Calcium carbonate deposits

A uniform coating of calcium carbonate deposited on the metal surfaces physically segregates the metal from the corrosive environment. To develop the positive LSI required to deposit calcium carbonate, it is usually necessary to adjust the pH or calcium content of the water. Soda ash, caustic soda, or lime (calcium hydroxide) may be used for this adjustment. Lime is usually the most economical alkaH because it raises the calcium content as weU as the alkalinity. 

Theoretically, controUed deposition of calcium carbonate scale can provide a film thick enough to protect, yet thin enough to allow adequate heat transfer. However, low temperature areas do not permit the development of sufficient scale for corrosion protection, and excessive scale forms in high temperature areas and interferes with heat transfer. Therefore, this approach is not used for industrial cooling systems. ControUed calcium carbonate deposition has been used successhiUy in some waterworks distribution systems where substantial temperature increases are not encountered. 

Figure 4.16 Thick calcium carbonate deposits on condenser tube and copper transfer pipe. Note the stratification.

An increase in carbonate-ion concentration moves the equilibrium in favour of calcium carbonate deposition. Thus one secondary effect of cathodic protection in seawater is the production of OH , which favours the production of CO, , which in turn promotes the deposition of CaCOj. Cathodically protected surfaces in seawater will often develop an aragonite (CaCOj) film. This film is commonly referred to as a calcareous deposit. 

Polyphosphate, often with sodium chloride. This is a very low-tech approach, relying primarily on the threshold mechanism of polyphosphate to prevent calcium carbonate deposition at the membrane-water interface. Products based on this simple technology are subject to many limitations and probably are inappropriate to most industrial RO situations. 

Layer, or layers, of minerals (especially calcium carbonate) deposited, by the throwing down, or precipitation, onto a heat transfer sur-... 

C06-0134. The reaction that forms calcium carbonate deposits in caves is as follows ... 

In addition to climate change, the increased atmospheric concentration of C02 is likely to have wide-spread ecological effects in various environments, since C02 is a physiologically active gas, in plants as well as animals. The acidic nature of C02 will also lead to changes in the chemistry of the ocean s surface, which is in equilibrium with the atmosphere. Once the shift in the oceanic chemical balance becomes significant, it will affect ecosystems. It has been shown, for example, that doubling C02 concentration in the atmosphere will reduce the rate of calcium carbonate deposition in coral reefs by 30-40% (Langdon et al., 2000). 

After equilibrium, filtration, and pH adjustment, the residual calcium ion concentration was then titrated by EDTA solution. A higher residual calcium ion concentration indicates better inhibition activity and, therefore, more effectiveness in controlling calcium carbonate deposition in the treated water. As shown in Table III, at dosages of 1 to 5 ppm, the polyacrylic acid was more effective than the acrylic acid/N-(hydroxyalkyl)-acrylamide copolymers. 

Acid rain has resulted in the loss of life in a number of lakes. The ability of a lake to withstand the impacts of acid rain is related to the geology of the lake s basin. In areas with limestone (calcium carbonate) deposits, a lake has a natural buffering capacity. The buffering capacity refers to the ability to resist changes in pH. In well-buffered lakes, calcium carbonate reacts... 

Many observations have shown that calcium carbonate deposition in wells is only troublesome when the first level of boiling is in the well and that the deposition is strongest just above this level. The deposit is most often calcite. However, aragonite, which is slightly more soluble than calcite, has also been identified. It seems that aragonite forms at the expense of calcite, when C02 degassing causes strong oversaturation (Arnorsson 1989). 

The following calcium carbonate deposits have been reported to occur in biological... 

Fig. 4.2 Calcium carbonate deposits can occur as layer-upon-layer of scale inside a pipe or on a metal surface, reducing both the flow of water and heat-transfer rates. The deposition is often a combination of both amorphous scale and crystalline scale (as depicted in the sketch)...

In practice, at the time, it was usual to destroy most or all of the alkalinity and reduce the pH to below 7.0 and often to as low as 6.0 because of the effectiveness of chromate inhibitor barrier films, it was not considered necessary to additionally practice a controlled calcium carbonate deposition program. 

Many cooling system operators use either lean water, with very low natural hardness, or ion-exchange softened water as a makeup source. Also, there are some authorities that require softened water makeup, say less than 5 to 10 ppm total hardness (TH), for comfort cooling systems on the basis that this will minimize the risk of calcium carbonate deposits, the presence which could in turn increase the risk of Legionella proliferation. 

Layer or layers of minerals (especially calcium carbonate) deposited, by the throwing down, or precipitation, onto a heat-transfer surface, reducing its U value. Scales are often hard and dense and difficult to remove. The scale can be crystalline in nature (a solid body having a characteristic internal structure, with symmetrically arranged plane surfaces and definite angles), or amorphous (lacking any characteristic crystalline shape). 

The same could be true for molluscs, except that here calcification started at an earlier geologic age. From what we have presented about the mechanism of calcium carbonate deposition and its close association with the activities of the Golgi apparatus, one cannot escape the conclusion that the whole operation was originally conceived only for the rapid and efficient removal of calcium and perhaps some other elements from within the cell. The unique role of calcium in so many physiological activities307 must also allow for a valve system for the discharge of unwanted surplus. Mineralization probably started as an accident but once a reality it became explored by the organisms. 

Goreau, T. F. Calcium carbonate deposition by coralline algae and corals in relation to their roles as reef-builders. Ann. N. Y. Acad. Sci. 109, 127—167 (1963). 

VARIABILITY IN CALCIUM CARBONATE DEPOSITION IN DEEP SEA SEDIMENTS WITH TIME... 

Variability in calcium carbonate deposition in deep sea sediments with time... 

Bramlette M.N. (1958) Significance of coccolithophorids in calcium carbonate deposition. Geol. Soc. Amer. Bull. 69, 121-126. 

Cloud P.E., Jr. (1962a) Environment of calcium carbonate deposition west of Andros Island, Bahamas. US. Geol. Surv. Profess. Papers 350, 1-138. 

Hard waters contain high concentrations of calcium and bicarbonate and steel samples exposed to hard waters result in the deposition of calcium carbonate deposit on the steel... 

The carbonate ions recombine with calcium ions to form calcium carbonate deposits. These deposits form a coating on heating elements in kettles and boilers, and build up inside hot water pipes. The coating is commonly called boiler scale. (See Figure 9.9.) It not only reduces the flow of water in pipes, but it also increases the cost of heating the water. 

Recent pelagic sediments containing over 30% calcium carbonate, by dry weight, cover a quarter of the surface of the earth (see Figure 1). These sediments make up a vast and chemically reactive carbonate reservoir which has a major influence on the chemistry of the oceans and atmosphere. In order to have a predictive understanding of the natural carbon dioxide system and the influence of man on it, the chemical dynamics of calcium carbonate deposition in the deep ocean basins must be known in detail. 

Calcite and whitings (RI 1.5-1.7) are naturally occurring calcium carbonate deposits. The lowest cost grades are ground limestone or the mixed calcium magnesium carbonate ore, dolomite. Synthetic calcium carbonate is also used as an extender, but it is more expensive. 

In the case of carbon isotopes the 6 C is the complex result of the relative importance of organic carbon sequestration or release by weathering compared to calcium carbonate deposition since the former has a 6 C of about —25 and the latter a 6 C of —0. A further effect can be the episodic release of extremely light carbon (6 C... 

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