Gypsum Determination

For a given slurry, the maximum filtration rate is determined by the minimum cake thickness which can be removed—the thinner the cake, the less the flow resistance and the higher the rate. The minimum thickness is about 6 mm (0.25 in) for relatively rigid or cohesive cakes of materials such as mineral concentrates or coarse precipitates like gypsum or calcium citrate. Sohds that form friable cakes composed of less cohesive materials such as salts or coal will usually require a cake thickness of 13 mm (0.5 in) or more. Filter cakes composed of fine precipitates such as pigments and magnesium hydroxide, which often produce cakes that crack or adhere to the medium, usually need a thickness of at least 10 mm (0.38 in). 

The hardness of a mineral as measured by the Mohs scale is a criterion of its resistance to crushing [Fahrenwald, Trans. Am. In.st. Min. Metall. Pet. Eng., 112, 88 (1934)]. It is a fairly good indication of the abrasive character of the mineral, a factor that determines the wear on the grinding media. Arranged in increasing order or hardness, the Mohs scale is as fohows 1, talc 2, gypsum 3, calcite 4, fluoride 5, apatite 6, feldspar 7, quartz 8, topaz 9, corundum and 10, diamond. 

Bacterial activity often plays a major part in determining the corrosion of buried steel. This is particularly so in waterlogged clays and similar soils, where no atmospheric oxygen is present as such. If these soils contain sulphates, e.g. gypsum and the necessary traces of nutrients, corrosion can occur under anaerobic conditions in the presence of sulphate-reducing bacteria. One of the final products is iron sulphide, and the presence of this is characteristic of attack by sulphate-reducing bacteria, which are frequently present (see Section 2.6). 

Gypsum is a relatively soft rock made of calcium sulfate. Rainwater percolates through g q)sum, dissolves some of the rock, and eventually becomes saturated with Ca ions and SOq ions. A geochemist takes a sample of groundwater from a cave and finds that it contains 8.4 X 10 M SO4 and 5.8 X 10 M Ca. (The ratio is not 1 1 because other sulfate rock contributes some of the SOq ions to the solution.) Use these data to determine the solubility product of calcium sulfate. 

Sedimentary rocks that are most likely to meet the first three criteria are unfractured shale, clay, siltstone, anhydrite, gypsum, and salt formations. Massive limestones and dolomites (i.e., carbonates with no continuous fracturing and solution channels) can also serve as confining layers. Then-suitability must be determined on a case by case basis. The fourth criterion has no relationship to lithology. 

Fig. 8.6. Solubility of gypsum (CaSCU 2H2O) at 25 °C as a function of NaCl concentration, calculated according to the Harvie-M0ller-Weare and B-dot (modified Debye-Hiickel) activity models. Circles and squares, respectively, show experimental determinations by Marshall and Slusher (1966) and Block and Waters (1968).

When the activity of each species in a reaction is known, we can determine the temperature (or temperatures) at which the reaction is in equilibrium. As an example, we calculate the temperature at which gypsum (CaS04 2 H2O) dehydrates to form anhydrite (CaS04). The RXN commands... 

Dissolved Concentrations of Calcium and SO2 Species. The equilibrium dissolved concentrations of total calcium and SO2 (sulfite plus bisulfite) species are important because comparison of these equilibrium concentrations with actual measured values determines the degree of gypsum saturation, and hence the potential for gypsum scale formation in the scrubber. As a first approximation, the fraction gypsum saturation of a scrubber liquor, having specified pH and specified concentrations of magnesium and chloride, is proportional to the measured calcium concentration, and inversely proportional to the measured S02 concentration. 

For a liquor of known pH and magnesium and chloride concentrations, the degree of gypsum saturation can be determined by measurement of either the total dissolved calcium or the total dissolved SO2 (sulfite plus bisulfite). The chemical model has been used to obtain correlations for gypsum saturation, presented below. The correlations, Equations 7 and 9, are valid for a typical scrubbing temperature of 50 °C, and for the same ranges of pH, magnesium, and chloride as for Equations 1-4. 

For a typical limestone scrubber inlet liquor pH range of 5.2-6.0, and for liquors having a chloride-to-magnesium ratio of 0.2 mole/mole or less, the following simplified equation can be used to determine gypsum saturation from calcium measurements ... 

Gypsum Saturation from Measurements of Dissolved SOg. Use of measurements of dissolved calcium to determine gypsum saturation is relatively easy from a computational standpoint use of measurements of dissolved SO2 is more difficult. However, wet chemical analyses for calcium are frequently subject to interference by high concentrations of magnesium. For installations where a quick and reliable analysis for calcium is not available, the use of dissolved SO2 is preferred, and the following correlation applies ... 

Use of Equations 2, 3, 5, and 9 to determine gypsum saturation gives results that agree with the chemical model to within a standard error of estimate of 0.04 fraction gypsum saturation for saturations of 0.5-2.0. 

XRD allows the determination of the actual mineral species present. The bulk coal samples resulted in an x-ray pattern with a high background due to the coal matrix. The major mineral species were easily determined, however the minor species could not be detected. As with the SEM-BSE analysis, interpretation of results in a quantitative manner (in relation to the coal) is difficult. The major minerals detected were quartz, kaolinite, gypsum and marcasite. 

To determine the rate of dissolution of hemlhydrate crystals, the same vessel was used as for the crystallization study. The vessel was filled with the sulphate-rich solution (zero Initial calcium concentration). An amount of sieved hemlhydrate seed crystals, about 10% In excess of that required to saturate the solution, was added. At very short time Intervals, samples were taken using a similar procedure to that for the gypsum growth Investigation. Samples were separated Into crystals for size analysis (with a 190pm orifice) and crystal content and solutions for analysis. Further details are given by Mukhopadhyay (17). 

Both hemihydrate and gypsum crystals coexist in a crystallizer in the Nissan process. Samples can be readily taken. The fraction of hemihydrate is determined on a bulk sample by evaluating the amount of hydrated water in the sample (2 moles for gypsum, 0.5 for hemihydrate). 

Subsequent reaction of the monosulfate with gypsum produces acicular crystals of ettringite. Monosulfate apparently does not contribute to expansion, whereas the formation of ettringite involves expansion. A recently issued patent [75] covers the use of prehydrated high-alumina cement (H-HAC), lime and gypsum mixtures. Particle type, size, thickness of protective coating, and presence of moisture determine the rate and extent of expansion [Fig. 6.11]. 

To determine the nature of the base, Lavoisier added to a solution of the gypsum some concentrated fixed alkali, drop by drop. Immediately the vitriolic acid left its base to unite with the fixed alkali and form with it vitriolated tartar [potassium sulfate]. The solution became turbid and a white precipitate formed, which when washed proved to be a very pure cal-... 

In this way the coordinates of all reciprocal lattice points on the zero layer lying within the area ODBECF are directly determined. Fig. 102 shows the results obtained from a 90° oscillation photograph (Plate VIII) of a gypsum crystal set with its c axis inclined 8 ° to the axis of rotation in spite of the limited precision in the determination of", there is no doubt about where to draw the net. If the remaining. 

Fig. 102. hkO plane of reciprocal lattice of gypsum crystal, determined from iho photographs in Plate VIII. The length of each arc represent the possible error. 

Wallerius in 1747 used a finger nail, knife, file or diamond powder for hardness determination. Werner in 1774 rubbed a mineral against a finger nail, knife or piece of steel to determine the quantities of powder thus derived. Hatty (1801) used calcite and quartz as well as glass for this purpose. The first arbitrarily chosen scale of hardness, containing exclusively minerals, was devised in Sweden by Kvist in 1768. It covered diamond—20, topaz—15, zeolite—13, quartz—11, fluorite—7, calcite—6, gypsum—5 and chalk—2. 

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