Calcium carbonate, crystallisation

The calcium carbonate crystallises in different ways, according to the species. The crystals may be prismatic, columnar or in platelet form, and each layer may be different. In some shells the layers also differ in colour. In all these layers the crystals are held together by a fine web of organic matrix. 

Many molluscs have a shell with a pearly surface lustre. This is due to optical reflection effects finm calcium carbonate, crystallising in the... 

The calcium salt is thereby converted into the soluble potassium salt and calcium carbonate is precipitated. The latter is removed by filtration, as before, and the filtrate concentrated on the water-bath to a small volume until a drop of the liquid, removed on the end of a glass rod, crystallises at once on cooling. 

A single substance may crystallise in more than one of seven crystal systems, all of which differ in their lattice arrangement, and exhibit not only different basic shapes but also different physical properties. A substance capable of forming more than one different crystal is said to exhibit polymorphism, and the different forms are called polymorphs. Calcium carbonate, for example, has three polymorphs — calcite (hexagonal),... 

Calcium, Strontium and Barium Selenocyanates have been prepared7 by dissolving the corresponding carbonates in selenocyanic acid and evaporating over sulphuric acid in vacuo. The calcium salt crystallises in groups of stellate crystals the strontium salt separates out in well-defined prisms, while the barium salt is also crystalline. 

Calcium Dihydrogen Orthoarsenate has been obtained in the form of a monohydrate by adding nitric acid to a mixture containing equal proportions of calcium carbonate and arsenious oxide and allowing the solution to crystallise.3 It may also be prepared by adding excess of arsenic acid to calcium carbonate or to the normal or monohydrogen orthoarsenate.4 It forms colourless plates which lose their water at 180° C. and at 360° C. yield calcium metarsenate, Ca(As03)2, as a crystalline mass, insoluble in hydrochloric acid. 

To 100 c.cs. ethyl alcohol in a -litre round-bottomed flask are carefully added with cooling 40 c.cs. cone, sulphuric acid. A reflux condenser is attached and the mixture heated for an hour on the water bath, and then allowed to cool. The liquid is poured into litre of water in a porcelain basin, and to this is added chalk, with stirring, until effervescence ceases. The calcium sulphate is filtered off, and washed with a little warm water. To the filtrate, which contains ethyl calcium sulphate, is added saturated potassium carbonate solution until the liquid gives a faint alkaline reaction to phenolphthalein. The calcium carbonate is filtered off and washed with a little hot water. The filtrate is then evaporated until crystallisation begins, when it is set aside to cool. The crystals of ethyl potassium sulphate are filtered off and dried, and a further crop obtained by concentrating the mother liquor. 

Determination of the Sugars.—A mixture of flo grams of the substance with hot water containing calcium carbonate is treated exactly as indicated for crystallised fruit. 

Potassium Dichromate, KjCcjO, may be prepared from the sodium salt and potassium chloride, or by a direct method. In the latter ease, chrome iron ore (p. 7) is calcined in the air with calcium carbonate, and the mass extracted with water containing a little sulphuric acid. Potassium carbonate is then added, and, after filtration and addition of sulphuric acid, the potassium dichromate is separated by fractional crystallisation. An alternative method consists in heating chromite with calcium carbonate and potassium sulphate, and lixiviating the mass with water. Oxidation of chromite by means of fused potassium nitrate is sometimes convenient, especially upon a small scale. Potassium dichromate is formed by the addition of any acid to a solution of potassium chromate (see p. 4.4 ). ... 

In the case of calcium carbonate (CaC03), the effect on modulus is due to both adhesion between filler and matrix, and increase in the degree of crystallisation (X. 55% -> 65%). A similar effect is observed on the yield stress. 

Naka K, Chujo Y. Effect of anionic dendrimers on the crystallisation of calcium carbonate in aqueous solution. C R Chimie 2003 6 1193-1200. 

The process is complex and involves simultaneous dissolution of calcium hydroxide and carbon dioxide, and crystallisation of calcium carbonate. Carbonation is generally carried out in a series of reactors under closely controlled pH, temperature and degree of supersaturation, to produce the required PCC morphology and particle size distribution (see section 31.2.3). Crystallisation can occur on the surface of the calcium hydroxide particles (producing scalenohedral crystals), in the aqueous phase (producing rhombohedral crystals) and at the gas-liquid interface. 

Stalactites are icicle-like deposits of calcium carbonate, which hang from the roofs of limestone caverns. They are formed by crystallisation from ground-water. 

Stalagmites are conical deposits of calcium carbonate on the floors of limestone caverns, formed by crystallisation from groundwater. 

Vaterite is a metastable form of calcium carbonate with a hexagonal crystal structure, whieh can crystallise from highly alkaline lake waters. It is of no commercial interest. 

Fig. 5.4 Parameters observed during the crystallisation of composite calcium carbonate fillta- with colloidal morphology. AmCC amorphous calcium carbonate...

All the resultant polymers have been characterised in detail and used in studies of the crystallisation of calcium carbonate from supersaturated aqueous calcium hydrogen carbonate, as the first step towards synthetic organic/inorganic composites. 

Extracellular precipitation can also be described as biomineralisation. The mechanism can range from being very simple to extremely sophisticated. The most complex systems involve cells which can control the crystallisation of common minerals (for example calcium carbonate) to such an extent that exoskeleton structure and shells can be constructed [27]. However, these complex mechanisms have little use in effluent treatment to date. 

Several factors determine the level of reinforcement attained by adding filler. These include, the volume fraction of filler added, the surface area of the filler (related to particle size), particle shape, the level of adhesion between the filler and polymer [80], as well as the thickness and natnre of the interphase between the two phases. A linear correlation between yield strength and heat of crystallisation has also been reported in the case of PP filled with calcium carbonate [81]. It is well known that spherical fillers give least... 

Kang et al. (2003) studied the application of the TCR to crystallisation of calcium carbonate, exploiting the high mass transfer coefQcients (of carbon dioxide in this case) and, they found, enhanced nucleation. In fact, due to the mixing patterns in the reactor, the crystal nucleation and growth processes were found to be near completion in the entry region of the reactor, and the rest of the reactor only acted to reduce crystal size, presumably by attrition. 

Cebe and Rimt [56] used DSC to investigate the effect of inclusion size, shape and filler surface treatment with stearic acid on the crystallinity and gas permeability of HDPE composites filled with micron sized calcite or nanosized calcium carbonate or clay. It was foimd that spherical inclusions had no effect on gas permeability whereas the plate-like inclusions reduced gas permeability and that neither the calcium carbonate nor the clay nucleated the crystallisation of HDPE imder slow cooling conditions. 

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