Calcium carbonate rates

Qualitative examples abound. Perfect crystals of sodium carbonate, sulfate, or phosphate may be kept for years without efflorescing, although if scratched, they begin to do so immediately. Too strongly heated or burned lime or plaster of Paris takes up the first traces of water only with difficulty. Reactions of this type tend to be autocat-alytic. The initial rate is slow, due to the absence of the necessary linear interface, but the rate accelerates as more and more product is formed. See Refs. 147-153 for other examples. Ruckenstein [154] has discussed a kinetic model based on nucleation theory. There is certainly evidence that patches of product may be present, as in the oxidation of Mo(lOO) surfaces [155], and that surface defects are important [156]. There may be catalysis thus reaction VII-27 is catalyzed by water vapor [157]. A topotactic reaction is one where the product or products retain the external crystalline shape of the reactant crystal [158]. More often, however, there is a complicated morphology with pitting, cracking, and pore formation, as with calcium carbonate [159]. 

The abihty of algiaates to form edible gels by reaction with calcium salts is an important property. Calcium sources are usually calcium carbonate, sulfate, chloride, phosphate, or tartrate (20). The rate of gel formation as well as the quaUty and texture of the resultant gel can be controlled by the solubihty and availabiUty of the calcium source. 

Nonblack fillers such as the precipitated siHcas can reduce both rate and state of cure. The mechanism appears to be one of a competitive reaction between mbber and filler for the zinc oxide activator. Use of materials such as diethylene glycol or triethanolamine prevents this competition thereby maintaining the desired cure characteristics. Neutral fillers such as calcium carbonate (whiting) and clays have Httie or no effect on the cure properties. 

The kinetics of the formation of the magnesium hydroxide and calcium carbonate are functions of the concentration of the bicarbonate ions, the temperature, and the rate of release of CO2 from the solution. At temperatures up to 82°C, CaCO predominates, but as the temperature exceeds 93°C, Mg(OH)2 becomes the principal scale. Thus, ia seawater, there is a coasiderable teadeacy for surfaces to scale with an iacrease ia temperature. 

Scaling is not always related to temperature. Calcium carbonate and calcium sulfate scaling occur on unheated surfaces when their solubiUties are exceeded in the bulk water. Metallic surfaces are ideal sites for crystal nucleation because of their rough surfaces and the low velocities adjacent to the surface. Corrosion cells on the metal surface produce areas of high pH, which promote the precipitation of many cooling water salts. Once formed, scale deposits initiate additional nucleation, and crystal growth proceeds at an accelerated rate. 

Sa.tura.tion Index. Materials of constmction used in pools are subject to the corrosive effects of water, eg, iron and copper equipment can corrode whereas concrete and plaster can undergo dissolution, ie, etching. The corrosion rate of metallic surfaces has been shown to be a function of the concentrations of Cl ,, dissolved O2, alkalinity, and Ca hardness as well as buffer intensity, time, and the calcium carbonate saturation index (35). 

A typical fermentation medium for penicillin production contains lactose, com steep Hquot, and calcium carbonate (3,153,154). In most industrial processes the carbohydrate source, glucose, beet molasses, or lactose, is continuously added to the fermentation. The rate of glucose addition must be carefully monitored, by pH or rate of oxygen depletion, because the synthesis of penicillin is markedly reduced in the presence of excess glucose. 

The calcium cyanamide feed is weU mixed with the recycled slurry and filtrate ia a feed vessel. The calcium cyanamide is added at a rate to maintain a pH of 6.0—6.5 ia the cooling tank. The carbonation step can be conducted ia a turbiae absorber with a residence time of 1—2 min. After the carbonation step, the slurry is held at 30—40°C to complete the formation of calcium carbonate, after which the slurry is cooled and filtered. AH equipment for the process is preferably of stainless steel. The resulting solution is used directiy for conversion to dicyandiamide. 

Calcium Oxide. Also called lime or quicklime (4,5), calcium oxide [1305-78-8] (Class 1, nonregenerative), is relatively iaexpensive. It is prepared by roasting calcium carbonate (limestone) and is available ia a soft and a hard form according to the way ia which it was burned. For desiccant service, soft-burned lime should always be used. Calcium oxide is most commonly used to dehydrate Hquids and is most efficient when it can be heated to speed the reaction rate. The reaction product is calcium hydroxide, which cmmbles as it picks up moisture. 

Mineral fillers are used for light-colored compounds. Talc has a small particle size and is a semireinforcing filler. It reduces air permeabihty and has htde effect on cure systems. Calcined clay is used for halobutyl stoppers in pharmaceutical appHcations. Nonreinforcing fillers, such as calcium carbonate and titanium dioxide, have large particle sizes and are added to reduce cost and viscosity. Hydrated siUcas give dry, stiff compounds, and their acidity reduces cure rate hence, their content should be minimized. 

Schierholtz and Stevens (1975), Noor and Mersmann (1993) and Chen etal. (1996) determined nucleation rates by integrating the total crystal number formed over a period and related it to an estimate of supersaturation in the precipitation of calcium carbonate, barium carbonate and barium sulphate respectively. 

Jones, A.G., Hostomsky, J. and Zhou Li, 1992a. On the effect of liquid mixing rate on primary crystal size during the gas-liquid precipitation of calcium carbonate. Chemical Engineering Science, 47, 3817-3824. 

The pH was adjusted to 7.0 with sodium hydroxide and calcium carbonate was added at the rate of 1 g/l. 

In sea-water, the increase of pH adjacent to the surface of cathodes brought about by the reduction of oxygen leads to the deposition of films of calcium carbonate and magnesium hydroxide . Such film deposition often results in a gradual decrease in the rate of galvanic corrosion of the more negative members of couples immersed in sea-water. 

Reacting species, predominant, 80 Reaction coordinate, 133 Reaction heat, 135 additivity of, 111 measurement of, 111 Reaction rates, 124 factors affecting, 125 Reactions, 38,129 acid-base, 188 balancing, 42, 217, 219 calcium carbonate decomposition, 143... 

As additional experiments, investigate the decomposition of calcium oxalate in a static air atmosphere and in a nitrogen atmosphere at a flow rate of 10 mL min -. Compare the final stage of the decomposition, i.e. the conversion of calcium carbonate to calcium oxide, using different furnace atmospheres. 

Dual or triple media filters are used to provide a coarse to fine filtration facility. Where an anthracite-sand media bed is employed, the anthracite, which is half the density of sand, acts as a roughing filter prior to the finer sand bed. The larger and irregular voids within the anthracite bed are particularly suitable for the removal of floes, and filtration occurs deep within the bed. Where fine crystalline precipitates of calcium carbonate or calcium phosphate occur, smaller grain sizes may be employed without risk of high head loss or low filter rates. 

The mechanisms described above tell us how heat travels in systems, but we are also interested in its rate of transfer. The most common way to describe the heat transfer rate is through the use of thermal conductivity coefficients, which define how quickly heat will travel per unit length (or area for convection processes). Every material has a characteristic thermal conductivity coefficient. Metals have high thermal conductivities, while polymers generally exhibit low thermal conductivities. One interesting application of thermal conductivity is the utilization of calcium carbonate in blown film processing. Calcium carbonate is added to a polyethylene resin to increase the heat transfer rate from the melt to the air surrounding the bubble. Without the calcium carbonate, the resin cools much more slowly and production rates are decreased. 

Juwekar and Sharma [1] described the kinetics of the above reactions. The formation of calcium carbonate is non elementary reaction which involves the number of elementary steps as shown in Scheme 7.2, steps (iv) and (v) assumed to be instantaneous. Absorption of C02 gas and dissolution of Ca(OH)2 affects the nucleation step, both are considered as rate controlling steps. 

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