Title of Invention

"A PROCESS FOR THE PREPARATION OF CHROMIUM OXIDE GREEN"

Abstract

This invention relates to a process for the preparation of chromium oxide green. According to the process chromic acid lumps are heated to a temperature of 150-300 degree Celsius till it becomes a black solid mass. The black solid mass is broken into small lumps; the broken lumps are heated again in a kiln at a temperature of 800 - 1500 degree Celsius to get green coloured said mass. The green coloured mass is cooled at room temperature and subjected to the process of wet grinding. The ground mass is washed and dried. The dried mass is then pulverized to get chromium oxide green.

Full Text

FIELD OF THE INVENTION This invention relates to a process for the preparation of chromium oxide green for use for polishing/finishing the precious stones like garnet, granite, ruby, sapphire, opal, and amethyst. BACKGROUND OF THE INVENTION The processes for the preparation of chromium oxide green are known in the prior art. According to the process disclosed in the U.S. patent No. 4040860 chromium oxide green is prepared by heating stoichiometrically equivalent mixture of sodium dichromate and an ammonium salt for a period of 15 minutes to a pigment forming temperature of about 800° C - 1100° C. calcining the mixture at the same temperature and then separating the formed pigment from the salt. Another process known in the art is disclosed in the U.S. patent No. 4067747 which comprises heating a mixture of ammonium dichromate with at least the stoichiometrically equivalent quantity of a sodium salt for a period of 15 minutes pigment forming temperature of about 800° C - 1100° C in the presence of about 1-45% water based on the weight of the mixture. Calcining the mixture at the same temperature and then separating the formed pigment from the salt. DISADVANTAGES OF THE PRIOR ART There are certain disadvantages associated with the conventional processes. One of the main disadvantages is that the chromium oxide green prepared by the known process does not give satisfactory results for polishing/finishing purposes. Another disadvantage is that the chromium oxide green prepared by the conventional processes is of dull green colour. Yet another disadvantage is that chromium oxide green is not suitable for polishing/finishing the precious stones as mentioned herein above. OBJECTIVES OF THE INVENTION The main objective of this invention is to provide a novel process for the preparation of chromium oxide green, which obviates the disadvantages associated with conventional processes. Another main objective of this invention is to provide a novel process for the preparation of chromium oxide green, which is suitable for polishing/finishing precious stones and gives the satisfactory results. Yet another objective of this invention is to provide a novel process for the preparation of chromium oxide green, which is of bright green colour. STATEMENT OF THIS INVENTION According to this invention there is provided a process for the preparation of chromium oxide green comprising heating chromic acid lumps in a heating vessel at a temperature of 150° C -300° C till it becomes a black solid mass, breaking said solid mass into the lumps, heating said lumps in a kiln at a temperature of 800° C -1500° C to get the required green colour of said mass, cooling said mass at room temperature and subjecting the same to the step of wet grinding, washing said ground mass and drying the same by heating, and then said dried mass being subjected to the step of pulverization to get the said chromium oxide green. DESCRIPTION OF THE INVENTION The process for the preparation of chromium oxide green according to a preferred embodiment is herein described in detail with reference to the accompanying example. In accordance with the process of this invention chromic acid in the form of lumps containing 0.1 - 5% preferably 0.5 - 2% sulphate ions by weight are placed in a heating vessel. The lumps are heated to a temperature of 150° C -300° C for a period of 1 - 5 hours till the lumps are converted in to a black solid mass. The black solid mass is allowed to cool and then it is broken into solid lumps. These broken solid lumps are placed into a kiln and heated again at a temperature of 800° C -1500° C for a period of 2 - 10 hours depending on the amount of solid lumps loaded in the kiln. The solid lumps are heated in the kiln preferably at a temperature of 1000° C - 1200° C so as to get the required colour of the solid mass. The solid mass so obtained is allowed to cool at room temperature and the cooled mass is subjected to the process of wet grinding with water so as to get the slurry having the particle size of the ground mass less than 10 microns (10-5m). The whole mass (slurry) is taken out of the grinder and washed with water till it becomes free of chromate and sulphate ions present in the slurry. The washed slurry (solid cake) is dried, by heating in the stainless steel trays. The dried solid cake/mass is then pulverized to get the mass in the powdered form of the size of less than 10 microns. The fumes produced during the heating of the chromic acid in the heating vessel are passed through a scrubber so as to recover the chromic acid entrapped in the fumes by dissolving the same in the water to get the solution in order to purify the fumes being left in the environment. The water solution so obtained is subjected to the step of evaporation to obtain the black solid mass of chromic acid. The solid mass so obtained is also heated in the same kiln with the black solid mass taken from the heating vessel. The process is now described in the following example. EXAMPLE 5 kg lumps of chromic acid having 1.5% sulphate ion concentration were placed in a heating vessel and heated at a temperature of 250°c for 1.30 hours to get the black solid mass of chromic acid. This mass was allowed to cool at room temperature and was broken into lumps. These lumps were loaded in to a rotary kiln and again heated at a temperature of 1000°c for 3 hours to obtain the lumps/mass of the required bright green colour. The mass so obtained is allowed cool at room temperature and ground ed with water in a grinder so as to get the slurry having the size of the mass less than 10 microns. The slurry was subsequently washed with water till it becomes free of chromate and sulphate ions. The washed cake/mass was dried by heating and then subjected to the step of pulverization to get the powdered mass of less than 10 microns. The yield obtained was 3.6 kg. Approximately which was tested for polishing/finishing purposes and following results was found. • BRIGHTNESS VERY GOOD • SHINING VERY GOOD I CLAIM: - 1. A process for the preparation of chromium oxide green comprising heating chromic acid lumps at a temperature of 150° C - 300° C till it becomes a black solid mass, breaking said solid mass into the lumps, heating said lumps in a kiln at a terpperature of 800° C - 1500° C to get the required green colour of said mass, cooling said mass at room temperature and subjecting the same to the step of wet grinding, washing said ground mass and drying the same by heating, and then said dried mass being subjected to the step of pulverization to get said chromium oxide green. 2. A process as claimed in claim 1 wherein said step of heating in the kiln is carried on for a period of 2 - 10 hours. 3. A process as claim in claims 1 & 2 wherein said step of heating in kiln is carried on preferably at a temperature of 1000°C-1200°C. 4. A process as in claim 1 wherein said step of wet grinding is carried on till the said mass is grounded to a size of less than 10 microns. 5. A process as in claims 1 & 4 wherein said step of washing is carried on till the ground mass becomes free of chromate and sulphate ions. 6. A process as claimed in claim 1 wherein said chromic acid lumps comprise of 0.1 to 5 % sulphate ions. 7. A process for the preparation of chromium oxide green substantial as herein described and illustrated.

Documents:

47-del-2003-abstract.pdf

47-del-2003-claims.pdf

47-del-2003-complete specification [granted].pdf

47-del-2003-correspondence-others.pdf

47-del-2003-correspondence-po.pdf

47-del-2003-description (complete).pdf

47-del-2003-form-1.pdf

47-DEL-2003-Form-18.pdf

47-del-2003-form-2.pdf

47-del-2003-form-3.pdf