variety of sources, primarily limestone, cement rock, oyster shell marl or chalk, all of which are primarily calcium carbonate. In addition, silica, alumina and iron ore are needed. These are obtained from sand, clay, shale, iron ore and blast-furnace slag.
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The selection and amount of additional ingredients is a function of the desired properties of the cement produced. The raw material grinding and kiln steps can be performed by either of two production processes, wet or dry. The choice between the two depends on the water and chemical content of the raw-materials, the availability of process water and the cost and availability of fuel.
In the wet process the raw materials are ground with water and are fed into the kiln as slurry. In the dry process the raw materials are dried, dry ground and fed into the kiln pneumatically. The remaining steps are identical in the wet and dry processes.
Water Use:
The cement industry in terms of total water use does play an important role in most industrial economics. Since the cement industry is found in so many geographical locations the water use characteristics vary quite a bit from country to country.
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Water in the cement production process has three basic uses:
1. Cooling Water:
The major water use at most cement plants is for cooling. Cooling water is used for bearings on the kiln and grinding equipment, air compressors, burner pipes and finished cement. Most cooling is non-contact. Cooling use is approximately the same for both the wet and dry processes.
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Process water is needed only for the wet process. Here water is added to the raw materials to aid in grinding and to make slurry for feeding the ground material to the kiln. The process water enters the kiln as part of the slurry and is evaporated, providing no liquid wastes.
3. Service and Sanitary Water:
Water is needed in certain plants for the preparation of raw material, either washing or beneficiation. Water is also used in the disposal of collected kiln dust. In the wet process, kiln dust is leached of soluble alkalis to recover raw materials by mixing the dry dust with water to form slurry.
The slurry is then put into a clarifier. The treated dust slurry is returned to be used in the raw material slurry preparation. For both the wet and dry processes, collected dust can be mixed with water and fed to a settling pond where the settled solids are not reused and the clarified water discharged.
Although this method is practised, it is not recommended to discharge untreated waste-water. Large quantities of cement dust are generated due to grinding and handling of cement. Water is used to control cement dust by being sprayed on roadways and parking lots and used for washing trucks.
One possible use for water in the cement industry is for the prevention of air pollution. This use is not frequently employed now, but may become more important in the future. The air pollution from the kiln stacks can be treated by wet gas-scrubbers. The scrubbers will use large quantities of water (on the order of 10 times that of the total water use). Therefore, a former air pollution problem becomes a water pollution problem and the scrubber effluent must be treated.
Recycling and Reuse:
There is some potential for recycling and reuse of waste-water within the cement production process. Cooling water can easily be recycled by installing cooling towers or cooling ponds for the dissipation of waste heat.
For the wet process, cooling towers or pond blow-down can be used in raw material slurry preparation. In a wet or dry-process plant, if kiln-dust leaching is being used, the blow-down can be used in the preparation of kiln-dust slurry. In a wet-process plant, all other waste-water can be reused in the kiln process except the leachate effluent from the clarifier, which must be disposed of in a containment pond. In the dry process, there are limited numbers of reuse possibilities. However, leachate waste could be treated to produce water suitable for reuse by means of electrodialysis.
Sources and Characteristics of Waste-Water:
Similar wastes are produced by the wet and dry process. However, in the wet process the wastes enter the waste-water stream while in the dry process the wastes enter the atmosphere. In the wet process, spillage and overflow from slurry formation produce suspended solids, dissolved solids and alkalinity in the preparation and grinding stage.
However, the major sources of waste-waters are generally produced when addressing the dust or air pollution problem, i.e. the collection of kiln dust and its disposal. The amount of waste generated by the wet process is slightly more than the dry process due to more use of leaching.
Waste-Water Treatment Practices:
Few steps in the manufacture of cement directly produce liquid wastes. In non-leaching plants, contact of raw material or final product with water provides the major source of the waste load. These waste sources can be reduced through good cleaning and maintenance practices or by collecting these waste sources for treatment. One important area of waste is the storage of raw material and finished products.
Protection of these materials from precipitation and spillage on the plant grounds will reduce significantly pollutants entering the waste-water stream. For leaching plants, settling ponds, containment ponds and clarifying are used as pre-treatment processes.
The discharge of the containment pond is then treated by electrodialysis for final discharge or recycled for slurry formation. The cooling water can be recycled through cooling towers or ponds and the blow-down reused.
Ceramic Industry:
Manufacture of ceramic is an ancient art. In general ceramic can be defined as ‘the art and science of making solid articles by the action of heat on earthen materials, which have inorganic nonmetallic materials as their essential component’.
This definition includes not only materials, such as pottery, porcelain, refractories, structural clay products, abrasives, porcelain enamels, cement and glass but also non-metallic magnetic materials; ferrotrics manufactured single crystals, glass ceramic and variety of products which were not in existence a few years ago. Ceramic products are wide ranging.
Each of these products need different composition of raw materials different glazing materials and also to be fired to a definite maximum temperature, which differ for each product, and to a definite firing and cooling time temperature schedule.
Thus manufacturing processes for production of ceramics are equally wide ranging. However, the basic manufacturing process remains same. In general the manufacturing process can be divided in following steps.
Recommended Waste-Water Treatment Options for Units Manufacturing Various Ceramic Products:
Due to different nature of industrial (non-biodegradable) and domestic (biodegradable) waste-water from such units, segregation of the two is recommended.
After the initial step of segregation, the following treatment systems are recommended for these waste-waters:
1. For Industrial Waste-Water:
The waste-water from ball mill washing and propylene drum washing should be taken to independent settling tanks. The settled sludge should be periodically removed and dispose of to commensurate with applicable disposal practices, because the sludge from glaze preparation section would contain heavy metals and thus need to be disposed of in a secure manner.
The overflow from these settling tanks should be taken to the effluent treatment plant for final treatment. Such a system would help in reducing the load on effluent treatment plant.
The results of the samples collected at the inlet and outlet of settling tank are presented in Table 21.2.
The characteristics of the effluent show that the unit can dispose its water in municipal sewer. However, in the absence of any such facility, physico-chemical treatment of waste-water to remove high TSS and O/G concentration is required before its final disposal.
Sources of Waste-Water Generation:
The sources of waste-water generation in the unit are as follows:
Industrial:
About 52.5 m3/day (estimated) of industrial waste-water is discharged to the municipal sewage every day. Major industrial usage of raw water is in the pickling section and in ball mills section.
Total discharge of waste-water from the pickling section is estimated to be of the order of 22.5 Kl/day. The degreasing tank is reported to be cleaned after every 3-4 months and the effluent is utilised in manufacturing scouring powder (used for washing of utensils) by mixing with rice husk ash and sold to local market.
The pickling and neutralisation tank are cleaned every 15 days and 7 days respectively and the waste-water from both the tanks flow to a collection sump. Thus neutralisation takes place in the collection sump in course of time and the neutralised effluent is discharged to the municipal drain. Expected pollution parameters in the waste-water are pH TSS, and grease and Fe.
Domestic:
Domestic water is used in the toilets and for drinking purpose. It is reported that about 18 m3/day of domestic waste-water is discharged to the municipal sewer.
Waste-water treatment system- The unit does not have any waste-water treatment system either for industrial or for domestic waste-water, and discharges their effluent waste-water (of the order of 45 m3/day) to the municipal sewer.
Waste-water characteristics- Spot samples of water have been collected from the pickling unit and the ball mill section and analysed in government approved laboratories, for pH, TSS, oil and greases, metals, results of which are summarised in Table 21.3.
Water Pollution Prevention Techniques:
In ceramic industries water is used mainly in following areas:
1. Wet grinding of raw materials.
2. Preparation of slip for moulding the required shape.
3. Glaze preparation.
4. Glazing (in spray glazing).
5. Flour washing and equipment/container washing.
6. Domestic use.
The water pollution prevention techniques can be divided in two types of measures as discussed below:
1. Housekeeping Measures:
It is a common practice that hose pipes used for floor washing, equipment washing are kept open, resulting increase in total volume to be treated in effluent treatment plant, i.e. higher capital investment and recurring cost.
A simple measure, i.e. usage of self-closing type water hose pipes will reduce the avoidable capital and recurring cost of the effluent treatment plant. Careful handling of fuel oil to prevent spillage will help in bringing down the oil and grease in waste-water.
2. Process Modifications Operational:
Wet grinding is done mostly in ball mills, from where the slurry is sent to underground blungers for blungering. In production of potteries, stone wares, sanitary wares, porcelain fire bricks, etc. after Hungering, the excess water is removed in filter presses. In many cases, the tank capacity does not commensurate the wash water hence part of it overflows down the drain.
Sometimes part of the water is intentionally drained for quality reasons. Efforts should be made to maximise utilisation of wash water. At least the filter press wash water could be completely recycled without any adverse effect on quality.
Prepared glaze is normally stored in PVC or metallic containers. In case of dip-glazing there is no water discharge, however in case of automatic spraying, the un-utilised glaze from spray which normally is washed down the drawing can be recycled. If not fully, at least the first wash from the drain could be recycled for use.
In ball mill and other equipment washing (in raw material handling sections), it is a common practice to discharge the whole wash water, which is quite substantial. It is recommended that, the equipment should first be rinsed with a little quantity of water and wash water should be collected separately and recycled. If process permits then subsequent wash water can also be recycled.
Otherwise it can be taken to a settling tank and overflow to the effluent treatment plant. The wash water is the major source of TSS and heavy metals. The data from one such unit shows that the TSS concentration from such washing is more than 25,000 mg/l. Avoidance of wash water will not only reduce the pollution load, but also reduce the capital and recurring cost of effluent treatment plant.
Water Pollution:
The water discharged from potteries, porcelain, small scale sanitary ware manufacturing units, decoration wares, fire bricks, stone wares is very low. However, in big units and specially sanitary wares, tiles manufacturing units the water discharged is substantial.
The characteristic of the waste-water analysis shows the presence of high TSS concentration along with heavy metal depending upon the glaze. The waste-water should be treated before its final disposal. For proper design of waste-water treatment system, it is recommended to segregate non-biodegrade industrial waste-water from bio-degradable domestic waste-water of the unit.
The segregated industrial waste-water is then recommended to be treated by dosing alum singly or in combination with polyelectrolytes, followed by sedimentation. For domestic waste-water anaerobic treatment in a septic tank is recommended before its final disposal. However, if sewer facilities exist the domestic water can be directly discharged into sewer without any treatment.