Аuthor: Jagvir Goyal
Источник:http://www.tribuneindia.com/2000/20000511/science.htm

     Effective utilisation of flyash generated by coal-based thermal power stations has been a major area of concentration for the scientists and engineers, especially for the last 15 years or so. Large quantities of flyash produced as a by product of coal-based power have been viewed as a serious environmental problem. Its constituents being the oxides of silicon aluminium, iron, calcium and magnesium along with traces of some highly toxic elements, its simple disposal has not been considered safe from environment-protection point of view. Thus its consumption in production of useful building materials has been suggested as the best solution to reduce its bulky presence on the earth.

     A lot of research on the subject of flyash utilisation has been made by scientific organisations such as CBRI, I.I.Sc., Bangalore and various pollution control research institutes. Many suggestions have been put forth CBRI has brought out many data-sheets. I.I.Sc. has held a number of seminars. On the whole, three major uses suggested have been the use of flyash in production of cement, bricks and in embankments. The utilisation made in actual however is yet to be listed.

     Gazette notification:

     On September 14,1999, the Ministry of Environment and Forests, Govt of India had issued the final notification (№563) on flyash to conserve top soil, prevent dumping of flyash and to make its effective and time-bound utilisation. The notification lays down that no person, within a radius of 50 km from a coal or lignite based thermal plant, shall manufacture clay bricks or tiles or blocks for use in construction activities without using at least 25 per cent of ash (flyash, bottom ash or pond ash) with soil on weight-to-weight basis. This notification is yet to see its full implementation.

     However, a brick kiln set up at Lehra Mohabat to produce clay-flyash bricks is fast chasing a target of producing two crore such bricks of first-class quality. This is apart from the second-class and third-class bricks that get produced along with. The kiln has already produced more than 1.6 crore of first class clay-flyash bricks of which about 1.5 crore stand consumed by now. The 2.0 crore target is likely to be achieved by June/July, 2000. Thereafter, further target shall be fixed keeping in view the future requirements.

     It is a common occurrence that while shaping theory into practical, a number of variations are noted on the ground because of change in properties of local materials, atmospheric conditions, prevailing working conditions and the type of machinery being used. A countdown of whatever is happening in actual rather than standard theory therefore proves quite useful to those connected to the profession. As far as clay-flyash bricks are concerned, such a detail may boost many others to shed unfounded fears and adopt use of flyash in bricks thus contributing towards environmental protection.

     The kiln setup at Lehra Mohabat is a normal, continuous type Bull’s kiln that bakes the bricks at 1020°±20°C. It has a capacity of producing eight lakh bricks per month. The quantity of flyash mixed with clay is 30% by volume. It has been noted that a further increase in quantity to flyash tends to make the bricks brittle which though having high crushing strength get broken at the edges during transportation. It is important to note here that the quantity of unburnt coal in flyash falls in the range of 6% – 12%. The saturation moisture content of local flyash is about 47% on an average basis while density of dry flyash is about 0.8 t/cu.m.

     A normal time cycle of 48 days has been observed. However, the activities of weathering of clay, preparing the mix, moulding and drying of bricks run parallel to the activity of baking of bricks. Thus a kiln having a capacity of 9 – 10 lakh bricks will produce about 8 lakh first class bricks per month. The bricks are made by extrusion process. Under this process, the bricks are machine moulded by installing brick-moulding-machines at the kiln site. Clay is weathered in open bins before sending it along with flyash to primary mixers where the two ingredients are well mixed before transportation to a pug-mill by means of a conveyer belt. In the pug-mill, required quantity of water is added to the mix. The pug-mill well kneads the mix and sends it to the extruder which pushes it into the mould to get the shape of a running column. The column further runs across the cutting table where wire-cutters are used manually to cut it into bricks of required size. The moulded bricks are then shifted to trays made for the purpose and transported to the sheds for drying. Further process is similar as followed in a conventional brick-kiln.

     Test results

     Clay-flyash bricks, when tested for various requirements laid under IS 13757 for these bricks, give satisfactory results. These bricks need to be tested for compressive strength, water absorption and efflorescence. It can be observed that compressive strength of these bricks is very high in comparison to the minimum strength requirement of 105 kg/sq. cm prescribed by PWD specifications. The water absorption and efflorescence are also significantly lower than those noted in the conventional clay bricks normally used in the building. Overall, the test results strongly advocate addition of flyash to clay for the manufacture of bricks.

     Not only that, a saving in fuel consumption has also been noted while manufacturing these bricks. A fuel consumption of 11 to 12 tonnes of coal for production of one lakh its clay-flyash bricks has been noted. This is three to four tonnes lesser than the normal corresponding consumption of coal. The variety of coal also determines it consumption. If good Assam coal is used, the consumption may be further reduced. This saving occurs due to presence of unburnt coal in the flyash used. The brick manufacturer therefore gets benefited in many ways – he saves fuel; he saves clay and better test results of bricks help in easy sale of bricks. Then he is serving a cause also – by putting flyash to use and helping in saving the environment.

     More than 12000 cubic metres of flyash stand consumed in manufacture of bricks at Lehra Mohabat. Though it makes little percentage of the flyash generated by the thermal plant yet it is a significant quantity. Setting up of more such kilns may not only help in saving earth’s crust but solve the flyash problem also.

     Though issuing of notification by the Government of India binds the brick-kiln-owners to use flyash in the manufacture of bricks, tax incentives such as excise duty exemption and sales tax relief should be given to those complying with the notification. The brick kiln owners will then use flyash by free will and not by force. More facilities such as grant of low-interest-loans and power connections on priority should also be thought of.