CaSO precipitation

Many of these reactions are in the direction needed to close the marine mass balances for major ions (Fig. 2.4). The exceptions are that they supply an unnecessary additional siiik for SO4 (CaSO precipitation) and a vast additional source of K+. The additional sink for SO4 does little damage to the marine SO4 mass balance in Fig. 2.4 because its removal affects ordy Ca + and only at the level of about 15% of the Ca + riverine inflow. The hydrothermal source for K+ cannot be rationalized as easily, because there is no adequate sink in the marine environment. Research into the sources and sinks of alkali metals reveals that K+ (and other alkali metals) that are released from basalts at high temperature are reincorporated back into basaltic rock on the sea floor at low temperature. Thus, is recycled in the vicinity of hydrothermal vents. The rates of release and incorporation are uncertain enough to obscure whether the net K+ flux is into or from the ocean in these regions. It is possible that the low-temperature removal of K+ to basalt represents a net sink large enough to accommodate the river inflow. 

SO2 absorbed in tower with NaOH—Na2S02 recycle solution. CaOH or CaCO added externally to precipitate CaSO, regenerate NaOH make-up NaOH or Na2C02 added. Process attempts to eliminate scaling/plugging problems of limestone slurry scmbbing. 

Two main categories of the wet process exist, depending on whether the calcium sulfate is precipitated as the dihydrate or the hemihydrate. Operation at 70—80°C and 30% P20 in the Hquid phase results in the precipitation of CaSO 2 filterable form 80—90°C and 40% P20 provide a filterable CaSO O.5H2O. Operation outside these conditions generally results in poor filtration rates. A typical analysis of wet-process acid is given in Table 4. For more detailed discussion of the wet-process acid, see Fertilizers. 

Seawater. Salt extraction from seawater is done by most countries having coastlines and weather conducive to evaporation. Seawater is evaporated in a series of concentration ponds until it is saturated with sodium chloride. At this point over 90% of the water has been removed, and some impurities, CaSO and CaCO, have been crystallized. This brine, now saturated in NaCl, is transferred to crystallizer ponds where salt precipitates on the floor of the pond as more water evaporates. Brine left over from the salt crystallizers is called bitterns because of its bitter taste. Bitterns is high in MgCl2, MgSO, and KCl. In some isolated cases, eg, India and China, magnesium and potassium compounds have been commercially extracted, but these represent only a small fraction of total world production. 

Precipitation of CaSOs I/2H2O under normal oxidation conditions usually results in 10-15% of the sulfite hemihydrate being oxidized to sulfate bemihydrate, incorporated in the crystal product as a crystalline solid solution. In order to obtain a representative SO3/SO4 crystal product, a sulfate concentration of 15,000 ppm was maintained in the feed liquor, thus simulating industrial conditions. 

Naturally, the principles of chemical equilibrium can be applied to any reaction or process. When a solid substance, such as CaSO, is dissolved in water, the reaction initially proceeds towards the right side. As a result, the concentrations of ions in the solvent increase. But, as time passes, the reverse reaction will start to occur and an equilibrium (dissolution-precipitation) is established. 

There are 0.05 M Ca and 0.03 M Ba " ions in a solution. If a SO anion is added to this solution gradually, what will be the concentration of Ba " ions when CaSO starts to precipitate ... 

Note that when water is added to CaSO f Os, the latter dissolves until its rate of dissolution is equal to its rate of precipitation. This is, by definition, the chemical equilibrium point. If, at this point, a certain amount of NaCl (a very water-soluble salt) is added, it suppresses the single-ion activity coefficient of Ca2+ and SO2". Hence, the... 

In a 1.0 L sample of 0.01 M potassium sulfate, K,SO, what is the minimum number of moles of calcium chloride, CaCl, that can be added to the solution before the precipitate calcium sulfate forms Assume that the addition of calcium chloride has a negligible effect on the total volume of the solution. K for CaSO = 2.4 x KT ... 

Subsequent laboratory continuous stirred tank reactor (CSTR) tests showed that in generating the waste solids CaSOi was being precipitated along with the CaSC>3 even though the solution was not saturated with respect to CaSOi. Thus suggesting the possibility that the precipitation of CaSO involved the formation of some sort of solid solution in the CaSC>3 1/2 H2O lattice. 

Not least of the many difficulties in the modelling of crystallisation fouling is the generally unknown extent of the induction or initiation period. The problem is illustrated by data published by Ritter [1981]. Table 8.5 gives the ranges of experimental induction times he obtained for CaSO and Li O precipitation. The... 

The precipitate of CaSO thus obtained is filtered off, the precipitate is washed with hot water (65°C) and the filtrate and wash water then combined. 

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