The Cement Making Process

For waste types (1), (3) and (4) the use in cement kilns can be regarded as a recovery operation. For waste type (2), however, there is no real benefit of using the material in the cement making process. The kiln is merely being employed as a means for a (thermic) waste disposal operation. 

In the present writer s opinion, this summary of selected papers indicates that a detailed understanding of the raw feed mineralogy-particle size relationship and the effects of the temperature profile in a kiln is absolutely mandatory for continued quality control. These papers, and many others of similar subject matter, demonstrate the practicality of cement plant microscopy. The microscope, perhaps better than any other instrument of analysis, and certainly as a corroborative tool, provides the means for visual appreciation of the cement-making process. But what kind of training is necessary What are the essential microscopical observations and the "standard" microscopical procedures that one can use to help ensure a quality product Knowing the microscopical nature of the raw materials is the first step. 

In the early 1960s, fired dolomite bricks had been developed for use in the burning zone of rotary cement kilns. Dolomite is chemically compatible with the cement-making process and readily acquires a protective coating with the clinker in the burning zone. The need for chrome-free linings in the cement-making process added impetus to penetration of dolomite refractories in this application. 

Wet process kilns. A schematic of the wet cement making process is shown in Fig. 8. 

The air-entraining action is known to involve adsorption at the air-water or solid-water interfaces. The soluble surfactant ions are adsorbed on cement particles making them hydrophobic, so that as bubbles are generated during mixing, they adhere to the cement. This process stabilizes bubbles, preventing their coalescence. Lowering of surface tension may also stabilize bubbles. 

In ancient India, a steel called wootz was made by placing very pure kon ore and wood or other carbonaceous material in a tightly sealed pot or cmcible heated to high temperature for a considerable time. Some of the carbon in the cmcible reduced the kon ore to metallic kon, which absorbed any excess carbon. The resulting kon—carbon alloy was an excellent grade of steel. In a similar way, pieces of low carbon wrought kon were placed in a pot along with a form of carbon and melted to make a fine steel. A variation of this method, in which bars that had been carburized by the cementation process were melted in a sealed pot to make steel of the best quaUty, became known as the cmcible process. 

By-Products. The biomass from the fungal fermentation process is called mycellium and can be used as a supplement for animal feed since it contains digestable nutrients (25,26). The lime-sulfuric purification and recovery process results in large quantities of calcium sulfate cake, which is usually disposed of into a landfill but can find limited use in making plaster, cement, waUboard, or as an agricultural soil conditioner. The Hquid extraction purification and recovery process has the advantage of Htde soHd by-products. 

Thomsen, J. C. (1931). Dental cement and process making the same. US Patent 1,792,200. 

The two other main types of human-made building cements, lime cement and gypsum cement, have been and still are used in many areas of the world. Both these cements require quite elaborate thermal procedures for producing their main components, which are slaked lime in lime cement and plaster of Paris in gypsum cement. Making them involves the calcination of an appropriate type of stone, a process that has been practiced since prehistoric times. Slaked lime is made by the calcination of limestone plaster of Paris, by the calcination of gypsum (see Textbox 33) (Cobum et al. 1990 Lea 1962). 

Cementation (1) The earliest known process for making steel from iron. It originated in the Iron Age but the first written description was published in Prague in 1574 and the earliest known ironworks using the process was that in Nuremburg, Germany, in 1601. The process was operated in Europe between its induction and around 1950 when it was finally... 

Both fossil fuels and hazardous waste fuels used in Southdown cement kilns contain metals. The raw materials (limestone, clay, sand) used to make cement clinker also contain metals. In fact, certain metals, such as iron and aluminum, are essential components of the final product. While metals cannot be destroyed, the Southdown cement kiln process effectively manages them in the following ways (a) cement kiln operators limit emissions by carefully restricting the metals content in wastes accepted for recycling (b) dust particles containing metals are returned to the kUn through closed-loop mechanisms, where metals are chemically bonded into the cement clinker (c) particles not returned to the kiln are captured in state-of-the-art pollution control devices and (d) small amounts are emitted from the stack in quantities strictly hmited by USEPA s BIF mle. 

Lime kilns are frequently associated with cement-making.7 The lime-sand cements in use since Roman times gain mechanical strength from the slow reaction of Ca(OH)2 with CO2 of the air to form interlocking crystals of CaCC>3. The sand acts primarily as a matrix around which this process occurs. 

Gypsum may be a potential source of sulfur and sulfuric acid. Some European plants make Portland cement and sulfuric acid from gypsum or anhydrite. In the Muller-Kuhne process, gypsum is mixed with clay and silica in quantities necessary to make cement, along with coke to reduce CaSC>4 to CaO. In equipment similar to that for portland-cement manufacture, the SOi is driven off and converted to sulfuric acid by the contact process,... 

The portland cement production process is extremely energy intensive (from 4 to 6 million Btu s (MMBtu s) are required to make a ton of product) therefore, alternative and cost-effective fuel options are of great interest. Waste tires have been tried as a supplemental fuel in veil over 30 cement kilns and in at least one rotary lime manufacturing kiln. Currently, tires are in use, either on a trial or permanent basis, in 11 cement kilns and one lime kiln. 

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