5.7.1 Safety Issues
In cement manufacture and use, safety relates to the effect the processes and materials involved in cement manufacture have on humans and measures that must be taken to mitigate the effects when they occur. For instance, when cement is mixed with water a high alkali solution (pH of about 13) is formed by the disso- lution of calcium, sodium and potassium hydroxides. When this solution comes into contact with any part of the body it stands to damage it. Again, the cement tech- nology is associated with lots of dust particles which when inhaled stands to damage the respiratory tracts. Excessive noise due to vibration and operation of machine parts could damage the eardrums if measures are not taken to protect them.
To avoid any damage occurring, gloves, goggles, filter masks and ear covers are used for protection. Cement can cause serious burns on the skin if contact is prolonged or if the skin is not washed properly.
In several countries, chromium levels in cement are not allowed to exceed 2 ppm as chromium is thought to be toxic and a major skin irritant. The major safety concerns associated with cement use and production and measures to deal with the problems are listed in Table5.14.
5.7.2 Environmental Effects
The environment can be adversely affected at every stage during the manufacture of cement. Materials and processes that adversely affect the environment during cement manufacture and ways to mitigate their effects are:
i. Emission of air-borne pollution in the form of dust particles and gases during quarrying and manufacture. This can be mitigated by the installation of
Table 5.14 Potential safety concerns of cement use and mitigation measures
Problem area Cause Mitigation
Noise pollution Machine/engine operation
Wear ear mufflers Respiratory tract Dust particles Wearfilter mask Skin and body
damage
Cement/water mixture Wear goggles, gloves and protective clothing
Skin irritation Chromium in cement Keep chromium 2 ppm Body toxicity Chromium in cement Keep chromium 2 ppm
Accidents Faulty
equipment/carelessness
Well-maintained equipment/well-trained workforce
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equipment to reduce dust during quarrying and manufacture. Also, equipment to trap and separate emission gases and dust particles may be installed.
ii. Damage to flora, fauna and land during quarrying. This is dealt with by the institution of land/soil reclamation and restoration measures to re-integrate quarries into the countryside after they have been closed down.
iii. Release of large quantities of CO2into the atmosphere as a result of burning large quantities of fossil fuel. The CO2contributes to the greenhouse effect.
The CO2 associated with Portland cement manufacture falls into four cate- gories. In thefirst instance, there is the CO2derived from decarbonation of limestone. Secondly, there is CO2from kiln fuel combustion. Thirdly, there is the CO2released from electricity generated for use during cement production, especially during grinding of raw materials and cement, and lastly, CO2 is released into the environment from the vehicles used in cement plants and distribution.
Thefirst source of CO2is fairly constant, from a minimum of around 0.47 kg CO2/kg of cement to a maximum of 0.54 kg CO2/kg cement. The second source varies with the efficiency of the plant. An efficient pre-calciner plant emits 0.24 kg CO2/kg cement; a low-efficiency wet process plant emits 0.565 kg CO2/kg cement;
a typical modern plant emits about 0.30 kg CO2/kg cement. The last source is almost insignificant at 0.002–0.005 kg CO2/kg cement. In sum, therefore, typically, 0.80 kg CO2/kgfinished cement is released into the environment. This value does not include CO2associated with electric power generation, where the power gen- erated by coal is about 0.09–0.15 kg CO2/kgfinished cement. CO2emission can be mitigated by modification of the chemistry of cement by use of wastes, and by adopting more efficient processes.
5.7.3 Other Pollutants in Cement Production
Other pollutants released into the environment during cement manufacture and are therefore controlled include nitrogen oxides, sulfur dioxide and dust. Carbon monoxide, volatile organic compounds (VOCs), polychlorinated dibenzodioxins (PCDD), dibenzofurans (PCDF) and heavy metals are also released in trace amounts.
i.Dust Emissions and Control
Large quantities of dust are released into the environment during the mining, transportation and preparation of raw materials used for making cement. Dust is also generated in kilns used in the production of clinker, and finally during the grinding and bagging of cement.
The best available techniques (BATs) for reducing dust emissions in the cement industry are electrostatic precipitators and bag filters. Electrostatic precipitators (ESPs) use electrostatic forces to separate dust from gaseous streams. An electro- staticfield is generated across the path of a particulate matter in the air stream. The 5.7 Safety and Environmental Effects of Cement Manufacture and Use 159
particles acquire negative charges and move in the direction of positively charged plates where they are collected. Dust accumulated on the collection plates is dis- lodged and collected by electrode rapping. Bagfilters for de-dusting gaseous waste streams comprise a fabric membrane that is permeable to gasflowing toward and through it but which will retain particulates. As dust accumulates on the fabric membranes of the ‘bag filters’ the resistance to gas flow increases. Therefore, regular cleaning is required to maintain performance. The most common cleaning methods include reverse airflow, mechanical shaking, vibration and compressed air pulsing.
ii.NOxEmissions and Control
NOx is a general term used, collectively, to refer to all the gaseous oxides of nitrogen, namely nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrous oxide (N2O) and nitrogen tetroxide (N2O4).
NOxis formed by the oxidization of the molecular nitrogen of the combustion air as well as the nitrogen compounds in the fuels and raw materials during the combustion of fuel. The NOxformed in the kiln burning zone at temperatures above 1200 °C is called the thermal NOx. The amount of thermal NOxproduced in the burning zone is related to both burning zone temperature and oxygen content. The NOxgenerated by the combustion of the nitrogen present in the fuel is called fuel NOx. Fuel NOx is formed in the pre-calciner where the prevailing temperature is in the range 850–900 °C, which is not high enough to form thermal NOx.
NOx emissions can be reduced via primary process optimization measures, including optimization of clinker burning process, computer-based expert system for kiln operation, optimization of the main burner and multi-stage combustion for in-line pre-calciners. NOxemissions in the kiln flue gas can also be abated using either the selective non-catalytic reduction (SNCR) or the selective catalytic reduction (SCR) methods. During SNCR, NOxinflue gases is converted into N2 and water (H2O) by reaction with NH4–X compounds injected into theflue gas. The ammonia-containing solution may be supplied in the form of anhydrous ammonia, aqueous ammonia or urea. The SCR method involves the injection of ammonia-based reagents into theflue gas stream in the presence of a catalyst. The common reagents used include anhydrous ammonia, aqueous ammonia and urea.
NOx compounds are converted to N2 and water and the catalyst allows these reactions to occur at lower temperatures, between 300 °C and 400 °C, than those required for SNCR.
NOx emissions can also be reduced through primary process optimization measures, including optimization of the clinker burning process, computer-based expert system for kiln operation, optimization of the main burner (low NOxburner) and multi-stage combustion for in-line pre-calciners.
iii.SOxEmissions and Control
SO2is the most common form of the oxides of sulfur released into the environment during cement manufacture. Sulfur dioxide is formed during the combustion of fuel in the burning zone of the kiln and the pre-calciner. Sulfur dioxide is also formed as
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a result of the oxidization of pyrite and organic sulfur in raw feed materials in the pre-heater or the kiln inlet of long wet or dry kilns. The SO2 formed by the combustion of fuel is absorbed in the basic feed material in the pre-heaters of modern cement kilns. Therefore, in modern kilns, any SO2 emissions from the cement plant are predominantly due to oxidation of pyrite in the feed materials in the upper stages of the pre-heater.
The methods applied in the cement industry to reduce SO2emission are:
Use of absorbents
The addition of dry or wet absorbents, such as slaked lime, quicklime or activated fly ash with high CaO content to the exhaust gas of a kiln can absorb SO2. Where emissions are below 1,200 mg/m3the most efficient way of utilizing slaked lime is to add it to the kiln feed prior to clinker burning and let the SO2absorption occur in the kiln, rather than inject it into theflue gases after sintering. Where SO2 con- centrations are higher this is not economically feasible.
Dry scrubbers
At emissions levels in excess of 1,500 mg/m3, a separate scrubber may be required to reduce emissions below 500 mg/m3. A dry scrubber can be installed to precede the raw meal entering the kiln.
Wet scrubbers
These are the most commonly used method forflue gas desulfurization in coal-fired power plants. A slurry or an aqueous solution of calcium carbonate, hydroxide or oxide is used to absorb SO2 from the flue gases. The slurry is sprayed in a counter-current direction to the exhaust gas and collected in a recycle tank at the bottom of the scrubber where the formed sulfite is oxidized with air to sulfate and forms gypsum, CaSO42H2O. The gypsum is usually separated and used as a set controlling agent during cement milling, while the water is returned to the scrubber.
Activated carbon
SO2can be adsorbed fromflue gases using activated carbon. The flue gases pass through an activated carbonfilter constructed as a packed bed and the used acti- vated coke is periodically replaced with fresh adsorbent.
Primary optimization processes
SO2emissions can also be reduced via primary process optimization measures such as reduction in sulfur content in fuels and kiln feed, optimization of the clinker burning process, and addition of slaked lime to the kiln feed with the use of a dry scrubber if necessary.
iv.Trace Emissions
Other emissions from the cement industry which occur in trace quantities include carbon monoxide, volatile organic compounds (VOCs), polychlorinated dibenzo- dioxins (PCDDs), dibenzofurans (PCDFs) and heavy metals.
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