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US 4,900,533 AGranted
Alkaline earth metal carbonates
AssigneeEnglish Clays Lovering Pochin & Company Limited
InventorPeter J. Malden
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AssigneeEnglish Clays Lovering Pochin & Company Limited
InventorPeter J. Malden
Estimating the term…
A precipitated alkaline earth metal carbonate is produced by calcining raw alkaline earth metal carbonate, for example a calcium carbonate, under conditions such that substantially all the chemically combined carbon dioxide is driven off to form an oxide, slaking the oxide in water to form a suspension of the corresponding alkaline earth metal hydroxide, allowing the suspension to cool to 45.degree. C. or below, and then carbonating the hydroxide in suspension in water with substantially pure carbon dioxide in the presence of a dithionite bleaching reagent to form a precipitate of an alkaline earth metal carbonate of increased brightness and purity. The precipitate may then be separated from the aqueous medium by, for example, filtration.
In a process for producing a precipitated alkaline earth metal carbonate which process comprises the steps of (a) calcining a raw alkaline earth metal carbonate under conditions such that substantially all the chemically combined carbon dioxide is driven off to form an oxide, (b) slaking the oxide in water to form an aqueous suspension of the corresponding alkaline earth metal hydroxide, (c) carbonating the aqueous suspension of the hydroxide with substantially pure carbon dioxide under conditions which result in the formation of a precipitate of an alkaline earth metal carbonate and (d) separating the precipitate from the aqueous medium, the improvement which comprises allowing the product of step (b) to cool to about 45.degree. C. or below, and then carrying out step (c) at an alkaline pH and in the presence of a dithionite bleaching agent whereby there is obtained a precipitate of an alkaline earth metal carbonate of increased brightness and purity.
A process for producing a precipitated alkaline earth metal carbonate which process comprises the steps of (a) calcining a raw alkaline earth metal carbonate under conditions such that substantially all the chemically combined carbon dioxide is driven off to form an oxide, (b)(i) slaking the oxide in water to form an aqueous suspension of the corresponding alkaline earth metal hydroxide, (b)(ii) allowing the suspension of the alkaline earth metal hydroxide to cool to a temperature not greater than about 45.degree. C., (c) carbonating the aqueous suspension of the alkaline earth metal hydroxide with substantially pure carbon dioxide in the presence of a dithionite bleaching agent and under conditions resulting in the formation of a precipitate of an alkaline earth metal carbonate, (d) dewatering the product of step (c) to form a cake of the alkaline earth metal carbonate, and (e) washing the cake of the desired alkaline earth metal carbonate at least once with clean water.
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Carbonates: preparation of calcium carbonate by carbonation of aqueous solutions and characterised by an additive other than CaCO3-seeds
Compounds of the metals beryllium, magnesium, aluminium, calcium, strontium, barium, radium, thorium
Term ended 13 Feb 2007 (estimated)
USPTO file history isn’t available for this patent, so only indexed dates are shown.
This invention relates to alkaline earth metal carbonates and, more particularly, is concerned with a process for producing precipitated alkaline earth metal carbonates.
The precipitated alkaline earth metal carbonate which is manufactured in the largest quantities is precipitated calcium carbonate, optionally associated with varying proportions of magnesium carbonate. Precipitated calcium carbonate and calcium/magnesium carbonate are white pigments used extensively in paper coating compositions and as fillers or extenders for paper, paint, rubber or plastics.
One known method of preparing a precipitated alkaline earth metal carbonate is to calcine a naturally occurring alkaline earth metal carbonate, such as limestone or dolomite, in order to drive off chemically combined carbon dioxide and leave the alkaline earth metal oxide. The alkaline earth metal oxide is slaked in water to form a suspension of the alkaline earth metal hydroxide, and then pure carbon dioxide is passed under controlled conditions through the aqueous suspension of the hydroxide. This process has the disadvantage that, if the raw alkaline earth metal carbonate contains hydrated iron oxides as impurities, the iron is retained throughout the process and is present in the final product, with the result that the product has a yellowish discoloration and a relatively low brightness, or reflectance to visible light. This problem has been recognized for a long time and several attempts have been made to overcome the problem, see for example U.S. Pat. Specification No. 1,994,271 (Church et al) and U.S. Pat. Specification No. 2,520,850 (Minnick). However, the results achieved in practice have not been satisfactory and, for this reason, much naturally occurring limestone and dolomite has been considered unsuitable for use as a raw material for making precipitated alkaline earth metal carbonates.
The invention is illustrated by the following Examples.
A sample of crude limestone was crushed and screened on a No. 60 mesh B.S. sieve (nominal aperture 0.25 mm) and 50 g of the material which passed through the sieve was heated for 1 hour at a temperature of 1000.degree. C. These conditions were sufficient to convert substantially all of the calcium carbonate to calcium oxide.
The calcium oxide was allowed to cool and then added to 200 ml of water in a beaker, the contents of the beaker being stirred continuously by hand to ensure even dissipation of the heat generated during the slaking operation. When a uniform suspension of calcium hydroxide had been achieved 0.2 g of sodium dithionite was added with stirring to the suspension in the beaker and carbon dioxide gas was passed through the suspension until the pH of the suspension had fallen to below 10.5 (preferably about 9.5). During carbonation the temperature of the suspension was maintained at around 45.degree. C. When carbonation was complete the suspension was filtered and the precipitate washed once by passing 200 ml of water through the cake on the filter.