Doloma

S.L.C. da Silva, Improvement of the hydration resistance of magnesia and doloma using organosilicon compounds, Ph.D. Thesis, UMR, Ceramic Engineering Dept., 2000. 

Crystal structure. The crystal structures of both calcium oxide and magnesium oxide are cubic. The sum of the ionic radii of CaO is 2.4 A while that of MgO is 2.1 A, which accounts for doloma (CaO MgO) having a greater density than calcium oxide [13.1]. 

Specific heat. The mean specific heats at 20 °C for CaO and doloma are 0.182 and 0.198 cal/g °C respectively. The values at higher temperatures for CaO are given in Table 13.1, with integrated values over temperature ranges being given in Fig. 16.18 [13.5]. 

Lime fusion is based on the reaction of zircon with calda, or doloma, which yields calcium zirconium silicate, calcium zirconate, calcium silicate, zirconium oxide, calcium magnesium silicate and the mixtures thereof, according to the reaction conditions ... 

Composition (wt%) Sintered MgO clinker Sintered doloma clinker Fused MgO... 

The product resulting from this relatively low-temperature calcination is highly porous and reactive and is known as calcined dolomite or simply doloma or dolime (i.e., CaO + MgO). Like lime, most dolime is produced either in vertical shaft kilns (Europe and UK) or rotary kilns (USA). Dolime is used in the extractive metallurgy of magnesium metal by the sili-cothermic process. 

Doloma. Calcined dolomite, i.e. a mixture of the oxides CaO and MgO the term was introduced by the Basic Furnace Linings Committee of the Iron and Steel Institute (I.S.I. Spec. Rept. 35, 1946). 

Basic products consist of magnesia, doloma, magnesia-chrome, chrome ore, and spinel. 

Another source of magnesia is seawater. Seawater contains 0.2% magnesium chloride. To separate it, dolomite is first calcined to produce doloma. 

Doloma is a mixture of calcia and magnesia. This is added to seawater. The oxides are converted to their hydroxides by the following reactions ... 

The increased usage of lime as a flux in steel making led to the replacement of doloma with magnesia, because poorly calcined magnesia was both more refractory- and more hydration-resistant than the low-purity doloma. Additionally, the doloma was no longer needed as a lime source for steel production. At this time, doloma was mainly used as a low-purity repair material for magnesia hearths. 

II. HIGH-PURITY DOLOMITE FOR THE PRODUCTION OF DOLOMA REFRACTORIES... 

Currently, high-purity doloma is produced in several European countries, Canada, and the United States, based on natural dolomite synffietic dolomite is only produced in Japan and China. There are approximately 10 producers worldwide of all types of doloma. 

Silica, iron oxide, and alumina are the most common impurities in high-purity doloma. Since the impurity levels are low and the melting points of the liquid phases are so high, the dolomite can be difficult to calcine. Temperatures in excess of 1850°C are usually required in order to achieve satisfactory density and hydration resistance. The microstructure of a typical high-purity doloma as shown in Fig. 1 consists of magnesia crystallites (3-5 microns in size) embedded in a matrix of lime. 

The typical chemistry of dolomites used for doloma production are shown in Table 1. 

The chemistry of Dolomite 2 corresponds to the upper limit of impurities that would be suitable for the production of doloma for refractory brick purposes. This dolomite would have to be dead-birmed in such a way that there would be no significant pickup of impruities during the process (i.e., requires high-purity fuels). 

The carbonate (dolomite) is converted to the oxide (doloma) and sintered to the required density in either a rotary kiln or a shaft kiln operating at 1850°C or greater. There are basically two different production routes ... 

Tabie 2 Chemical and Physical Properties of a Typical High-Purity Doloma... 

IV. CHARACTERISTICS OF REFRACTORIES BASED ON HIGH-PURITY DOLOMA... 

In the case of cement, one of the big advantages of doloma-based refractories is to provide chrome-free lining configurations. 

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