Gypsum saturation calcium

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. 

Gypsum Saturation From Measurements of Dissolved Calcium. Equation 7 below can be used with Equation 4 to calculate gypsum saturation from measurements of total dissolved calcium. 

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 ... 

The degree of gypsum saturation of a 1imestone/magnesia scrubbing liquor can be calculated from either dissolved calcium or dissolved S02 (sulfite plus bisulfite) analyses. 

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. 

If the fluid had been initially richer in calcium than carbonate (MCa++ > MhcOj )> 38 noted by Hardie and Eugster (1970), it would have followed a distinct reaction path. In such a case, calcite precipitation would deplete the fluid in carbonate, allowing the calcium concentration to increase until gypsum (CaS04 2H2O) saturates and forms. The point at which the calcium and carbonate are present at equal initial concentration (MCa++ = Mhco3 ) is known as a chemical divide. 

Geochemical speciation modelling indicated saturation with respect to gypsum and several carbonates, slight under-saturation with respect to calcium arsenate (Ca3[As04]2) and ferrihydrite. 

In order to demonstrate the power of the Solver in Excel, let us return to the problem mentioned in the introduction to this chapter (p.31) What is the solubility of calcium sulphate but this time taking into account activity coefficients. As it turns out, they are far from zero, even in a saturated solution of only slightly soluble gypsum. 

Liquor saturated with both calcium sulfite and gypsum... 

Figure 3. Equilibrium SOi partial pressure as a function of pH and magnesium concentration for liquors saturated with calcium sulfite and gypsum at 50°C with...

For liquors saturated with both calcium sulfite and gypsum, the concentration of dissolved sulfite species increases linearly with dissolved magnesium concentration, decreases linearly with dissolved chloride concentration, and is only weakly affected by pH. 

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). 

A,A, A" Calcium concentrations, operating, satd. with gypsum, satd. with hemlhydrate (Figure 4). nucleatlon rate, /min. 1 o slurry, calcium concentration g Ca /I soln. under saturation, g Ca /I soln. equilibrium calcium solubility, g Ca Vg soln. equlHbrium calcium concentration at transition temperature g Ca /g soln. 

Chemically pure reagents were used. Cadmium was added as its sulfate salt in concentrations of about 50 ppm. Lanthanides were added as nitrates. For the experiments with other metal ions so-called "black acid from a Nissan-H process was used. In this acid a large number of metal ions were present. To achieve calcium sulfate precipitation two solutions, one consisting of calcium phosphate in phosphoric acid and the other of a phosphoric acid/sulfuric acid mixture, were fed simultaneously in the 1 liter MSMPR crystallizer. The power input by the turbine stirrer was 1 kW/m. The solid content was about 10%. Each experiment was conducted for at least 8 residence times to obtain a steady state. During the experiments lic iid and solid samples were taken for analysis by ICP (Inductively Coupled Plasma spectrometry, based on atomic emission) and/or INAA (Instrumental Neutron Activation Analysis). The solid samples were washed with saturated gypsum solution (3x) and with acetone (3x), and subsequently dried at 30 C. The details of the continuous crystallization experiments are given in ref. [5]. 

Monosulphate is troublesome to prepare. Mylius mixed 500 ml of a supersaturated calcium aluminate solution (649 mg CaO and 969 mg Al203l ) at 18°C, with shaking, with 2924ml of saturated CH (1.25gCa01" ) and 334ml of saturated gypsum (1.93gCaS041 ). After 30 min, crystallization was complete the mixture was filtered, washed four times with a little water followed by 96% ethanol and ether, and dried over CaClj and soda lime without evacuation. Monosulphate may also be prepared hydrothermally (A8). 

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