Calcium carbonate, 1.21

Metal carbonate compounds are among the most common on the earths surface. Of the rock-forming carbonates, calcite (CaCOj) and dolomite (Ca(Mg,Fe)(C03)2) are the most abundant accounting for more than 90% of natural carbonates [55,57]. 

The most widely used reinforcing filler in the thermoplastic industry is calcium carbonate. Much of the calcium carbonate used derives from limestone, which occurs widely throughout the world. Usually only the white and bright limestones are mined for applications as polymer fillers. Limestone is either wet or dry ground and is then classified on the basis of particle size. 

The finer grades of calcium carbonate are produced chemically by a precipitation method. These form CaCOj from Ca(OH)2 (milk of Kme) via addition of CO2. The particle size and shape can be controlled by process variables. 

Typical commercial calcium carbonate particles are shown in Fig. 1.8. They are roughly spherical. 

Calcium carbonates are commercially supplied in five forms water-ground, dry-ground, ultra fine-ground, precipitated, and surface-treated. Calcium carbonates have abroad mean particle size range. Some precipitated carbonates have a mean particle size of less than 0.1 micron, while some dry-ground types have a mean particle size of over 20 micron. 

Calcium carbonate scaling is perhaps the most common type of problem, with the possible exception of microbial fouling, that RO membranes experience. Fortunately, it is fairly easy to detect and handle. Basically, if the ion product (IP) of calcium carbonate in the RO reject is greater than the solubility constant (Ksp) under reject conditions, then calcium carbonate scale will form. If IP Ksp/ scaling in unlikely. The ion product at any degree of saturation is defined as  

IP = ion product [cation] = cation concentration [anion] = anion concentration superscripts  

Langelier Saturation Index (LSI) is used to determine the scaling potential of calcium carbonate. (Note that LSI is used up to about 4,000 ppm TDS higher concentrations rely on the Stiff-Davis Saturation Index.) The LSI is calculated using the following formulas 

A positive LSI means that scaling is favored a negative LSI means that corrosion is favored. It is desirable to keep the LSI near zero (or below) in the RO concentrate to minimize calcium carbonate scaling. This is usually accomplished by feeding acid to lower the pH or feeding an antisealant (see Chapter 8.2.3). Care must be given if sulfuric acid is used to adjust the pH, as this may exacerbate sulfate-based scales, such as calcium sulfate, barium sulfate, and strontium sulfate. 

Alternatively, antisealants can be used to control calcium carbonate scale at LSI values as high as 2.0-2.5, depending on the specific antisealants. Calcium also forms scales with fluoride, sulfate, and phosphate. The LSI will not help predict these scales analysis of water quality, using the ion product and solubility constants, is required to determine the potential for scaling with calcium fluoride or calcium phosphate. Antisealants currently available can address calcium fluoride and calcium sulfate scale they do not address calcium phosphate scale (although newer antisealants will be available in the near future to address this scale). 

Calcium carbonate (also known as chalk) is the most commonly used filler for PVC. This material is mined as calcite mineral and ground to a particular particle size range. It may also be precipitated from solution to give a fine particle size suitable for use in high performance areas. 

Particle size is important and, for some applications requiring good weathering and impact performance (window profile), the ultrafine milled, high whiteness, natural version is normally used. To ease dispersion, the filler is usually coated with stearic acid. Coated ultrafine and precipitated calcium carbonates are claimed also to have a positive effect on impact properties in impact modified formulations (52, 294, 462). The abrasive wear of calcium carbonate, on melt processing equipment, is not significant but increases with increasing levels (177). 

Tensile strength properties of PVC-U, filled with precipitated and ground calcium carbonate, have been investigated (381, 404). 

Calcium carbonate nanoparticles are commercially available and are claimed to give a cost effective way of increasing impact strength (113). Their use in impact modified PVC has improved mechanical properties (57). 

Chalk fillers also have extensive use in PVC-P applications where the particle size restriction is not so essential. Higher addition levels can also be accommodated. They have extensive use in wire and cable where they assist, in combination with other additives to reduce HC1 generation in a fire situation. 

Calcium carbonate is another inexpensive filler used in rubber compounding as an economic diluent. 

The most common form of the calcium carbonate filler used in rubber is from the simple grinding of limestone into fine-particle-size filler, usually with an average particle size down to about 2 micrometers in diameter. This type of filler is very inexpensive but can be a degrading filler, deteriorating rubber compound properties. These types of calcium carbonate fillers are generally not used in tire technology or in rubber compounds used to make parts where dynamic properties are important. 

Sometimes ground marble can be used as a feedstock to make calcium oxide (quicklime). This precipitated form of calcium carbonate is finer in particle size and of a significantly higher purity and cost than the ground limestone. Still, these precipitated calcium carbonates are not reinforcing fillers in that they do not significantly improve the physical properties of the cured rubber compound. 

Ground calcium carbonate Precipitated calcium carbonate 

Precipitated caicium carbonate is dependent on iimestone, heat, water, and sodium carbonate. 

In terms of weight, calcium carbonate is the most important filler for plastics and it is also widely used in rubber and paints. Calcium carbonate is, in fact, much more than chalk (as it is universally described in the plastics industry). The term covers natural chalk, limestone, and marble - and also precipitated calcium carbonate, which has a very fine particle size, is relatively expensive, and offers some interesting properties in polymer compounds. 

Among recent developments are grades claiming 10-40% improvement in moulding productivity as a result of faster cooling. Electrical grades for 

Recent modified grades of calcium carbonate (from ECC International) include an ultrafine stearate-coated grade made from pure Italian marble, giving high whiteness in rigid PVCs and optimization of the cost-performance of titanium dioxide. 

High-whiteness calcium carbonate (derived from pure Italian marble) comes in various grades  

Chalk whitings are used as general purpose extenders. 

Applications of fine-grained CaCOj as a white filler in paints, in the paper industry etc. 

Economic Importance The current worldwide extraction of limestone and other calcium carbonate-containing minerals such as dolomite is estimated to be ca. 3-10 t/a, of which 950 10 t/a is in the USA. In the rock and earth extraction industry, limestone is in second place, only the mining of sand and gravel being more important. The extraction of limestone in 1995 in Western Europe was 850 10 t. The 1995 production of synthetic (precipitated) calcium carbonate in the USA was 1.483 10 t/a. 

From the more recent reports cited below, further references to the extensive literature concerned with calcite decomposition may be traced. Other modifications of CaC03 (aragonite and vaterite) undergo solid phase transitions to calcite at temperatures of 728 K and 623—673 K respectively [733], below those of onset of decomposition ( 900 K). There is strong evidence [742] that the reaction 

In reviewing reported values of E for calcite decompositions, Beruto and Searcy [121] find that most are close to the dissociation enthalpy. They suggest, as a possible explanation, that if product gas removal is not rapid and complete, readsorption of C02 on CaO may establish dissociation equilibria within the pores and channels of the layer of residual phase. The rate of gas diffusion across this barrier is modified accordingly and is not characteristic of the dissociation step at the interface. 

Where large samples of reactant are used and/or where C02 withdrawal is not rapid or complete, the rates of calcite decomposition can be controlled by the rate of heat transfer [748] or C02 removal [749], Draper [748] has shown that the shapes of a—time curves can be altered by varying the reactant geometry and supply of heat to the reactant mass. Under the conditions used, heat flow, rather than product escape, was identified as rate-limiting. Using large ( 100 g) samples, Hills [749] concluded that the reaction rate was controlled by both the diffusion of heat to the interface and C02 from it. The proposed models were consistent with independently measured values of the transport parameters [750—752] whether these results are transfenable to small samples is questionable. 

Chapter 18 Metal Oxide Filled Micro and Nano Natural Rubber 

2 Metal Oxides as Reinforcing Fillers in Natural Rubber 

3 Structure and Morphology of Metal Oxide Filled NR Composites 

1 Mechanical Properties Dynamical Mechanical Thermal Analysis (DMTA) 

Thermal Properties Electrical Properties Magnetic Properties Thermal Conductivity and Thermal Diffusivity 

Chemical formula CaCO, Chemical functionality none in the main compound  

Decomposition temp., °C 1150 Loss on ignition, % 45-47 Snrface tension, mJ/m 40 

Particle shape irregular Crystal structure see below (a) Hegman flness 2 

Sieve analysis, residue on 325 mesh sieve 0.005-0.2% Specific surface area, mVg 20-35 

MAJOR POLYMER APPLICATIONS acrylics, epoxy resins, ethylene-vinyl acetate copolymer, polycarbonate, polyester, polyethylene, polypropylene, styrene butadiene rubber  

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Chlorides of sodium, magnesium and calcium are almost always the prevailing compounds, along with gypsum and calcium carbonate. 

Rainwater for instance will pick up atmospheric COg and react with calcium carbonate (limestone) to form a soluble substance, calcium bicarbonate. This reaction gives water its natural hardness . 

Surface water Is usually undersaturated in calcium ions (Ca ). Where (even saturated) surface water mixes with sea water, mixing zone corrosion will dissolve calcium carbonate. Evidence of this occurring may be seen on islands. 

A further important reaction is the replacementot the Ca + ion in calcium carbonate by a magnesium ion. The latter is smaller, hence space or porosity is created in the mineral lattice by the replacement. The resulting mineral is dolomite and the increase in effective porosity can be as high as 13%. The process can be expressed as... 

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 alkali metals of Group I are found chiefly as the chlorides (in the earth s crust and in sea water), and also as sulphates and carbonates. Lithium occurs as the aluminatesilicate minerals, spodimene and lepidolite. Of the Group II metals (beryllium to barium) beryllium, the rarest, occurs as the aluminatesilicate, beryl-magnesium is found as the carbonate and (with calcium) as the double carbonate dolomite-, calcium, strontium and barium all occur as carbonates, calcium carbonate being very plentiful as limestone. 

Some carbonates are important industrial chemicals. Calcium carbonate occurs naturally in several forms, including limestone, and is used in the production of quicklime, calcium oxide CaO, slaked (or hydrated) lime, calcium hydroxide Ca(OH)2 and cement. 

When heated, sodium hydrogencarbonate readily decomposes evolving carbon dioxide, a reaction which leads to its use as baking powder when the carhon dioxide evolved aerates the dough. In the soda-ammonia process the carbon dioxide evolved is used to supplement the main carbon dioxide supply obtained by heating calcium carbonate ... 

To prepare the potassium salt, the mixture of ethanol and sulphuric acid is boiled under reflux, cooled, and treated with an excess of calcium carbonate. 

Required Rectified spirit, 20 ml. sulphuric acid, 8 ml. (15 g.) calcium carbonate, 12 g. 

The addition of the calcium carbonate should take about 20... 

Dichloroacetic acid is conveniently prepared by the action of calcium carbonate in the presence of a little sodium cyanide upon chloral hydrate, followed by acidification with concentrated hydrochloric acid ... 

Fit a 1500 ml. bolt-head flask with a reflux condenser and a thermometer. Place a solution of 125 g. of chloral hydrate in 225 ml. of warm water (50-60°) in the flask, add successively 77 g. of precipitated calcium carbonate, 1 ml. of amyl alcohol (to decrease the amount of frothing), and a solution of 5 g. of commercial sodium cyanide in 12 ml. of water. An exothermic reaction occurs. Heat the warm reaction mixture with a small flame so that it reaches 75° in about 10 minutes and then remove the flame. The temperature will continue to rise to 80-85° during 5-10 minutes and then falls at this point heat the mixture to boiling and reflux for 20 minutes. Cool the mixture in ice to 0-5°, acidify with 107-5 ml. of concentrated hydrochloric acid. Extract the acid with five 50 ml. portions of ether. Dry the combined ethereal extracts with 10 g. of anhydrous sodium or magnesium sulphate, remove the ether on a water bath, and distil the residue under reduced pressure using a Claiseii flask with fractionating side arm. Collect the dichloroacetic acid at 105-107°/26 mm. The yield is 85 g. 

A good 3ueld of 5-iodo-2-aminotoluene may be obtained by intimately mixing o-toluidine hydrochloride, iodine and calcium carbonate, and then adding water to the mixture. The liberated hydriodic acid reacts at once with the Calcium carbonate and the lij driodide of the base is not formed. 

Triturate 20 g. of dry o-toluidine hydrochloride and 35 5 g. of powdered iodine in a mortar and then grind in 17 -5 g. of precipitated calcium carbonate. Transfer the mixture to a conical flask, and add 100 ml. of distilled water with vigorous shaking of the flask. Allow the mixture to stand for 45 minutes with occasional agitation, then heat gradually to 60-70° for 5 minutes, and cool. Transfer the contents of the flask to a separatory funnel, extract the base with three 80 ml. portions of ether, diy the extract with anhydrous calcium chloride or magnesium sulphate, and remove the excess of solvent. The crude 5-iodo-2-aminotoluene separates in dark crystals. The yield is 32 g. Recrystallise from 50 per cent, alcohol nearly white crystals, m.p. 87°, are obtained. 

Place 45 g. (43 ml.) of benzal chloride (Section IV,22), 250 ml. of water and 75 g. of precipitated calcium carbonate (1) in a 500 ml. round-bottomed flask fltted with a reflux condenser, and heat the mixture for 4 hours in an oil bath maintained at 130°. It is advantageous to pass a current of carbon dioxide through the apparatus. Filter off the calcium salts, and distil the filtrate in steam (Fig. II, 40, 1) until no more oil passes over (2). Separate the benzaldehyde from the steam distillate by two extractions with small volumes of ether, distil off most of the ether on a water bath, and transfer the residual benzaldehyde to a wide-mouthed bottle or flask. Add excess of a concentrated solution of sodium bisulphite in portions with stirring or shaking stopper the vessel and shake vigorously until the odour of benzaldehyde can no longer be detected. Filter the paste of the benzaldehyde bisulphite compound at the pump... 

Preparation of palladium - calcium carbonate catalyst. Prepare 60 g. of precipitated calcium carbonate by mixing hot solutions of the appropriate quantities of A.R. calcium chloride and A.R. sodium carbonate. Suspend the calcium carbonate in water and add a solution containing 1 g. of palladium chloride. Warm the suspension until all the palladium is precipitated as the hydroxide upon the calcium carbonate, i.e., until the supernatant liquid is colourless. Wash several times with... 

Dissolve 7 g. of pure oleic acid in 30 ml. of dry ethyl chloride (chloroform may be used but is less satisfactory), and ozonise at about —30°. Remove the solvent under reduced pressure, dissolve the residue in 50 ml. of dry methyl alcohol and hydrogenate as for adipic dialdehyde in the presence of 0 5 g. of palladium - calcium carbonate. Warm the resulting solution for 30 minutes with a slight excess of semicarbazide acetate and pour into water. Collect the precipitated semicarbazones and dry the... 

Both objectives have been met by designing special hydrogenation catalysts The most frequently used one is the Lindlar catalyst, a palladium on calcium carbonate combi nation to which lead acetate and quinoline have been added Lead acetate and quinoline partially deactivate ( poison ) the catalyst making it a poor catalyst for alkene hydro genation while retaining its ability to catalyze the addition of H2 to the triple bond... 

Lindlar catalyst (Section 9 9) A catalyst for the hydrogenation of alkynes to as alkenes It is composed of palladium which has been poisoned with lead(II) acetate and quino line supported on calcium carbonate... 

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