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cement dust - an overview | sciencedirect topics

cement dust - an overview | sciencedirect topics

Environmental issues are a major concern of all industries and readymixed concrete is no exception. Emissions of cement dust are strictly controlled by the use of dust-extraction systems in loading areas, cement silo filter systems, restrictions on powder-blowing pressures and the provision of high-level silo alarm systems.

Many plants now have water-recycling systems which minimize the amount of water being discharged from the site. All wash-out and surface water is stored, any excess solids removed and the water re-used in concrete.

Dust, consisting of solid particles that are (a) entrained by process gases directly from the material being handled or processed, like cement dust or grain from grain elevators; (b) direct offspring of a parent material undergoing a mechanical operation, like sawdust from woodworking; and (c) entrained materials used in a mechanical operation, like sand from sandblasting. Dust particles are between 0.1 micron and 10 mm in diameter; they can be relatively large.

Fume, that is, solid particles formed by the condensation of vapors by sublimation, distillation, calcination, or other chemical reactions. Examples include zinc and lead oxides resulting from the oxidation and condensation of metal volatilized in a high-temperature process. Fume particles are from 0.03 to 0.3 m in diameter.

Smoke, which consists of solid particles formed by incomplete combustion of carbonaceous materials. Although hydrocarbons, organic acids, sulfur dioxide, and oxides of nitrogen are also produced by combustion processes, only the solid particles resulting from incomplete combustion of carbonaceous materials are called smoke. Smoke particle diameters are between 0.05 m and approximately 1 m.

Virtually every industrial process is a potential source of dust, smoke, or aerosol emissions, including waste incineration, coal combustion, combustion of heavy (bunker-grade) oil, and smelting. Agricultural operations are a major source of dust, especially dry-land farming, as are demolition and construction. Traffic on roads, even when they are completely paved, is also a major source. In 1985, 40% of the total suspended particulate matter in the air of Seattle's downtown industrial center was identified as crustal dust raised by traffic on paved streets.

Fires are a major source of airborne particulate matter, as well as of hydrocarbon emissions, CO, and dioxin-related compounds. Forest fires are usually considered a natural (nonanthropogenic) source, but fires for land clearing, slash burns, agricultural burns, and trash fires contribute considerably. Since 1970, most communities in the United States have prohibited open burning of trash and dead leaves, and the Resource Conservation and Recovery Act of 1976 (RCRA) regulates the management of municipal waste landfills so that dump fires are a thing of the past.

Wood-burning stoves and fireplaces also produce smoke that contains partly burned hydrocarbons, aromatic compounds, tars, aldehydes, and dioxins, as well as smoke and ash. A growing number of cities in the United States now permit only low-polluting stoves (certified stoves) and prohibit the use of wood stoves and fireplaces altogether during unfavorable weather conditions.

The most important chemically identifiable particulate air pollutant is lead, usually lead oxide or lead chloride. Vehicle exhaust is the source of most airborne lead in the United States except in the vicinity of nonferrous smelters, although demolition of structures in which lead paint was used also puts lead dust in the air. The Bunker Hill lead smelter at Kellogg, Idaho, the largest in the United States, was shut down in 1980, eliminating a significant source of lead pollution in that city. Lead from vehicle exhaust is deposited for several hundred yards downwind of highways. At concentrations of freeway interchanges, in urban areas, deposited lead has been identified as a source of lead intoxication.

PM is classified into two categories: PM directly related to materials in and around rigs and PM in the emissions from the use of diesel fuel. The former is primarily cement dust and fine particles from the handling and storage of sand or other proppant. This type of PM is unlikely to have any impact off the site, and onsite exposure is covered by US Occupational Safety and Health Administration rules.

Diesel PM emissions are from either stationary or mobile sources. Almost all fracturing pumps operate on diesel fuel. Their emissions occur intermittently, during the fracturing portion of well operations, which is typically a few days. Fracturing is followed by approximately the same number of days of rig movement and drilling operations. Then fracturing will recommence. This is in normal operations. In batch drilling, several wells are drilled but not completed, known in the industry as DUC (pronounced duck) wells. This offers the ability to defer the expensive fracturing step. In mid-2016, it was estimated that several thousand DUC wells were on hold waiting for better pricing before being fractured. In such a design, the emissions from the fracturing pumps are concentrated in time. Wind direction and speed determine the spread to the local area. The stationary (but distributed) PM monitors in the area are unlikely to be useful in identifying local impact.

The mobile source is diesel trucks transporting materials to support the operation. Where local surface water is not available, fresh water is brought in tankers. If the wastewater disposal sites are located elsewhere, as they probably are in the case of disposal in Underground Injection Control Class II wells, wastewater is hauled away in tankers. Thus, mobile sources will vary depending on whether local water is needed or flowback water is used. However, transport of sand or other proppant applies to all operations, and variation lies only in the amount used per stage. There is a trend toward using more, in the belief that it increases recovery.

Truck traffic is episodic. Long convoys produce bursts of emissions over short periods. Information on the frequency and volume of traffic could be acquired by the local municipality from the operator, but existing monitoring sites are unlikely to cover the route taken or the drilling site. Because the objective is to estimate the impact on the local population, the adequacy of PM measurements in other settings could be instructive. Except for a few studies, some of which are ongoing, most reporting has been about retrospective or anecdotal incidences of health outcomes to individuals. This parallels the situation for secondhand tobacco smoke and urban automotive discharge into the air.

The body of literature on PM measurement for evaluation of health outcomes appears unanimous on one point: Results of studies of concentrations measured closely proximal to individuals differ substantially from those of studies of concentrations measured at fixed points, with the latter being the current practice. Only the most evenly distributed contaminants provide similar results from both methods. Sensors close to the breathing zone are the gold standard.3 This is expensive even if appropriate wearable sensors were available (discussed later). Accordingly, researchers emphasize the need for complementary fixed location testing for cost-effective measures of cohort exposure over long periods.

The shale oil and gas setting needs epidemiologic studies that allow clear lines to be drawn between cause and effect. Cost-effective measurements with fit for purpose accuracy and resolution are needed. Investigators have shown that in these situations, examination of the most exposed portion of the cohort is required.4 This design is not only sounder but also would define the relative risk between exposed and unexposed populations. In our case, mere identification of the most exposed portion of the cohort may prove challenging because of the mobile source and episodic nature of the releases.

Prospective hydrocarbon areas where little or no activity has commenced offer the opportunity for baseline testing of pollutants. Examples are in North Carolina, New York, and countries such as the United Kingdom, Brazil, and Argentina. Even if the potential exposure areas are identified accurately, which in most cases is likely, the locations of monitors will require a high level of sophistication because of the uncertainties in the source, as noted previously. One such study is underway in North Carolina, near the city of Sanford. The likely production area is well defined, and most of the bluff bodies housing the sensors are set up downwind from that area, with one proximal to an existing state-directed air emissions measurement site to facilitate calibration. An effort was also made to estimate the likely routing of trucks carrying materials for the drilling site. Bluff bodies were placed at road intersections and in a school on the route. The sensing is for VOCs and PM. For the latter, the instrument is a MicroPEM, which is a newly developed portable device described later in this chapter. It is one of a group of devices developed in response to the generally held belief that cost-effective measures were not readily available.5 The National Institute of Environmental Health Services launched an initiative in 2006 to meet this need.6 A recent review details the state of the art.7

As with wastewater, successful and cost-effective air pollution control has its foundation in complete awareness of all of the individual sources, fugitive as well as point sources. The process of cataloging each and every individual air discharge within an industrial manufacturing or other facility is most efficiently done by first developing detailed diagrams of the facility as a whole. Depending on the size and complexity of the facility, it may be advantageous to develop separate diagrams for point sources and sources of fugitive emissions. Next, a separate block diagram for each air discharge source should be developed. The purpose of each block diagram is to illustrate how each manufacturing process and wastewater or solid wastes treatment or handling process contributes unwanted substances to the air. Figures 1-6 through 1-8 are examples that pertain to a facility that manufactures cement from limestone.

At this particular facility, cement, manufactured for use in making concrete, is produced by grinding limestone, cement rock, oyster shell marl, or chalk, all of which are principally calcium carbonate, and mixing the ground material with ground sand, clay, shale, iron ore, and blast furnace slag, as necessary, to obtain the desired ingredients in proper proportions. This mixture is dried in a kiln, and then ground again while mixing with gypsum. The final product is then stored, bagged, and shipped. Each of the individual production operations generates or is otherwise associated with dust, or particulates, and is a potential source of air pollutant emissions exceeding permit limits.

Figure 1-7 illustrates that raw materials are received and stockpiled at the plant and are potential sources of particulate emissions due to the fine particles of dust generated during the mining, transportation, and loading and unloading processes. Their susceptibility to being blown around if they are out in the open is also a factor. To control fugitive emissions from these sources, it is necessary to conduct all loading, unloading, grinding, and handling operations within enclosures that are reasonably airtight but are also ventilated for the health and safety of employees. Ventilation requires a fresh air intake and a discharge. The discharge requires a treatment process. Candidate treatment processes for this application include bag houses, wet scrubbers, and electrostatic precipitators, possibly in combination with one or more inertial separators. Each of these treatment technologies is discussed in Chapter 8.

A very important aspect of air pollution control is to obtain and then maintain a high degree of integrity of the buildings and other enclosures designed to contain potential air pollutants. Doors, windows, and vents must be kept shut. The building or enclosure must be kept in good repair to avoid leaks. In many cases, it is necessary to maintain a negative pressure (pressure inside building below atmospheric pressure outside building) to prevent the escape of gases or particulates. Maintaining the integrity of the building or enclosure becomes very important, in this case, to minimizing costs for maintaining the negative pressure gradient.

As further illustrated in Figure 1-7, the next series of processing operations constitutes the cement manufacturing process itself, and starts with crushing, then proceeds through mixing, grinding, blending, and drying in a kiln. Each of these processes generates large amounts of particulates, which must be contained, transported, and collected by use of one or more treatment technologies, as explained in Chapter 8. In some cases, it may be most advantageous from the points of view of reliability or cost effectiveness, or both, to use one treatment system for all point sources. In other cases, it might prove best to treat one or more of the sources individually.

Continuing through the remaining processes illustrated in Figure 1-6, the finished product (cement) must be cooled, subjected to finish grinding, cooled again, stored, and then bagged and sent off to sales distribution locations. Again, each of these operations is a potential source of airborne pollutants, in the form of particulate matter, and it is necessary to contain, transport, and collect the particulates using hoods, fans, ductwork, and one or more treatment technologies, as explained in Chapter 8.

The next step in the process of identifying each and every source of air pollutant discharge from the cement manufacturing plant being used as an example is to develop a block diagram for each individual activity that is a major emission source. Figure 1-8 illustrates this step. Figure 1-8 is a block diagram of the process referred to as the kiln, in which the unfinished cement is dried using heat. This diagram pertains to only the manufacturing process and does not include sources of emissions from the physical plant, most of which are sources of fugitive emissions.

Figure 1-8 shows that the inputs to the kiln include partially manufactured (wet) cement and hot air. The outputs include dry partially manufactured cement and exhaust air laden with cement dust, or particulates. The diagram then shows that there are four candidate technologies to treat the exhaust gas to remove the particulates before discharge to the ambient air. The four candidate technologies are:

Each of these technologies is worthy of further investigation, including investigation of technical feasibility and cost effectiveness. Also, each of these technologies results in a residual, which must be handled and disposed of.

For instance, the bag house technology produces a residual that can be described as a dry, fine dustessentially, raw cement. This material can be stored in a dust bin (the dust bin must be managed as a potential air pollution source), and from there many options are possible. The dust could be:

The first of the above options is only a partial solution at best, since there must be some blow down, if only to maintain quality specifications for the finished product. Burial is a final solution, but it must be accomplished within the parameters of good solid waste disposal practice. Water slurry is only an interim treatment step. Forming a water slurry transforms the air pollution potential problem to a water pollution potential problem (a cross-media effect). The slurry can be transported to another location without risk of air pollution, but once there, it must be dewatered by sedimentation before final disposal within the bounds of acceptable solid waste and wastewater disposal practices.

The foregoing example illustrates how an entire manufacturing facility must be analyzed and diagrammed to define each and every source of discharge of pollutants to the air as an early step in a technically feasible and cost-effective air pollution control program. The next steps are presented below.

The air filtering system must have the capability to eliminate 99.95% of all particulates above 20 m, since these particles are the main source for compressor erosion. Installations located in the desert, in coastal areas or near dust emitting sources (coal dust, cement dust, fly ash, and pollen) should be equipped with self-cleaning systems that have an effective dust removal system. The ambient air for many combined cycles located close to refineries contains air-borne heavy hydrocarbon matter that eventually could sinter on compressor blades and cause non-recoverable degradation. In the absence of an efficient and proper designed inlet system, the ingestion of humid and salty air at a seaside location may lead to erosion and corrosion in the compressor as well as in the turbine section of the gas turbine. Finally, it is recommended that an ambient air quality analysis be conducted during initial site development, as extremely useful information.

The cement industry is one of the main industries necessary for sustainable development. It can be considered the backbone for development. The main pollution source generated from cement industry is the solid waste called cement by-pass dust, which is collected from the bottom of the dust filter. It represents a major pollution problem in Egypt where around 2.4 million tons per year of cement dust is diffused into the atmosphere causing air pollution problems because of its size (1-10 microns) and alkalinity (pH 11.5).

Cement by-pass dust is naturally alkaline with a high pH value and represents a major pollution problem. The safe disposal of cement dust costs a lot of money and still pollutes the environment. The chemical analysis for the by-pass dust is shown in Table 13.7.

Because of the high alkalinity of the cement by-pass dust, it can be used in the treatment of the municipal sewage sludge, which is considered another environmental problem in developing countries since it contains parasites such as Ascaris and heavy metals from industrial waste in the city. Although sludge has a very high nutritional value for land reclamation, it might contaminate the land. The safe disposal of sludge costs a lot of money and direct application of sludge for land reclamation has a lot of negative environmental impacts and is very hazardous to health.

Mixing the hazardous waste of cement by-pass dust with the environmentally unsafe sewage sludge will produce a good quality fertilizer. Cement by-pass dust will enhance the fermentation process of the organic waste and kill all microbes and parasites. The high alkalinity cement bypass dust fixes the heavy metals present in the product and converts them into insoluble metal hydroxide. Hence preventing metal release in the leachate. Agricultural wastes must be added to the mix to adjust the carbon to nitrogen ratio as well as the pH value for better composting (El Haggar 2000). The produced fertilizer from composting is safe for land reclamation and free from any parasites or microbes that might exist in raw sludge.

According to cleaner production techniques and the industrial ecology concept, a number of alternatives can be demonstrated to utilize the cement bypass dust as a raw material in another industry or another process such as:

On-site recycling in cement production process (most efficient in wet process) and more research required to optimize the percentage of bypass dust to be recycled without affecting the cement properties.

On-site recycling of bypass dust within the industrial process can be done as a raw material to produce clinker. The wet process was found to be efficient and economic where 87% of the alkaline salts were removed by this method. On the other hand, the dry process requires treating the dust to remove the salts before mixing with the raw materials. The dust treatment process can be done by one of two ways:

Adding the dust to water basins with mechanical stirring and sometimes with heating using gases from the kiln; the solution is then left for a specific residence time before filtering the excess water.

The treated dust can then be recycled in the production line by 16% of weight from the raw materials used or if dried can be added to clinker with 8% by weight to be able to have the same standards of the produced cement. However, recycling the cement bypass dust will not utilize all the quantity of dust generated daily in cement factories.

By pressing the cement bypass dust in molds under a certain pressure, bricks/interlock/tiles can be formed with a breaking strength directly proportional to the pressure used to form them; sometimes the breaking strength is even higher than the pressure used. In the Turah cement factory in Egypt, experiments were conducted on the following:

Using 100% cement bypass dust with a pressure of 200 kg to form cylindrical cross-section bricks of 50 cm2 area were the breaking pressure of these bricks reached 120 kg/cm2. In addition, chemical treatment of these bricks during the hydraulic molding can achieve a breaking pressure of 360460 kg/cm2 for the 100% cement dust bricks.

Using cement bypass dust with clay to produce bricks has proven to reduce the weight of the bricks along with reducing the total linear drying shrinkage. In addition, this opportunity can utilize very high percentages of the bypass dust. However, this will still depend on the market needs and the availability of easily transported bypass dust.

Using bypass dust as a main raw material (4550%) along with silica and sandstone and melting the mix at temperatures ranging between 1,250 and 1,450C, glass materials were obtained. The glass product has a dark green color with high durability due to the high calcium oxide (CaO) content in cement bypass dust. It can be used for bottle production for chemical containers. This step was then followed by treatment for 1530 minutes at temperatures ranging between 750 and 900C to form what is known as ceramic glass. This new product, unlike glass, has a very high strength and looks like marble. The produced ceramic glass is highly durable and can resist chemical and atmospheric effects. Consequently, this new product opens the way for utilizing huge quantities of bypass dust in producing architectural fronts for buildings, prefabricated walls, interlocks for sidewalks, and many other engineering applications.

Because of the high alkalinity and pH value of cement kiln dust, it can be used in stabilizing municipal sewage sludge. Municipal sludge contains bacteria, parasites, and heavy metals from industrial wastes. Therefore, if used directly as a soil conditioner, it will cause severe contamination to the soil and the environment and may be very hazardous to health.

Two types of sludge from sewage treatment plants can be used; the first one from a rural area where no heavy metals were included and the second from an urban area where heavy metals might exist depending on the level of awareness and compliance.

Due to the high alkalinity of cement bypass dust, when it is mixed with municipal sludge, it enhances the quality of sludge by killing the bacteria and viruses (e.g. Ascaris) in the sludge. Also, it will fix the heavy metals (if they exist) in the compost and convert them into insoluble metal hydroxides thus reducing flowing of metals in the leachate. Agricultural waste, which is considered a major environmental problem in developing countries as discussed in Chapter 7, can be added to the compost to adjust the carbon to nitrogen ratio and enhance the fermentation process. Agricultural waste will also act as a bulking agent to improve the chemical and physical characteristics of the compost and help reduce the heavy metals from the sludge.

The uniqueness of this process is related to the treatment of municipal solid waste sludge which is heavily polluted with Ascaris eggs (a most persistent species of parasite) using a passive composting technique. This technique is very powerful, very efficient with much less cost (capital and running) than other techniques as explained in Chapter 5. First, primary sludge is mixed with 5% cement dust for 24 hours. Second, agricultural waste as a bulking agent is mixed for passive composting treatment. Passive composting piles are formed from sludge mixed with agricultural waste (bulking agent) and cement dust with continuous monitoring of the temperatures and CO2 generated within the pile. Both parameters are good indicators of the performance and digestion process undertaken within the pile.

Passive composting technology has shown very promising results, especially by adding cement dust and agricultural wastes. Results show that Ascaris has not been detected after 24 hours of composting mainly due to the high temperature elevations reaching 70 to 75C for prolonged periods, as well as the high pH from cement dust. Also, the heavy metal contents were way beyond the allowable limits for both urban sludge as well as rural sludge.

As a result of previous discussion, three major wastes (cement bypass dust, municipal wastewater sludge (MWWS) from sewage treatment plants as well as agricultural waste) can be used as byproducts to produce a valuable material instead of dumping them in landfills or burning them in the field. This technique will protect the environment and establish a new business. If cement bypass dust does not exist, quick lime can be used to treat the MWWS. Sludge has a very high nutritional value but is heavily polluted with Ascaris and other pollutants depending on location. Direct application of sludge for land reclamation has negative environmental impacts and health hazards. Cement bypass dust is always considered a hazardous waste because of high alkalinity. The safe disposal of cement dust costs a lot of money and still pollutes the environment because it is a very fine dust with a high pH (above 11) and has no cementing action. Agricultural waste has no heavy metals and contains some nutrients, which will be used as a bulking agent. The bulking agent can influence the physical and chemical characteristics of the final product. It will also reduce the heavy metal content of the sludge and control C/N ratio for composting.

Three applications can be used to utilize cement bypass dust through road pavement layers. The first application deals with the subgrade layer, the second application deals with the base layer, while the third application deals with asphalt mixture as will be explained in this section.

Base layer: It is quite well known that limestone is used in the base layer (which is located right below the asphalt layer) for road paving. Also good binding and absence of voids in this layer is crucial to maintain strength and to prevent settlement and cracking. Therefore, due to its softness adding cement bypass dust as filler material to the base layer fills the voids formed between rocks. This helps increase the density (weight/volume) of this layer due to increase of weight and fixation of volume improving the overall characteristics of binding especially if base layers of thickness more than 25 cm are required. Also, the absence of voids in the base layer prevents the negative impact of acidic sewage water and underground water which work on cracking and settling the base layer.

Asphalt mixtures: Asphalt is a mix of sand, gravel, broken stones, soft materials, and asphalt. In Marshall's Standard Test* for designing asphalt mixtures, it was found that the percentage of asphalt required can be reduced as the density of the mixture increases. Therefore, adding cement bypass dust which has very fine and soft particles improves the mixture efficiency by filling the voids. Also, the bypass dust contains high percentages of dry limestone powder and some basic salts which in nature decrease the creeping percent of the asphalt concrete, enhance the binding process, and reduce the asphalt material required, which is very desirable in hot climates.

This process was implemented in the road joining the stone mill of Helwan Portland Cement Company and the company's factory in Egypt. The results of binding the base and subgrade layers assured that adding the cement bypass dust to the layers improved the overall characteristics of the road. The road is still operating and is in perfect condition even though the trucks using the road have load capacities not less than 100 tons.

If the transformer is oil filled, a check is made of the liquid level and for any leaks. The transformer may have an oil level gauge, or the level will be marked inside the tank (Figure 3-1). Larger transformers have a gauge, and smaller types have a level mark.

If the coolant level is low and a leak is suspected, locate the exact point of the leak by cleaning the surface of the transformer with a solvent. When the surface is dry, dust the area with a dry powder such as talcum, lime, or cement dust. The exact point of the leak can be readily located in this manner.

If the leak is above the oil level, an inert gas such as nitrogen can be inserted at 3 to 5 psi. Use a solution of soapy water on all joints and welds. The location of the leak will show up as a fine stream of bubbles. Leaks above the oil-level mark can occur because the coolant rises in the tank when it is heated during normal operation.

Once the leak has been located, it can be repaired if the transformer has already been accepted. Small leaks in welded joints can usually be repaired by peening them with a ball peen hammer. Larger leaks in joints need to be welded, brazed, or soldered. Leaks around gaskets can normally be stopped by tightening the bolts. If not, the gasket will need to be replaced. Leaks around threaded joints can be repaired by applying a joint compound that will not dissolve in oil. In the event this does not work, the joints need to be re-threaded.

Once repairs have been effected, the oil level must be replenished. Be sure the replacement oil is the same as that in the transformer. If mineral oil is mixed with any of the synthetic oils, it is almost impossible to separate them. The entire transformer may have to be drained of the contaminated oil and refilled with the recommended coolant. A 1000 kVA transformer may hold from 500 to 1000 gallons of oil, depending on its voltage rating.

Transformers should not be overfilled. In operation the oil becomes hot and expands, building up pressure inside sealed transformers. If the transformer is not sealed, the oil may spill out under normal operating conditions.

The motor enclosure is chosen to give adequate protection to persons against contact with live or moving parts inside the enclosure and to the machine against the ingress of solid foreign bodies. Adequate protection is also required against the harmful ingress of water and other liquids. The most vulnerable part of an electric motor to contamination is the electrical insulation of the windings and electrical terminations and it is important that the correct choice of enclosure is made in order to ensure good reliability. The ingress of coal dust, cement dust, water, steam, oil or other contaminants could cause premature electrical or mechanical failure, or overheating of the motor due to restriction of ventilation circuits.

A primary consideration when choosing an enclosure is to examine the environment and decide whether it is suitably clean and dry for direct cooling, using a ventilated type of machine, or whether there is any risk from airborne dust, water, steam, oil or saline atmosphere, which will require total enclosure of the motor. A significant factor in this choice is capital cost, which is usually higher with totally-enclosed types. Other factors which can influence the choice of enclosure are whether the motor is situated indoors, or is outdoors and exposed to the weather. Saline environments, such as those encountered with coastal power stations, are particularly arduous. Site conditions during the construction and commissioning of the power station have also to be taken into account, since the risk is greater, often for long periods of time.

The general trend is for the greater use of totally-enclosed type motors and they are invariably used for boiler auxiliaries, due to higher levels of contamination. There is also an increasing tendency to use them on turbine-generator auxiliaries, in view of the risk of contamination from steam, water or oil, or from activation of fire protection equipment. With small power motors up to approximately 10 kW, the technical and economic advantages of ventilated-type motors are so marginal that totally enclosed motors are invariably used. For outdoor applications, totally-enclosed weatherproof motors are used with special features provided, such as bearing seals, gaskets between flanged joints and protective finish to bare metal surfaces, such as shaft extensions, etc. Pipe- or duct-ventilated motors have occasionally been used in special applications, such as circulating water pumps. These have advantages, particularly for larger machines, where a source of clean air is readily available (inlet duct), or where the discharge of relatively large quantities of hot air could affect the ambient temperature to the detriment of operating staff or other plant (outlet duct). However, the more usual types of enclosure for circulating water pump motors are drip-proof screen-protected or totally-enclosed, closed-air-circuit, air cooled with an increasing tendency towards the latter, particularly for coastal stations due to the risk of saline contamination.

The type of enclosure is defined in BS4999, Part 20 (IEC 345) and contains an international code, consisting of the letters IP followed by two numerals. The first digit (0 to 5) signifies the degree of protection against contact by persons with live or moving parts inside the enclosure and of machines against ingress of solid foreign bodies. The second digit (0 to 8) signifies the degree of protection against harmful ingress of water. In general, the higher the number, the higher is the degree of protection. Table 7.3 lists some typical motor auxiliary drives for a CEGB power station and gives the types of enclosure used.

Mineral carbonation is a promising and safe approach for permanent sequestration of CO2 via the transformation of CO2 into various carbonates. There are several elements that can be carbonated, but alkaline earth metals in terms of calcium and magnesium are the most suitable for carbonation due to their abundance and insolubility in nature (Sipil etal., 2008). Natural minerals rich in calcium or magnesium, for example, olivine (Mg2SiO4), serpentine (Mg3Si2O5(OH)4), and wollastonite (CaSiO3), are used as the feedstock for providing the Mg and Ca for the formation of carbonates. However, it could be very energy intensive for the processes of mining, mineral pretreatment (i.e., crashing, grinding, and milling, etc.), kinetic enhancement on the carbonation via temperature elevation, or acid dissolution of the natural minerals. Iron could also be employed for carbonation, but considering it is a valuable mineral resource for other industrial applications, it is less suitable for large-scale carbonation.

In addition to the natural minerals rich in Mg and Ca, there are also some industrial solid wastes containing large amounts of Mg, Ca, and even Fe. The industrial wastes include fly ash, various types of iron and steelmaking slags, carbide slag, cement dust, etc. In comparison to the natural feedstock of Mg- and Ca-containing minerals, the industrial wastes are more suitable for economical CO2 sequestration. This is because the industrial wastes are more kinetically unstable and hence are more reactive to carbonation, and therefore require less pretreatment and less energy-intensive carbonation conditions. In addition, the industrial wastes are always near the CO2 intensive point, providing a possible way for in-situ sequestration, which in turn cuts the transportation cost.

Iron and steelmaking slags are byproducts produced during the manufacturing processes of iron and steel respectively. Blast furnace slag (BFS) is a product of iron production, which has been widely investigated and utilized, particularly as a supplementary cementitious material for cement or alkali-activated material, owing to its high hydraulic property or alkali activation reactivity. Steelmaking slag is a byproduct produced in the process of refining the iron to steel in various furnaces (Shi, 2004). For a ton of steel, around 0.130.2ton of slag is produced (Yu and Wang, 2011). According to the US Geological Survey, the global production of steelmaking slag is estimated to be on the order of 170 million to 250 million tons (USGS, 2018). Unlike the iron slag, the steel slag exhibits much lower hydration reactivity and poorer hydraulic properties. Furthermore, it usually contains high content of free-CaO and periclase, which can produce excessive volume expansion as a result of hydration and therefore induce volume instability. In addition, the grinding of steel slag is energy intensive due to its poor grindability. In general, the utilization of steel slag is quite limited or of less economic value; for instance, in China only approximately 10% of steel slag has been used. Nonetheless, more approaches for the use of steel slag are still under seeking. It is found that under CO2 rich environment, the steel slag is very carbonation reactive, and hence has a big potential to be used as the feedstock for CO2 sequestration. This has attracted increasing attention from researchers and abundant work has been focused on this. As reported, up to 13 and 21.5wt% of CO2 (by weight of steel slags) could be sequestrated into stainless steel slag and basic oxygen furnace slag (BOFS) respectively (Baciocchi etal., 2009; Chen etal., 2016). To further reduce the energy consumption and cut the relevant cost for the CO2 sequestration in steel slag, more researches are conducted to integrate CO2 sequestration with value-added products development, for example, development of construction materials. This provides a novel and promising way for the economic CO2 sequestration and valorization treatment of steel slag.

clinkerization - cement plant optimization

clinkerization - cement plant optimization

The process of clinkerization signifies conversion of raw meal into clinker minerals mainly consisting of C4AF(Aluminoferite), C3A(Aluminite), C2S(Belite) and C3S (Alite) phases along with small percentage of free lime CaO, MgO, Alkalies, Sulphates etc. The conversion taking place in kiln system as raw meal is heated gradually to clinkerization temperature (1450 0C) as shown below in table 1.

Kiln system has seen a sea of development since 1950s to till date, from vertical shaft kilns to modern pre-calciner kiln. Capacity has increased from as low as 50 tpd to as high as 12000 tpd from kiln. Heat consumption reduced from 1400 kcal/kg to 670 kcal/kg of clinker. Specific heat consumption of various kiln systems is tabulated (Table 2) below to assess the progress in the development of clinkerization technology.

The overall process of conversion from raw meal to clinker being endothermic demands a theoretical heat of about 380-420 kcal/kg-clinker. However, the rest of the specific heat consumption as tabulated above constitutes heat losses from preheater exhaust gases, clinker, cooler exhaust gases, preheater dust and radiation losses. Heat loss distribution across different elements can be established through heat balance and process audit of pyro section. Fuels used commonly to provide heat for the conversion processes are coal, fuel oil, and natural gas. Alternative fuels like petcoke, rubber tyres, wood chips, etc. have been introduced to economize cement making process.

Lime Saturation Factor (LSF) is the ratio of the actual amount of lime in raw meal/clinker to the theoretical lime required by the major oxides (SiO2, Al2O3 and Fe2O3) in the raw mix or clinker. It is practically impossible to complete the reaction to 100%, in a reactor like rotary kiln, therefore there will always be some unreacted lime (CaOf) known as free lime. The amount of free lime in clinker indicates incomplete burning and needs to be monitored. When coal is used as fuel, the ash content and its composition should be considered in raw mix design. LSF of clinker lies in the range of 92-98. Higher LSF at controlled free lime content translates to better quality of clinker (high C3S), difficult clinkerization, high heat consumption.

Silica Modulus (SM) is the ratio of content of oxides of silica to the oxides of alumina and iron. SM signifies the ratio of solid content to the melt content. Therefore, when SM is too high, nodulization becomes weak and clinkerization reaction (C3S formation) rate slows down, kiln becomes dusty and difficult to operate. While as when SM is too low, more melt is formed in kiln, issues like thick kiln coating, kiln melting, snowman formation in cooler are more prone. Normal range of SM is 2.3-2.7.

Alumina Modulus (AM) is the ratio of content alumina oxide to iron oxide. AM signifies the temperature at which liquid formation starts, the nature of liquid formed and the color of clinker formed. The lowest temperature is obtained at AM equal to 1.6, which is the optimum for clinker formation and nodulization. Higher the AM, lighter the color of clinker (cement). Normal range of SM is 1-2.5.

MgO is commonly present in raw meal. Some of the MgO (2%) is accommodated into the clinker mineral structure, while as extra MgO forms a crystal called periclase and causes mortar expansion. MgO up to 4 % is found common in clinker. Rapid cooling of clinker can mitigate the expansion problems, however higher MgO causes ball formation in kiln, increases melt phase etc. and therefore, can disturb kiln operation.

Alkalies A part of alkalies Na2O and K2O combines chemically with clinker minerals, while as the major part remains as water soluble and affects adversely cement strength (28 Day Strength). If alkalies are not balanced by sulphates, volatile recirculation phenomenon starts disturbing kiln operation due to kiln inlet, bottom cyclone coating. Alkali content is generally expressed in terms of sodium equivalent as under:

SO3 in clinker comes from raw materials and fuel. Sulphate can form a stable compound with potassium and comparatively lesser stable compound with sodium as potassium sulphate (K2SO4) and sodium sulphate (Na2SO4) respectively. Sulphur in raw meal increases SOx emission and causes build-up in preheater. Sulphates needs to balance alkalies in kiln system. Excess sulphates can be calculated as:

Cl chlorides can come from raw materials and fuel. Chlorides form stable compounds with alkalies and are highly volatile. 1% of chlorides is generally considered the maximum in hot meal sample. Excess chlorides needs to be bypassed at kiln inlet through bypass duct. Clinker can contain about 0.012 to 0.023 % cl.

Liquid Phase (%) mainly consists of the aluminium, iron and magnesium oxides. However, alkalies and sulphates also contribute to liquid phase. The liquid phase plays an important role in coating formation and nodulisation. The liquid percentage at 1450 0C can be estimated using the formula

Burnability is a reference value for raw meal indicating how difficult it is to burn. Hard burning is indicated from incomplete burning in terms of free lime content. Although the burning atmosphere in kiln is different from a laboratory oven, nevertheless it is believed that under similar conditions of temperature and residence time, free lime content will depend only on the physical and chemical characteristics of raw meal. Different cements groups have been using different ways to estimate burnability. In FLSmidth the following procedure is followed.

Raw meal samples are placed in laboratory oven at 1400 0C, 1450 0C and 1500 0C respectively for 30 minutes and free lime is measured for each. The FLS burnability is indexed with 100 at following free lime values 3.6% for the sample at 1400 0C, 2.6 for the sample at 1450 0C and 1.6 for the sample at 1500 0C.

Degree of Calcination: is determined by loss on ignition (LOI) of hot meal sample. To reduce the influence of alkalies, sulphur etc. the loss should be measured at 950 0C. Formulas used to approximate degree of calcination are as under.

Clinker free lime (CaOf) should be as high as possible to avoid hard burning of clinker, but safely below value, inviting mortar expansion; normally, between 0.5% and 1.5%. Free lime indicates incomplete clinker burning, therefore should be monitored regularly and maintained closely in the acceptable range. Kiln feed rate fluctuations and composition inconsistency makes difficult to control free lime in clinker

Clinker litre weight (grams/litre): A convenient supplement for free lime measurement is the more rapid determination of litre weight of clinker sample from the cooler discharge to approximately +6/-12 mm and weighing a standard 1 litre volume. Normal range of litre-weight is 1100-1300 g/L. Low litre weight means high free lime and dusty clinker in general (for higher AM free lime can be higher instead of high litre weights).

Kiln Speed should be such that volumetric loading is within the range 10-15% and heat transfer is maximized. Pre-calciner kilns generally rotate at 3.5-4.5 rpm. Under normal conditions, kiln should be run with as high rpm as possible. Higher kiln rpm improves clinker mineralogy and grindability. Speed control is used to take care of usual kiln disturbances like coating fall down with the other controlling parameters like, fuel rate, preheater fan rpm and kiln feed rate.

Fuel Rate is frequently used as a controlling parameter in kiln operation. Fuel is regulated in kiln and precalciner to maintain required temperature. O2 and CO must be considered first before increasing fuel rate.

Feed Rate is generally maintained in a stable kiln operation. When the control actions, like kiln speed, fuel rate and air control fails or is expected to be insufficient to control kiln disturbance, feed rate is changed as required.

Preheater Fan Speed is varied to fulfill air requirement in kiln system and maintain oxidizing conditions in kiln. ID fan speed is not changed frequently in normal kiln operation, unless feed or fuel changed significantly.

Kiln Inlet Analyser gas composition reveals the process (kiln) stability and combustion efficiency. With a good flame in kiln O2 at kiln inlet will be about 1-2% and CO less than 200 ppm, while as it has been observed that an unstable flame may yield in excess of 500 ppm CO with even 3% O2. NOx measurements at kiln inlet gives an early indications of changing burning zone temperatures conditions, before it is reflected in kiln torque trend. It is important to mention that kiln inlet gas analyser probe position should be inside the kiln to avoid leakage air through inlet seal to be sucked with sample gas.

PC-Gas Analyser is generally installed in the outlet duct from the bottom cyclone to avoid frequent jamming of gas filter due to high dust load in PC outlet duct. Oxygen level should as between 0.5-1.5 at CO less than 100 ppm.

Preheater outlet Analyser: In preheater down-comer analyser serves both purposes, to measure leakage across the tower and the overall combustion conditions in kiln system. Moreover, it serves as a safety equipment for all critical equipments in upstream gas circuit like, ESP, Bag house etc. Oxygen content of 1.5 -2.5% is considered good at preheater outlet. Prompt action is recommended if CO increases more than 0.5%.

Lower Cyclone Temperature is considered most important and stable temperature in preheater to control pre-calciner fuel rate. It is generally maintained manually or by PID loop in the range of 10 0C, in the range 850-900 0C to ensure calcination between 90% to 95 %.

Burning Zone Temperature is monitored by radiation pyrometer. Maintaining constant burning zone temperature means, clinker of constant quality and grindability from a consistent kiln feed. Radiation pyrometer gives a relative value of temperature on the basis of visibility (color) in burning zone and can be used as a decisive parameter in stable kiln operation.

Secondary air Temperature should be as high as possible, It reflects the stability of clinker bed in cooler and the heat recuperation from hot clinker. The higher the best, in the range of 800-1050 0C.

Cyclone Cone Drafts. In operation of kiln it is a life line to monitor all preheater drafts, particularly cyclone cone drafts. Cone drafts in preheater cyclone gives an important indication of cyclone jamming along with the other parameters like temperature.

Kiln Back End Temperature indicates the overall stability of kiln operation. It is generally maintained very closely. Variation in kiln back-end temperature indicates either change in burning zone or a change in calciner, hence is of pivotal importance to infer both areas of interest. Back-end temperature is normally maintained at 1050 0C.

Flame Geometry will determine the flame length and therefore, burning zone length. Flame should be as shot as possible, But, care should be taken to avoid thermal abuse of refractory due to shorter and one sided flames.

Kiln Hood Draft should be slightly negative and must be maintained closely between 0 to -2 mm H2O preferably by PID control loop with Cooler vent fan speed. More negative will increase cold air leakages into the kiln through outlet seal and hood, while as positive pressures are unsafe.

Cooler bed height, Undergrate Pressures. Maintaining constant clinker bed height is a key to stable cooler operation. Undergrate pressure reflects bed resistance and changes with clinker size. To maintain constant Undergrate pressure cooler speed is varied manually or in auto-mode by PID control loop. Constant bed height ensures stable secondary and tertiary air temperatures.

Cooling Air Quantity is maintained to ensure cooling of clinker and heat recuperation from hot clinker from kiln. Specific air usage is generally considered as key performance indicator of cooler. New generation coolers can cool clinker to the temperatures to as low as 65 0C over ambient with a specific air consumption of 1.7 kg/kg-clinker. Generally cooling fans are designed at 2.7 kg-air/kg clinker.

Preheater: Preheaters as name implies serves the purpose of heating raw meal to a temperature where calcination or dissociation of CO2 begins in calciner. Preheater consists of 4-6 low pressure cyclones one over the other. Number of cyclones depends on the natural humidity (moisture) in raw materials, in other words the drying capacity required to dry out raw materials in raw mill. Five stage-cyclones are commonly existing in cement plants. In order to increase heat utilization in kiln system, six stage cyclones are as well installed in many cement plants. However, increasing cyclone stages beyond six does not look economic any more, as the quantum of heat saving is not significant to justify it, moreover the increased pressure drop across preheater outbalances the improvements due to additional cyclone.

Raw meal enters (at 50 0C) in the riser duct of second cyclone (from top) and is picked up with the hot gases to first (top) cyclone, where raw meal is separated from gas stream and passed down to second cyclone. Heat transfer takes place in suspension phase between hot gases and raw meal. In this way raw meal passes from top cyclone to lower cyclone (bottom cyclone but one) and enters calciner at about 800 0C. As a whole the heat transfer process is counter current as feed moves from top to bottom and hot gases from bottom to top, however in actual the whole heat transfer takes place in co-current heat transfer mode. Rate of drying, dehydration and calcination are governed by heat transfer rate. Efficient heat transfer can be generally assessed from the difference of the gas and material temperature of cyclones.

With the evolution of low pressure cyclone, it became feasible to go from 4-stage cyclone preheater to 6-stage preheater and harvest more heat economy in kiln section. For reference are tabulated the pressure drop values across preheater of 4,5 and 6-stage preheater.

Brick Lining of Cyclones: All preheater components need to be lined from inside with appropriate refractory to save shell/components from heat and to hold heat inside for process use. Refractory castable, Bricks, Insulation Bricks are used in preheater.

Calciner.Calciner serves the purpose of decomposition of carbonates into reactive oxide calcium oxide. Calcination is an endothermic process and needs heat energy of about 420 kcal. Raw meal is taken in calciner from the last but one stage of preheater. Heat for calcination is supplied through secondary firing in calciner and combustion air is taken from cooler through tertiary air duct. Various configurations of calciner are existing in modern kiln systems. Fundamentally calciner can be either in line with kiln or can be a off-line, thus, taking only air from cooler in different configurations in itself. With coal as a fuel the recommended retention time in calciner should be at least of 3.3 seconds to ensure fuel combustion in calciner. With the development of cement technology, 60% of the fuel required is fired in calciner and 90 to 95 % of calcination duty is done outside the kiln.

Rotary Kilns.Rotary kiln is a rotating cylinder, installed at an inclination of 3.5 to 4 % to facilitate material movement. Length and diameter of kiln is decided for the required capacity throughput. Main factors dictating size of kiln are the retention time (25-30 minutes) of material in kiln, degree of filling (10-17%) and thermal loading of burning zone (2.8-4.8 x 106 kcal/h/m2). Pre-calciner kilns are shortest in length, as 90-95 % calcination is completed outside the kiln. L/D of three tyre kiln is between 14-17 and for new kiln like Rotax kiln it is only 12-13. Kilns are commonly supported on three supporting stations. Each supporting station has 2 rollers and 4 bearings. All rollers are mounted on one fabricated bed plate. Tyre rests on rollers which have an angle of about 30 degrees at the center of the kiln. Kiln is lined with refractories bricks of 150-250 mm thickness, depending on the diameter of kiln. Basic bricks are preferred in burning zone, however, 75 % alumina bricks are still used for cost consideration. Rest of the kiln is lined with ~ 45 % alumina bricks.

Clinker cooler serves two main objective of cooling clinker from temperature of about 1350 0C to the temperature (65-150 0C) where it can be handled by conveyors like pan conveyors, chain, Elevators etc. and heat recovery from hot clinker coming out of kiln. A huge development has happened in clinker coolers designs and types as well. Grate cooler with a take-off for pre-calciner is generally required for pre-calciner kilns. Cross bar coolers are used in new plants to achieve cooling efficiencies (>70%) and less maintenance burden. New coolers are designed for the capacity to be handled with the loading of 40-55 tpd of clinker cooled/m2 of grate area. Cooling air requirement is generally designed at 2.2-2.5 nm3/kg-clinker. Either hammer crusher or roller crusher is used to break lumps of clinker before coming out from cooler. Water spray or Air to Air heat exchanger is used to cool down cooler vent air before de-dusting in ESP or bag filter.

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