Industry Description and Practices
The preparation of cement includes mining ; crushing and grinding of raw materials (principally limestone and clay); calcining the materials in a rotary kiln; cooling the resulting clinker; mixing the clinker with gypsum; and milling, storing and bagging the finished cement. The process results in a variety of wastes, including dust, which is captured and recycled to the process. The process is very energy-intensive and there are strong incentives for energy conservation. Gases from clinker cooler are used as secondary combustion air. The dry process, using preheaters and precalciners, is both economically and environmentally preferable to the wet process because the energy consumption (200 joules per kilogram (kg) is approximately half of that for the wet process.
Certain solid waste products from other industries, such as pulverized fly ash (PFA) from power stations, Slag, roasted pyrite residues, and foundry sand can be used as additives in cement production.
The generation of fine particulates is inherent in the process, but most are recovered and recycled. Approximately 10-20% of the kiln feed can be suspended in the kiln exhaust gases, captured, and returned to the feed, other sources of dust emissions include the clinker cooler, crushers , grinders, and material handling equipment . When the raw materials have high alkali or chloride content, a portion of the collected dust must be disposed of as solid waste, to avoid alkali buildup. Leaching of the dust to remove the alkali is rarely practiced. grinding mill operations also result in particulate emissions. Other materials handling operations, such as conveyors, result in fugitive emissions.
Pollution Prevention and Control
The priority in the cement industry is to minimize the increase in ambient particulate levels by reducing the mass load emitted from the stacks, from fugitive emissions, and from other sources. Collection and recycling of dust in kiln gases is required to improve the efficiency of the operation and to reduce the atmospheric emissions. Well designed, operated and maintained units normally can achieve less than 0.2 kilograms (kg) of dust per metric ton (kg/t) of clinker using dust recovery systems. NOx emissions should be controlled by the use of proper kiln design, low NOx burners and use of an optimum level of excess air.
The NOx emissions from a dry kiln with preheater and precalciner is typically 1.5 kg/t of clinker compared to 4.5 kg/t for the wet process. The NOx emissions can be reduced further to 0.5 kg/t of clinker by after burning in a reducing atmosphere and energy of the gases recovered in a preheater/precalciner.
Monitoring and Reporting
Frequent sampling may be required during start-up and upset conditions. Once a record of consistent performance has been established, sampling for the parameters listed above should be as detailed below.
Equipment for continuous monitoring of opacity levels (or particulates in the stack exhaust whichever is cost-effective) should be installed. Sulfur content of raw materials , Direct measurement of particulate, SO2 and NO2 levels at the plant boundary levels should be carried out at least annually. When operational upsets occur, the opacity of kiln and clinker cooler exhaust gases should be measured directly and corrective actions taken to maintain the opacity level of the stack gases below 10% (or an equivalent measurement).
The pH and temperature of the wastewater effluent should be monitored on a continuous basis. Suspended solids should be measured monthly if treatment is provided.
Monitoring data should be analyzed and reviewed at regular intervals and compared with the operating standards so that any necessary corrective actions can be taken. Records of monitoring results should be kept in an acceptable format. These should be reported to the responsible authorities and relevant parties, as required, and provided to MIGA if requested.