There are three rayon grade pulp units and one paper grade pulp unit in the private sector. The total installed capacity of the three rayon grade pulp units till 1977-78 was 117,000 tonnes per year and during 1976-77, production was 118,000 tonnes. The export and re-export of paper. This clearly indicates that there has been an almost exponential increase in import of paper, suggesting the necessity for further expansion of the paper industry in India. The figure for import is much higher in comparison to that for export. The projected consumption is given indicating considerable scope for further growth.
With growing literacy and standard of living, the paper consumption will increase, resulting in more pollution from the paper industry. Suitable measures should be taken to contain pollution while considering establishment of newer paper industries. The paper industry is a highly capital-intensive industry. A new integrated pulp and paper mill of a capacity of 100 tonnes per day, producing writing and printing paper, would need an investment of Rs. 700 million, including all costs, on the basis of a capital cost of Rs. 20,000 per annual ton.
This would take into consideration the possible escalation in costs in the coming years as it takes four to five years for a paper mill to commence production. Naturally, an investment on pollution abatement without much return has to be treated as a social obligation. The problem becomes more acute in this country when it is realized that our raw materials contain a high percentage of silica as compared to the Western World and that the quality of limestone, available in this country, near the paper mills, is of very poor quality containing a large percentage of silica. Our major cellulosic raw materials for paper industry are bamboo, woods, bagasse, grasses and straws. A majority of the mills use 70-80 percent bamboo, the balance being one or more of the above mentioned raw materials. Bacteria and fungi also interact with paper making materials and eventually lead to their biodegradation.
The ash mainly contains silica which, as will be seen later, increases the problems of pollution. The amount of dust produced during chipping of bamboo is 2 to 5 percent. Utilization of this dust is a major problem because of its high silica content. Laboratory experiments have been conducted, by screening it, and rejecting the fines, containing 18-20 percent silica and utilizing the accepted material, containing 6-8 percent silica along with normal chips for pulping.
The rejects containing 18-20 percent of total bamboo can be used for making manure after composting. Processes have been patented for making soil aids from dust containing cellulose. An Australian company has developed microbes capable of composting raw dust in 6-8 weeks. The above rejects could also be used for making cattle feed. The normal process consists in steaming the material in an autoclave, at a pressure of 10-15 atmospheres, for 2 hours, followed by treatment with ammonia.
The rejects can also be used as a fuel in boilers. However, because of the high ash content, briquettes will have to be made. The dust could also be used for making compressed boards, using synthetic resin as a binder. However, at the moment, the dust is a real problem in Indian paper mills. The chipper house is generally, full of bamboo dust, particles of which fly in the air, affecting eyes, lungs and the human respiratory system. A typical analysis of bamboo dust is: Ash 2.
7-3.5 percent, Si02 2.6-3.7 percent, A1203 0.2-0.25, CaO 0.26-0.
36 percent. In the case of paper mills, using bagasse as the raw material, pith which constitutes 30 percent of bagasse, is another source of pollution, as it cannot be used for pulping purposes. Because of the high silica content, the chemical consumption is high and it chokes the wire mesh of the machine wire. A lot of it flies in the air with bagasse fibres, during depithing operation, and causes ‘bagassis’ to the workers in the section. It has low calorific value and, hence, cannot be used as a fuel but it could be used for making Boards using synthetic resins binder. Recently, a biological method of depithing has been developed in South Africa to counteract these difficulties (Ritter’s Process).
Considering that about 25-30 percent of pith is separated in the process of bagasse pulping, it is evident that proper use for this has to be found if the cost of paper making has to be reduced. It could be used for making cattle feed. While no data are available on the nutritive value of pith as cattle feed, it may be safely assumed to be similar to corn cobs which are considered in U.
S.A. to have 60 percent of the feeding value of corn shows the analysis of bagasse screening along with other common roughages used in India at present. Pith can be used as a fuel. It could be mixed with pulverized coal or bagasse and fired in coal stoker boiler furnaces. For this purpose, a spreader stoker system has been devised in which the larger particles are burnt on a step grate, while fine pith is burnt in suspension. Another possible field of application is in absorption of nitroglycerine for the manufac¬ture of dynamite, because of its highly absorbent qualities.
It can be used as soil conditioner. It will promote humus accumulation in soil, hold water and retain humidity, thereby, adding to the bulk. About 4-5 percent pith, added to superphosphate, would prevent its caking. Some of the Indian Paper Mills partially use hard woods for pulping. This produces about 10-15 percent bark, depending on the age, height and species of the tree. Some of the mills also use soft woods as per their availability. Although bark could also be used for pulping with chips, it results Moreover, there is a limit up to which it could be used. It constitutes a disposal problem and the only recourse is to use it as a fuel or compost.
It could be used for making particle boards. However, because of the high capital investment, the industry does not bother about small quantities, being wasted, and the barks are usually burnt. There are two methods generally applied for pulping (a) Sulphate process (b) Sulphite process. In this country, all mills, except one, use the sulphate process because of the import of sulphur. The sulphate process utilizes caustic soda, buffered with sodium sulphide as the pulping liquor and is known as white liquor. Black liquor is the waste product, obtained after pulping the raw material and contains residual caustic soda, sodium sulphide, carbonate, sulphate, silica, dissolved lignin, pentosans, etc. The whole economy of sulphate pulping depends on the recovery of pulping liquor black.
There is no chemical recovery system in smaller units (say below 50 tonnes per day) since it is uneconomical and the entire black liquor has to be drained away, causing serious pollution problems, besides affecting the economy of the system. The black liquor, from smaller mills, could be used as a potential source for the manufac¬ture of a variety of organic compounds since it contains lignin which is an organic polymer, mainly aromatic in nature, containing carbonyl, carboxylic, hydroxyl, methoxy groups as reactive groups. So, also there are hemicelluloses present, which could be recovered in some form or the other. Lignin could be precipitated by passing the flue gas (carbon dioxide) to precipitate it. The slimy precipitate, slow and difficult to filter, is washed with water and dried. Some of the important organic compounds that could be manufactured from lignin are vanillin and related products, organic sulphides lignin such as dimethyl sulphide (DMS).
Dimethyl vanillin acid for hydroxybenzoic acid, catechols, etc. Lignin could also be used as a source of nitrogenous fertiliser; it helps in retaining moisture in soil. The entire organic matter is burnt in the furnace. This is not a very satisfactory way of utilization of valuable raw material since some valuable organic compounds could be manufactured from the same. An alternative source of fuel should be used to avoid wastage. Leakage from pumps is another source of pollution, apart from spoiling the floor of the building.
However, improved types of packing are now available which avoid leakage through the glands. The flue gases from the furnace contain soda ash and sodium sulphate particles entrained with them. Although all modern mills have electric precipitators which work at over 99 percent efficiency, the precipitators are, once in a while, thrown out of gear, causing nuisance, due to escape of soda ash in the surrounding air, which is injurious to the health of the workers.
Lime sludge or filter cake produced in the caustic sing plant of the recovery section is another source of pollution. About 0.6-0.8 ton of this sludge is produced for every ton of pulp manufactured and its composition is as follows: In western countries, where the quality of lime stone is better, it is reburnt percent silica. Moreover, the high silica content of the raw material further aggravates the situation, and the western method of disposal cannot be adopted. Several possibilities for disposal have been suggested: 1. Use raw material for nitro-lime fertiliser.
2. Use for making building material, blocks and bricks. 3. Use ass agricultural lime in acidic soil. However the problem remains unsolved on a commercial scale.
The filters used in paper making (China clay, bentonite, titanium dioxide, precipitated CaC03, etc.), and the fibres, in the exit water, from the machines, are another source of pollutants in the paper mill. Different flocculants such as ferric alum, activated silica, alum, and sodium aluminates have been used to reduce the total loss to less than one percent. In western countries, cationic poly-aciylamide has been used, thereby, reducing the total loss to less than 0.2 percent.
The back water is treated with alum and sodium aluminate solution and settled. However, one percent solids are still present in the back water. This is treated with 2 percent bentonite slurry and, then, with one percent actylamide solution. The water is allowed to settle and the top layer is used as fresh water. Air pollution is caused by gases containing sulphur compounds emitted at different stages of pulping and chemical recovery. Hydrogen sulphide, methyl mercaptan and dim¬ethyl sulphide are released at different stages. The gaseous pollutants are controlled generally by chemical treatments.
The particulate emissions from recovery furnaces are frequently controlled by an electrostatic precipitator or venturi scrubber and some reduction in odorous sulphide emission occurs in the scrubber. The foul odours could be reduced by introducing black liquor oxidation prior to evaporation. In addition to minimizing the air-borne pollution, black liquor oxidation provides several other benefits: 1. Reduces corrosion problems and improves heat transfer in multiple effect evaporators. 2.
Produces fuel which is less sticky and which can be fired at higher total solids concentration without trouble at the primary airports of the furnace. 3. Sodium carbonate content of green liquor is partly replaced with active sodium sulphide, reducing consumption of lime per tonne of bleached pulp produced. 4. Increases sulphidity. 5. Improves pulp yield and its bleach ability and strength properties.
Fly ash, from coal fired boilers, is a source of pollution. The fly ash is carried by flue gases, and, unless electrostatic precipitators are provided, it escapes into the atmosphere. Noise in the mill is another source of pollution. The shrill sound of blowing the digester, the kut-kut sound of chippers and the recovery-furnace sound can also be heard from a distance, particularly at night time.