Clinker recycling

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. 

As seen in Table 4-5, particulate emissions were found to be emitted at a rate of 0.04 lb/ton clinker. In 1981, particulate emissions from the kiln, clinker cooler, and raw mill were estimated to be 0.027 lb/ton clinker burning only coal. When the raw mill was bypassed (i.e., kiln and cooler dust were not recycled), emissions from the kiln and cooler were estimated to be 0.051 lb/ton clinker.21... 

Calcium sulfate may be decomposed to cement clinker and sulfur dioxide gas in a coke-fired rotary kiln at 900-1400°C (Miiller-Kiihne).28 However, the unfavorable economics of this process relegate it to countries that do not have other sources of sulfur. Phospho-gypsum (gypsum produced by the acidulation of phosphate rock) may be decomposed in this way as a means of recycling the sulfur values in the large waste phosphogypsum piles at fertilizer plants (OSW-Krupp and FIPR/Davy... 

Tests showed that the slab from pressing of clinker at 120 bar and 20 percent recycle contained 97 percent finer than 2.8 mm, and 39... 

From experiments to grind cement clinker to —80 pm, as compression is increased from 100 to 300 MPa, the required recycle ratio decreases from 4 to 2.8. The energy required per ton of throughput increases from 2.5 to 3.5 kWh/ton. These data are for a 200-mm-diam-eter pilot-roll press. Status of 150 installations in the cement industry is reviewed [Strasser et al., Tiock Products, 92(5), 60-72 (1989)]. In cement clinker milling, wear is usually from 0.1 to 0.8 g/ton, and for cement raw materials it is between 0.2 and 1.2 g/ton, whereas it may be 20- to 40-in ball mills. 

Materials that are fed via the sinter plant are mainly oxidic and there is a limit to how much can be accepted without a deterioration occurring in the performance of the sinter machine. This can be overcome to a certain extent by, for example, using clinkered Waelz oxide so that it becomes a replacement for recycled sinter fines. This means that less lunq) sinter output will need to be crushed for recycling so that the output of lump sinter increases. 

The results of microscopy by the Ono method are considered an important part of the data base in a computerized multivariate analysis conducted routinely for cement quality control in plants of the Onoda Cement Company in Japan (Aizawa, 1985). Microscopical data are said to be helpful in making frequent adjustments in kiln parameters, clinker silo operation, and adjustment of cement fineness. The continually updated analysis involves the following process variables burning zone temperature, kiln outlet clinker temperature, secondary air temperature, extraction air temperature, kiln rotation torque, nitrous oxide, percent clinker passing the 5-mm screen, and liter weight. In some Japanese cement plants, clinkers are recycled until the microscopical targets have been met, according to Uchikawa (1992). 

Dusty clinker high porosity, breaded nodules, agglomerated fine particles, alite-rich, large alite crystals, and relatively scarce liquid phase (a) Decomposition of outer clinker shell and concentration of liquid phase in clinker core (Allegre and Terrier, 1960) (b) Recycling of precipitator dust, reducing zones, lack of AljOj in raw slurry (Hofmanner, 1973)... 

Effects of common minor and trace elements derived from recycling waste materials in fuels and as raw materials for clinker production, as well as cement hydration, are summarized by Uchikawa and Hanehara (1997). Crystal size and optical property variations in clinker phases (alite, belite, aluminates, and ferrite), and their hydraulic reactivities, are shown to be related to concentrations of sulphm, magnesium, phosphorous, fluorine, chlorine, chromium, manganese, zinc, and many other elements. The cement industry is based in crystal chemistry. 

One well loiown potential use of some industrial wastes is through their incoi poration in the cement industry [8-11]. The recycling of waste in the cement industry is perfomied in two ways the waste is either a component of the clinker generated as a basis of the cement, or the waste is added to the previously generated clinker to make conunercial cement. 

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