Calcium carbonate precipitation

Caustic soda is removed from the carbonate—bicarbonate solution by treating with a slight excess of hard-burned quicklime (or slaked lime) at 85—90°C in a stirred reactor. The regenerated caustic soda is separated from the calcium carbonate precipitate (lime mud) by centrifuging or rotary vacuum filtration. The lime mud retains 30—35% Hquid and, to avoid loss of caustic soda, must be weU-washed on the filter or centrifuge. Finally, the recovered caustic solution is adjusted to the 10% level for recycle by the addition of 40% makeup caustic soda. 

Precipitated Calcium Carbonate. Precipitated calcium carbonate can be produced by several methods but only the carbonation process is commercially used in the United States. Limestone is calcined in a kiln to obtain carbon dioxide and quicklime. The quicklime is mixed with water to produce a milk-of-lime. Dry hydrated lime can also be used as a feedstock. Carbon dioxide gas is bubbled through the milk-of-lime in a reactor known as a carbonator. Gassing continues until the calcium hydroxide has been converted to the carbonate. The end point can be monitored chemically or by pH measurements. Reaction conditions determine the type of crystal, the size of particles, and the size distribution produced. 

Wachi and Jones (1991b) used a gas-liquid flat interface reactor as a semi-batch precipitation cell for the experimental measurement of calcium carbonate precipitation, as shown in Figure 8.15. 

Hostomsky, J. and Jones, A.G., 1993c. Ibid., Modelling of calcium carbonate precipitation in the reaction between gaseous carbon dioxide and aqueous solution of calcium hydroxide. Indem. pp. 2055-2059. 

The calcium carbonate precipitate was removed by filtration, and the filtered solution was found to contain 1,436 g of fructose as determined by optical rotation. A small amount of calcium bicarbonate was present as an impurity in solution and was removed by the addition of oxalic acid solution until a test for both calcium and oxalic acid was negative. The insoluble calcium oxalate precipitate was removed by filtration. 

In areas of the system where the heat gradient is less severe, calcium carbonate precipitates in both crystalline and amorphous forms. It may precipitate as a calcite or aragonite sludge, but more usually an aragonite scale is produced. Aragonite is hard and adherent, depositing in FW lines and various boiler surface components such as boiler tubes. 

Various theories have been postulated to explain the phenomenom that magnetic fields can change the way in which calcium carbonate precipitates and crystallizes, the most probable one being that the applied field distorts the electrical charge carried by small particles of calcium carbonate that have already formed in the water, thus affecting the way in which they adhere to form large particles. 

Two of the study systems, Lake Michigan and Pond 3513, exhibit cyclic behavior in their concentrations of Pu(V) (Figure 2 and 3). The cycle in Lake Michigan seems to be closely coupled with the formation in the summer and dissolution in the winter of calcium carbonate and silica particles, which are related to primary production cycles in the lake(25). The experimental knowledge that both Pu(IV) and Pu(V) adsorb on calcium carbonate precipitates(20) confirms the importance of carbonate formation in the reduction of plutonium concentrations in late summer. Whether oxidation-reduction is important in this process has not been determined. 

Inside newiy formed caverns, calcium carbonate precipitates from water dripping from the ceilings of the chambers. This happens when water that is saturated with carbon dioxide and calcium hydrogen carbonate comes into contact with air. Some of the dissolved CO2 escapes into the gas phase. This shifts the two equilibria to the left, and solid calcium carbonate precipitates ... 

The amount of calcium carbonate precipitating from any particular drop is imperceptibly small. Nevertheless, over the years, these deposits grow into translucent hollow tubes of CaC03 called soda straws (see photo inset). Soda straws lengthen as water drops fall from their tips. These delicate structures can reach lengths of several feet. In time, water flowing over the outside of the tube adds width to the growing formation, and the soda straw matures into the familiar stalactite. 

Much building material has been derived from two monomineral sedimentary rocks gypsum (composed of hydrated calcium sulfate) and limestone, which consists of calcite (composed mostly of calcium carbonate). Freshwater and seawater contain dissolved calcium carbonate and calcium sulfate. Most limestone and gypsum are formed when, as a consequence of the evaporation of water, calcium sulfate and calcium carbonate precipitate out of the water solutions as either gypsum or limestone. Limestone is also formed as a result of the activity of living organisms. Many sea- and freshwater animals, such as snails, clams, and corals, as well as some water plants, draw... 

Calcium carbonate is a common inorganic compound known as limestone. Calcium carbonate has many applications in industries such as medicine, agriculture, paint plastic and surface coatings etc. The vast majority of calcium carbonate used in industry is extracted by mining process. Pure calcium carbonate (e.g. for food or pharmaceutical use), is synthesized by passing carbon dioxide into a solution of calcium hydroxide slurry. In this process calcium carbonate precipitates out, and this grade of product is referred to as precipitate calcium carbonate (abbreviated as PCC). The common reaction is as follows ... 

A recent study in these laboratories (75) on calcium carbonate precipitation from Wyodak coal has confirmed the relationship between ion-exchangeable calcium and the appearance of calcium carbonates during liquefaction. These experiments were performed on samples of the subbituminous coal which had been demineralized, to ensure that all carboxylic acid groups were in the acidic form, and subsequently exchanged with varying amounts of calcium ions. 

Merseburg A process for making ammonium sulfate fertilizer from gypsum. The gypsum is slurried with water and ammonium carbonate solution added. Calcium carbonate precipitates and is removed, any excess of ammonium carbonate is neutralized with sulfuric acid, and the solution is concentrated until it crystallizes ... 

What is then the actual distribution of dissolved species and the amount of calcium carbonate precipitated from the solution ... 

Turner JV (1982) Kinetic fractionation of carbon-13 during calcium carbonate precipitation. Geochim... 

Ammonium sulfate also is manufactured from gypsum salt, CaS04 2H20. Finely divided gypsum is added to ammonium carbonate solution. Calcium carbonate precipitates out, leaving ammonium sulfate in solution. 

Dibasic sodium phosphate also is prepared by reacting dibasic calcium phosphate with sodium carbonate. The product calcium carbonate precipitates leaving dibasic sodium salt in the solution. The solution on cooling yields... 

First the aqueous C02 dissociates in water, producing carbonic acid, then the acid attacks the anorthite, leaching Ca2+ and neutralizing the acid, and finally, calcium carbonate precipitates. 

Calcium carbonate precipitation and sedimentation from mid-August through October (ref. 20 see also the following discussion of components) significantly enhanced P fluxes. The impact of calcite precipitation on P removal from the epilimnion is shown in Figure 8. Phosphorus flux into the... 

Scaling results when the solubility limit of calcium carbonate is reached, at which point precipitation onto tube surfaces occurs. The extent of calcium carbonate precipitation is a function of the composition of the water and the temperature. The alkalinity, dissolved solids and f>H determine the scaling characteristics. Decreasing the pH by the direct addition of acid or by carbonization will decrease the scaling tendencies of the water within limits. If a water is on the scaling side of equilibrium, increasing the temperature will increase the scale deposition. 

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