Cement Production

Cement Production. There are various types of cement, of which the most commonly used are the natural Roman cement (product of firing marls and dolomites) and portland cement, which finds the widest application; it is obtained by firing clayey marls or, more often, by firing a mixture of clay and lime (76-80% lime, 20-24% clay). During firing, CaCO₃ decomposes into CO₂ and CaO, which enters into chemical combination with the components of clay (SiO₂, Al₂O₃, Fe₂O₃, etc.). The production of portland cement in the USSR is concentrated mainly in large plants equipped with new mechanical installations. The production process consists of the following operations: crushing limestone in crushers and rollers, grinding into powder in special ball mills (cominors and roars), where lime is mixed with the added washed clay; the resulting mixture (the finest powder suspended in water) goes to firing, which is carried out at a temperature of 1,400-1,500° in various types of furnaces; in the furnaces, coal is burned, which has been previously ground in a mill to form the finest powder. The resulting clinker from firing goes to a cooler, then to ball mills, where grinding takes place; the ground cement goes to silos, from which it is automatically poured into barrels. Workers in various departments are exposed to different professional hazards, of which the meteorological factor and dust are the most significant. Workers at furnaces are in conditions of elevated air temperature (depending on the furnace system - from 18-25° to 40-48°) and exposure to radiant energy. Dust is released in all processes of crushing and grinding of limestone and coal, when unloading the furnace, subsequent transportation, and grinding of cement, and when pouring it into barrels. In these processes, very large amounts of dust are released; according to German research (Kelsh), its content in the air varies in different processes from 55 to 1,720 mg/m³, with 87-1,920 mg/m³ found in old plants and 55-450 mg/m³ in more modern ones; at the Yerinsky plant (1925), about 260 mg/m³ was found near the mill, at the Volzhsky plant (1927) - 133-504 mg/m³. This dust is very fine, the overwhelming majority of cement dust particles have sizes not exceeding 4-5 μm, particles larger than 10 μm are rarely encountered. Prolonged exposure to such large amounts of dust causes workers to develop lesions of various organs: skin, respiratory tract, conjunctiva, hearing organ, etc. In older literature, there are indications that cement plant workers very often suffer from dermatitis known as 'cement scabies'; at present, this disease occurs relatively rarely. Much more often in these workers, cracks form in the skin folds due to exposure to accumulated material, and microorganisms penetrating these cracks lead to purulent processes, which are especially common in warm weather. According to data from Thompson and Brundage, who in 1929 presented detailed material from a large American plant, workers in cement plants suffer particularly frequently from bronchitis, skin lesions, mainly furunculosis, conjunctivitis, diseases of the ear, nose, tonsils, rheumatism, and disorders of the gastrointestinal tract; examination of 422 workers at this plant revealed many cases of chronic pharyngitis, conjunctivitis, earwax plugs, and others. Data from studies of cement plant workers in the USSR (Gauzner - 1925, Potapov - 1927) also confirm the high morbidity among cement workers with rhinitis, otitis, skin lesions, and subcutaneous tissue, etc. The question of the frequency of tuberculosis and pneumoconiosis among cement workers has been raised repeatedly; data from both foreign and USSR studies indicate that tuberculosis among these workers is not more common than in the rest of the population. As for pneumoconiosis, it develops rarely in cement workers and does not cause significant changes in lung tissue or in the general condition of the body. Thus, during an examination of 263 workers at the Yashkinsky cement plant (Vishnevsky, Yablokov and Yushkevich, 1931), pneumoconiosis was found in 20 people with more than 11 years of experience, of which 14 people had stage I and 6 people had stage II. Prevention mainly consists of full mechanization of all production processes (fully implemented in new USSR plants), installation of good supply-exhaust ventilation with dust extraction at its source in dusty departments, complete sealing of equipment on which crushing, grinding, and transportation of lime, coal, and cement take place. Further measures are personal prevention: appropriate special clothing, installation of washbasins, showers, baths, etc.