Half-burned dolomite

Half-burned dolomite (CaCOa MgO) is produced in relatively small quantities in Germany by the controlled calcination of dolomite at 800 to 900 °C. Little appears to have been published about the process. A small scale process was developed pre-1940 using a rotary kiln in which steam was injected into the flame to moderate the temperatures and to reduce the CO2 partial pressure [16.6]. 

Half-burned dolomite (CaCOs MgO) is used in some treatment plants as a closely graded granular filter medium to raise the pH of water without the risk of over-dosing. 

As the particles of half-burned dolomite react, they become finer, and reduce the porosity of the bed. It then becomes necessary to back-wash the bed to remove the finest particles and to replace them with fresh material. 

European Standards are being prepared for chemicals used for the treatment of water intended for human consumption, including high-calcium lime and half-burned dolomite [28.3-28.5]. It seems likely that they will set the norm for lime products used in many environmental applications as well as in the water industry. 

Half-burned dolomite is specified [28.3] (Table 28.3) in terms of ... 

Table 28.3. Half-burned dolomite for use in water treatment — major and minor components (proposed limits)...

Half-burned dolomite (CaC03 MgO) is produced by the controlled calcination of dolomite at low temperatures to dissociate most of the MgCOs component and little of the CaCOs component. 

Abstract This study evaluated the fluorine removal capacity of half-burnt dolomite and examined the removal mechanism. As the result of examining the burning conditions of dolomite, it was found that dolomite burnt at 1023 K for 4 h has the highest performance. It turned out the fluorine removal mechanism that CaCOs does not contribute to reaction and that fluorine is sorbed with hydration of MgO. As the result of comparing removal capacity by changing the initial pH of fluorine solutions, it was found that performance is reduced in the strong alkaline solution. 

Thermally efficient calcination of lime dolomite and clay can be carried out in a multicompartmeut fluidized bed (Fig. 17-27). Fuels are burned in a fluidized bed of the product to produce the required heat. Bunker C oil, natural gas, and coal are used in commercial units. Temperature control is accurate enough to permit production of hme of very high availability with close control of slaking characteristics. Also, half calcination or dolomite is an accepted practice. The requirement of large crystal size for the hmestoue limits apphcatiou. SmaU-sized crystals in the hmestoue result in low yields due to high dust losses. 

Four qualities of calcined dolomite are produced — half-, light-, hard-, and dead-burned. 

According to Konopicky (1957), the expansion joints should, as a rule of thumb, be designed to absorb half the thermal expansion that develops up to the service temperature of the refractory lining. Approximate values to be adopted are 0.5% for fireclay and silicon carbide brick, and 1.2% for magnesite-chrome and dolomite brick. The expansion joints are filled with cardboard, which burns away when the kiln is heated, leaving the joints clear. In general, it is more favourable to provide a larger number of narrower expansion joints than a smaller number of wider ones. 

Atmospheric Fluidized Bed Combustors (AFBC), operating at nearly atmospheric pressure, are fluidized with superficial air velocities from 1 to 4 m/s. During operation, the bed consists of sorbent particles (limestone or dolomite), ash particles and coal. Coal particles burn at about 900°C, which is roughly half of the combustion temperature in pulverized coal-fired boilers. Although the coal particles constitute only about 1% of the total bed material, the bed temperature is uniform and about 100°C lower than the coal particle temperature, due to the excellent heat transfer characteristics generated by the... 

The rotary kiln has become extremely important in lime processing, and is used to burn a large portion of dolomitic quicklime production. More than half of all industrial quicklime is made in rotary kilns. The rotary is particularly useful for making the hard burned grade of quicklime that must contain a minimum amount of unburned limestone. 

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