Limestone magnesium content

Sulfur Emissicms Sulfur present in a fuel is released as SO2, a known contributor to acid rain deposition. By adding limestone or dolomite to a fluidized bed, much of this can be captured as calcium sulfate, a dry nonhazardous solid. As limestone usually contains over 40 percent calcium, compared to only 20 percent in dolomite, it is the preferred sorbent, resulting in lower transportation costs for the raw mineral and the resulting ash product. Moreover, the high magnesium content of the dolomite makes the ash unsuitable for some building applications and so reduces its potential for utilization. Whatever sorbent is selected, for economic reasons it is usually from a source local to the FBC plant. If more than one sorbent is available, plant trials are needed to determine the one most suitable, as results from laboratory-scale reactivity assessments are unreliable. 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Voluminous research on the "dolomite problem" (Chapters 7 and 8) has shown that the reasons for the high magnesium content of carbonates are diverse and complex. Some dolomitic rocks are primary precipitates others were deposited as CaCC>3 and then converted entirely or partially to dolomite before deposition of a succeeding layer still others were dolomitized by migrating underground waters tens or hundreds of millions of years after deposition. However, the observation that the magnesium content of pelagic limestones that have not been uplifted, or... 

Magnesium Content of Limestone. It has been generally accepted (until recently) that the magnesium content of the limestone should be minimized to prevent the presence of polluting soluble magnesium salts in the product solids. There is now a general trend, however, to stabilize... 

The steady state concentration of magnesium species in a scrubber system using dolomitic lime will depend on several factors, including magnesium content of the limestone, system purge rate, and limiting maximum solubility. Whatever the concentration obtained, it seems likely to have a significant beneficial effect on absorption. 

The factors which determine the reactivity of the limestone are not fully understood, but include the magnesium content, the aluminium and fluoride contents, and the crystal structure. The rate of reaction also depends on the particle size of the limestone. Thus, while a lower reactivity can be offset by finer grinding, it raises both operating and capital costs. Details of reactivity tests have been published, see [12.5,12.6]. 

What improvement in the procedure is required for analysing dolomite, or limestone with higher magnesium content ... 

When magnesium content of limestone is high, magnesium oxalate gets coprecipitated with calcium oxalate. In such a case, redissolve the precipitate of calcium oxalate on the filter paper and in the beaker in 20—25 ml of hot dilute HCl. Dilute to about 100 ml and precipitate with about 5 ml of hot 8% ammonium oxalate and proceed as per llie normal procedure. 

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