Chemistry of inorganic compounds – With additive
Reexamination Certificate
2001-10-31
2004-07-13
Hendrickson, Stuart (Department: 1754)
Chemistry of inorganic compounds
With additive
C423S430000
Reexamination Certificate
active
06761864
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a process for purification of calcium carbonate-rich by-product generated in an industrial process, particularly in nitrophosphate plants in the fertilizer industry, and converting the by-product to finely divided calcium carbonate.
The purification is carried out by thermal treatment of wet calcium carbonate-rich by-product, as available from a nitrophosphate plant. The thermal treatment is applied at a specific temperature, so as to remove moisture and volatile matter, and to decompose or sublime impurities present such as ammonium and nitrate salts. Special care is taken not to decompose the calcium carbonate. In addition, the finely divided calcium carbonate obtained may be coated by using an emulsion or solution of a fatty acid or of a salt of a fatty acid.
Calcium carbonate is used in the manufacture of paper, rubber, plastics, glass, textiles, putties, chalks, sealant, adhesives, paints, inks, varnishes, food, cosmetics, dentifrices, chemicals and pharmaceuticals.
Calcium carbonate produced according to the process of this invention is particularly suitable for use as filler in rubber, paint, plastics and PVC.
BACKGROUND OF THE INVENTION
Several physical and chemical processes have been suggested for removal of impurities from waste calcium carbonate. Physical processes requires drying and grinding, to a fineness sufficient that impurities can be removed by screening, classification, magnetic separation, hydrocyclone separation or floatation. The disadvantages associated with these physical methods of removing impurities are the requirement of specialized machines/devices and the maintenance of the specialized machines and devices. Moreover, these processes are complex and difficult to administer. Other disadvantages of physical processes include unpredictable process efficiencies, variation in the quantities of impurities removed and the expenses associated with employing such physical techniques for removing impurities.
Leaching or bleaching the dried and ground calcium carbonate with chemicals such as halogen, oxygen-containing acids of a halogen or salts thereof, ozone, hydrogen peroxide etc. are described in British patent No. 1285891 (1972) as a chemical purification process. Disadvantages of chemical processes include the requirement of special chemicals, and the number of unit operations to perform the treatments. In addition, the use of a single chemical may not be suitable to remove all the impurities. Removal and separation of excess chemicals remaining after the chemical process is another disadvantage.
Another previously known and widely used method for the treatment of waste containing calcium carbonate, generally known as lime sludge, is re-burning. Reburning consists of burning within the kiln and removing the burned lime after cooling in a material cooler fitted immediately after the kiln. The burned lime reaches a temperature of 1000-1200° C. during the treatment in the kiln. Many technological improvements have been made in kiln design. Such improvements include incorporating a drier crusher, e.g., a hammer mill type crusher, which is mounted before the kiln and cyclone separators. Such a plant, which is also known in other contexts, is described by L. Lenado and Ramon Aguillon (EP 0041113, 1982). The Lenado process is associated with many disadvantages, such as the incorporation of a new machine, the maintenance of the new machine, necessity of special steel which can withstand high temperature, high dust loss, the requirement of cyclone separators and caking in ducts due to the dust from the kiln being sticky in the temperature range 600-700° C. The product obtained is calcium oxide which must be further processed by hydration and carbonation, to produce industrially useful calcium carbonate. Otherwise, as the Lenado process is practiced, the product is recycled in a causticizing process in producing paper pulp.
Mattelmaki discloses a process (U.S. Pat. No. 5,110,567) wherein the dryer crusher has been replaced with a pneumatic drying device, which lowers the final temperature of the exhaust gas. However, this process has the main disadvantage that the material processed is not free flowing. The material demonstrates a dry sticking tendency which often causes ring formation in the preheating zone of the rotary kiln. In addition, this process produces calcium oxide and not calcium carbonate.
Mehaffey et al, (U.S. Pat. No. 4,892,649) disclose a process for purifying calcium carbonate ore by the removal of silicate impurities from the ore using reverse floatation. The process achieves high yields and low acid insoluble content in the calcium carbonate product by employing novel collectors which comprise organo-nitrogen compounds such as quaternary ammonium compounds. The disadvantages of this process are that a floatation unit is required which must be operated continuously. Disposal of separated impurities and removal of unused quaternary compounds raises concerns about pollution resulting from this process.
Theil Jorgen (U.S. Pat. No. 5,711,802) discloses a method and a processing plant for treatment of lime sludge formed by the caustizing process during the manufacture of paper pulp. According to the Theil method, the lime sludge is dried, pulverized and then preheated in a kiln at a predetermined temperature in the range 400-600° C. to obtain dry and poulvurized calcium carbonate. The dried material thus obtained is further calcined/burned in any type of suitable kiln to produce quick lime which in turn can be reused in causticizing process. The drawbacks associated with this invention are that the lime sludge must be dried, pulverized and preheated prior to calcination. The product obtained is calcium oxide and therefore can not be used in rubber, paint, plastics and PVC.
Drummand (U.S. Pat. No. 5,690,897), describes a method for purification of calcium carbonate by lowering the iron content. The method involves addition of an iron chelating agent to an aqueous calcium carbonate slurry, reducing the pH of the slurry by employing CO
2
containing gas, while maintaining the slurry in a temperature range of 20 to 100° C. The resulting slurry is stirred, filtered, washed and dried. The disadvantage of this invention is that it is limited to the removal of iron impurities. Moreover, continuously passing CO
2
gas and maintaining pH at constant value, while simultaneously maintaining the temperature is both difficult to accomplish and uneconomical.
Price et. al. (U.S. Pat. No. 4,793,985, disclose a process for producing ultra fine ground calcium carbonate in which a dry feed limestone is slurried to a solids content of 15 to 60% by weight. The slurry is dispersed with an organic polyacrylate (DISPEX N40) and other macromolecular dispersing agents. The dispersed slurry is fed to an attrition mill where it is bead ground to produce a material wherein 50-70% of the particles are less than 2 micron. The ground calcium carbonate is classified in a centrifuge into a product stream having from 70-99% of its particles less than 2 microns and an underflow stream of larger particles. The product is then treated with ozone which serves not only to increase the brightness of the product but to return the slurry to a non-dispersed state. The calcium carbonate slurry is then dewatered to produce a product containing 58-69% by weight solids, which is suitable for preparing a higher solids content slurry or for spray drying. The drawbacks associated with the Price process is that, ozone being a rare gas, is utilized for merely for improving brightness. Use of ozone also adds to the production cost.
Bunger et al. (U.S. Pat. No. 5,846,500) disclosed a process for the purification of calcium hydroxide using a highly dilute aqueous solution for dissolution of calcium hydroxide followed by settling and filtration for the removal of impurities. The Bunger process is applied particularly to removing impurities from hydrated carbide lime, which is used subsequently as a feed stock for a high value calcium carbonate. Prec
Ayyer Jayalekshmy
Badheka Yogiraj Mansukhlal
Chunnawala Jatin Rameshchandra
Jasra Raksh Vir
Oza Pravinchandra Mahasukhray
Council of Scientific and Industrial Research
Drinker Biddle & Reath LLP
Hendrickson Stuart
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