Carbonate precipitates

The dependence of the oxidizabiUty of Mn on the degree of alkalinity is an important factor in some processes for the removal of manganese from water and wastewater. Manganese(Il) compounds are fairly stable, although the hydroxide and carbonate precipitated from alkaline solution tend to... 

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. 

In the oil-weU drilling industry, the barite suspension used as drilling mud can be destabilized by the presence of soluble materials such as gypsum. Addition of barium carbonate precipitates the gypsum, inhibits coagulation, and thus permits the mud to retain the desired consistency and dispersion. 

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. 

Precipitation involves the alteration of the ionic equilibrium to produce insoluble precipitates. To remove the sediment, chemical precipitation is allied with solids separation processes such as filtration. Undesirable metal ions and anions are commonly removed from waste streams by converting them to an insoluble form. The process is sometimes preceded by chemical reduction of the metal ions to a form that can be precipitated more easily. Chemical equilibrium can be affected by a variety of means to change the solubility of certain compounds. For e.xample, precipitation can be induced by alkaline agents, sulfides, sulfates, and carbonates. Precipitation with chemicals is a common waste stream treatment process and is effective and reliable. The treatment of sludges is covered next. 

Calcimn carbonate (precipitated from calcimn cliloride)... 

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. 

Localized pre-boiler scale and corrosion debris deposits. Combination of New phosphate, iron, copper, and silica deposition Old re-deposited debris Transport of Fe, Cu, Ni, Zn, Cr oxides to HP boiler section, leading to deposition, fouling, and possible tube failures Transport of minerals and debris including malachite, ammonium carbamate, basic ferric ammonium carbonate Precipitation in FW line of phosphates, iron, and silicates... 

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. 

The spent salt from MSE is currently sent to an aqueous dissolution/carbonate precipitation process to recover plutonium and americium. Efforts to recover plutonium and americium from spent NaCl-KCl-MgCl2 MSE salts using pyrochemistry have been partially successful (3). Metallothermic reductions using Al-Mg and Zn-Mg alloys have been used in the past to recover plutonium and americium, and produce salts which meet plant discard limits. Attempts at direct reductions of MSE salts using... 

The solubility of a solid can be increased by removing one of its ions from solution acid can be used to dissolve a hydroxide, sulfide, sulfite, or carbonate precipitate and nitric acid can be used to oxidize metal sulfides to sulfur and a soluble salt. 

Oceanic surface water is everywhere supersaturated with respect to the two solid calcium carbonate species calcite and aragonite. Nevertheless carbonate precipitation is exclusively controlled by biological processes, specifically... 

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. 

Laboratory experiments have indicated that carbonate precipitation can alter the permeability of the core samples under reservoir conditions. The precipitation reduces the gas permeability in favor of the liquid permeability. This indicates that precipitation occurs preferentially in the larger pores. 

The reaction involving only carbon dioxide, carbon and carbon monoxide is popularly known as the Boudouard reaction. Its occurrence in the forward direction is favored at high temperatures. The reverse reaction, namely, carbon precipitation, occurs at lower temperatures. The precipitation of carbon is, however, known to occur only slowly at temperatures below 700 °C in the absence of a catalyst. 

Figure 5.31 Metal carbonate precipitation diagram (based on data for 25 °C).

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 ... 

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