Gypsum crystals

Gypsum board paper is a special three-ply paper manufactured from repulped newspapers. The face paper or cream face has the ply against the core unsized so that the gypsum crystals can grow into it, as this is the principal form of bonding between the core and facers. The middle ply is sized and the outer ply is more heavily sized and treated to control paint absorption. For the completed gypsum board system to work, the joint treatment and paper must absorb paint at the same rate. 

A. D. Randolph and D. Etherton, Study of Gypsum Crystal Nucleation and Growth Rates in Simulated Flue Gas Desulfurization Eiquors, EPRI Report CS1885, Electric Power Research Institute, Palo Alto, Calif., 1981. 

Common pollutants in a titanium dioxide plant include heavy metals, titanium dioxide, sulfur trioxide, sulfur dioxide, sodium sulfate, sulfuric acid, and unreacted iron. Most of the metals are removed by alkaline precipitation as metallic hydroxides, carbonates, and sulfides. The resulting solution is subjected to flotation, settling, filtration, and centrifugation to treat the wastewater to acceptable standards. In the sulfate process, the wastewater is sent to the treatment pond, where most of the heavy metals are precipitated. The precipitate is washed and filtered to produce pure gypsum crystals. All other streams of wastewater are treated in similar ponds with calcium sulfate before being neutralized with calcium carbonate in a reactor. The effluent from the reactor is sent to clarifiers and the solid in the underflow is filtered and concentrated. The clarifier overflow is mixed with other process wastewaters and is then neutralized before discharge. 

Prayon One of the Wet processes for making phosphoric acid by reacting phosphate rock with sulfuric acid. The byproduct is gypsum, calcium sulfate dihydrate. It uses a compartmentalized, multi-section, lined, concrete reactor, with finishing tanks in which the gypsum crystals mature. In 1990 one third of the wet process phosphoric acid made in the Western World was made in this way. The process was developed in 1977 by the Societe de Prayon, Belgium. Variations are known as PH2, PHI 1, and PH12. One variation uses solvent extraction with isopropyl ether and tri-n-butyl phosphate. 

Etherton studied the growth and nucleation kinetics of gypsum crystallization from simulated stack gas liquor using a one-liter seeded mininucleator with a Mixed Suspension Mixed Product Removal (MSMPR) configuration for the fines created by the retained parent seed. The effect of pH and chemical additives on crystallization kinetics of gypsum was measured. This early fundamental study has been the basis for later CSD studies. 

Etherton, D.L., "Experimental Study of Calcium Sulfate (Gypsum) Crystallization from Stack-Gas Liquors," M.S. Thesis, University of Arizona, Tucson (1980). 

Chang, J.C.S. and T.G. Brna, "Gypsum Crystallization for Limestone FGD," Chemical Engineering Progress, November 1986, p.51. 

A computer model has been generated which predicts the behaviour of a continuous well mixed gypsum crystallizer fed with a slurry of hemihydrate crystals. In the crystallizer, the hemihydrate dissolves as the gypsum grows. The solution operating calcium concentration must lie in the solubility gap. Growth and dissolution rates are therefore limited. 

Measurements were undertaken of the solubility of each phase in acid solutions, of the growth rate of gypsum crystals and the dissolution rate of hemihydrate. The growth rate depends on the square of the supersaturation and on temperature with an activation energy of 64 kJ/mol. The nucleation rate appears to vary linearly with supersaturation. 

It is the purpose of this study to model gypsum crystallization under Nissan process conditions. 

Gypsum crystals have an SG of 2.32, are colourless and belong to the monoclinic system. Figure 2 shows photographs of industrial and laboratory gypsum crystals. The crystals are elongated and can range up to 40 pm in size. 

Thus digestion of the rock phosphate would be undertaken above the transition temperature to form hemlhydrate crystals. The crystallizers however would be operated at a temperature below the transition value and so gypsum crystals are produced. 

A model for a gypsum crystallizer will require the following Information,... 

The growth rate of gypsum crystals from this work Is given by... 

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). 

As far as the gypsum crystals are concerned, the analysis is identical to that for a seeded MSMPR (34). The information required is the growth rate and the mean residence time. For the hemihydrate, the analysis is that for a continuous seeded MSMPR dissolver (35), which parallels that for the crystallizer. The information needed is the dissolution rate and the mean residence time. 

The operating calcium concentration for a gypsum crystallizer fed with hemlhydrate crystals must lie In the solubility gap between the solubilities of the two species. This places severe limits on the range of growth and dissolution rates possible. 

A sink-float technique has been developed for separating hemlhydrate from gypsum crystals for samples from mixed species crystallizers. 

Fig. 4. Phosphoric acid magma with low SO4 concentration showing good gypsum crystals and little unreated phosphate rock.

Fig. 5. Phosphoric acid magma with high SO4 concentration showing poor gypsum crystals and unreacted phosphate rock.

Fjg, 9. Satellites near the O—H valency band in the Raman spectrum of gypsum crystal... 

X-ray diffraction photographs of a gypsum crystal rotated round a direction m< hied H h> the c axis in an arbitrary direction. Above, complete rotation below, 90 oscillation... 

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. 

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