Title of Invention

AN IMPROVED PROCESS FOR THE PREPARATION OF A TANNING AGENT

Abstract

The present invention relates to an improved process for the preparation of a tanning agent. More particularly a novel chromium-silica compound is prepared by reacting a monovalent cation of hexavalent chromium salt with a silica salt by the addition of reducing agent and subsequent complexation with organic ligands either simultaneously or successively. The resulting separated slurry being subjected to conventional drying at 130-260°C to get the product of the chromium-silica compound. The product has the ability to produce leathers with shrinkage temperature more than 105°C and having excellent softness, suppleness, stretch, fluffy and smooth grain and strength at the same time obviating the negative characteristics associated with both chromium and silica tannage.

Full Text

The present invention relates to an improved process for the preparation of a tanning agent. More particularly, the present invention provides a process for the preparation of a novel chromium-silica compound. The compound has potential use predominantly as a self-tanning agent for hides and skins. It is envisaged to be able to produce leathers with excellent softness, stretch, suppleness, and fluffy grain with smooth surface. Conventionally skins/hides are stabilised against microbial degradation by tanning. Chrome tanning has been the most predominant method of tannage for the commercial purpose. The major limitation associated with this is that the chrome tanned leathers are empty in nature and hence need heavy retanning for its final performance. Unlike chamois leather, which is essentially oil tanned, chrome tanned leather do not provide any lubrication among the fibres. Hence strong fatliquoring becomes mandatory for the desired level of softness. The conventional chrome tanning practices employ basic chromium sulphate salt, a progenitor of several chrorriium species, having various charges and degree of polymerisation. The kinetic inertness of certain chromium species lends itself to poor exhaustion of chrome tanning. Thus the commercial chrome tanning activities release chromium in the range of 2000-5000 ppm. As reported by Bellavere et al (Environmental Technology Letters, 2, 119, 1981), the environmental consequences arising from the discharge of industrial wastewater containing chromium into water bodies are enormous. While Tsou et al (Chemical Research in Toxicology, 10, 962, 1997) have proved that chromium has the ability to cause cancer, Blankenship et al (Toxicology and Applied Pharmacology, 126, 75, 1994) have even reported cell death as the ultimate result occurred by the influence of chromium. The wide ecological concern as well as economic loss has posed the researchers to look for suitable alternatives for chromium. Several methods have been evolved over the years for better * management of chromium in leather industry, as reported by Chandrasekaran (Leather Science, 34, 91, 1987). Among them the chrome saver approach and the development of mineral alternatives to chromium have been gaining momentum. Covington (Chemical Society Reviews, 26, 73, 1997) has reported the use of metal ions like aluminium, titanium, zirconium and iron for tanning. However the major disadvantage associated with these products is that the resulting leathers exhibit incomplete tannage as well as very poor hydrothermal stability and do not withstand boil test. In addition, these products lack the ability to withstand pH conditions above 5.5, as reported by Heidemann (Fundamentals of Leather Manufacture, Eduard Roether KG publications 1st edition, Chapter 11, 1993). These limitations have forced the researchers to look for other alternatives for the purpose of tanning. The use of silica for tanning of skin/hide is an age-old process. The precipitation of gelatine by the addition of silicic acid forms the basis of silica tanning. Morin (French patent, 479,908, 1914) for the first time provided a silica tanning process, whereby the delimed skins were treated in a bath of sodium silicate, followed by the treatment with a dilute acid like acetic acid. However, the tanning was not completed since the polymerisation of silica occurred at initial stage, which had prevented the penetration of silica. Silica tannage was later attempted + by Hough (British Patent 17,137, 1915), who used for this purpose an acid solution of silica, prepared by reacting 30% sodium silicate with 30% hydrochloric acid, ensuring excess acid in the solution so as to neutralise the sodium silicate completely. The major limitation associated with all these conventional silica tannages is that the silica gets fixed on the external layers of the pelt, thereby lacking penetration of silica resulting in incomplete tannage. Several attempts have therefore been made to ensure complete penetration of silica during tanning. Bocciardo (French patent 781,988, 1934) provided a method of silica tanning in which the pelt had been pretanned with formaldehyde. Again, Benckiser (German Patent 906,135, 1954) used polyphosphates along with silicate for the purpose of tanning. Essentially they were based on incorporation of some other tanning agent in pretanning or in combination. Although silica tanned leathers exhibit good feel and excellent softness, the most important negative attributes associated with them are that they possess low hydrothermal stability and are very weak in mechanical strength. Silica tanned leather was reported by Wilson (The Chemistry of Leather Manufacture, Volume 2, Chemical Catalog Co., Inc. (Reinhold Publishing Corp.), New York, 2nd Edition, Chapter 26, 1929), to tear very easily within a few months of ageing, relating this phenomena to the probable action of the silicic acid on the leather fibres. * This has prompted the researchers to explore options of combining silica with other metals so as to make use of the positive features of the respective materials to meet the end requirement. Kirk (US Patents 2,383,653, 1945 and 2,395,003, 1946) used double silicates of sodium and another metal such as aluminium, iron, chromium and zirconium in tanning. Fernald and Her (US Patent 2,395,472, 1946) provided a process for the production of acid soluble double silicates of an alkali metal and a metal selected from the group consisting of Al, Cr, Fe, Zn and Zr, together with H-bonding donors capable of adapting the products for tanning. These products are especially sodium aluminium silicate with a ratio of AI2O3/SiO2 from 1:2 to 1:5 and a ratio of Na2O/ AI2O3 less than 1.5:1, with or without hydrogen bonding agent, as reported by Kirk (US Patent 2,383,653, 1945). However, they have used only a double salt of silica where the astringency of Al, Cr, Fe, Zn and Zr may not be enough to drastically increase the shrinkage temperature of the skins/hides. Chrome tanning by virtue of its nature imparts neither any lubricating effect nor fullness to the final leather. It is an empty tannage. But it has an ability to increase the shrinkage temperature to 120°C that cannot be achieved by any other currently available tanning agents. Chrome tanned leather also has very good dyeability and affinity towards other post tanning chemicals. On the other hand silica has an ability to preferentially fill the loose portion as well as lubricate * the leather due to its intrinsic gelling nature, thereby not only eliminating the need for using acrylic syntans but also reducing the offer of fatliquors. However silica tanning does not increase the shrinkage temperature beyond 90°C and silica tanned leather possesses low strength characteristics. The main objective of the present invention is to provide an improved process for the preparation of a tanning agent, which obviates the drawbacks stated above. Another objective of the present invention is to prepare a compound of chromium and silica, to make use of the advantages of the both, while at the same time obviating the negative attributes associated with them, having active ingredients capable of giving near zero wastage of the metal salt, by adding ligands. Still another objective of the present invention is to develop a tanning salt which when used in tanning gives leathers with shrinkage temperature more than 105°C. Yet another objective of the present invention is to develop a tanning salt which when used in tanning provides leathers with excellent softness, suppleness, stretch, smoothness, fluffiness and strength. Accordingly the present invention provides an improved process for the preparation of a tanning agent, which comprises: i) mixing a salt of silica and salt of monovalent cation of hexavalent chromium such as herein described in aqueous medium to have silica to chromium ratio in the range of 0.1-1.5 moles, ii) adding to the said reaction mixture, as formed in step (i), sulphuric acid, so as to have 1.6-4.0 moles of sulphuric acid per mole of chromium, iii) adding 0.1-0.2 moles of conventional reducing agent per mole of chromium, such as herein described, and conventional organic ligands preferably dicarboxylic acid and hydroxy dicarboxylic acid ranging from 0.1-1.0 moles per mole of chromium to the reaction mixture as formed in step(ii), iv) ascertaining the absence of chromium(VI) in the reaction mixture, as formed in step (iii), by known method, and adding additional reducing agent, if necessary, till complete reduction of chromium(VI), v) ageing the resulting mixture, as formed in step (iv), for 8-12 hrs followed by the adjustment of pH of the slurry in the range of 2.5-3.0 by conventional method, vi) removing the solids from the resulting slurry by known methods such as filtration, centrifugation, dried by conventional method to get the tanning agent in powder form. In an embodiment of the present invention, the salt of monovalent cation of hexavalent chromium used may be such as sodium dichromate, potassium dichromate, ammonium dichromate. In another embodiment of the present invention, the silica salt used may be such as sodium meta silicate, sodium aluminium silicate, potassium meta silicate, lithium meta silicate, sodium ortho silicate, zirconium ortho silicate. In yet another embodiment of the present invention, the amount of silica salt used may be in the range of 0.1 - 1.5 moles of silica per mole of chromium. In still another embodiment of the present invention, the amount of water added to the reaction mixture may be in the range of 15-30 moles of water per mole of chromium. In yet another embodiment of the present invention, the amount of sulphuric acid used may be in the range of 1.6 -4.0 moles per mole of chromium. In still another embodiment of the present invention, the reducing agent used may be such as molasses, glucose, sodium sulphite, sodium bisulphite, either individually or in combination. In yet another embodiment of the present invention, the amount of reducing agent may be in the range of 0.1 - 0.2 moles per mole of chromium. In still another embodiment of the present invention, the dicarboxylic and hydroxy dicarboxylic acids used as organic ligand may be such as phthalic acid, oxalic acid, adipic acid, citric acid, maleic acid, tartaric acid, lactic acid, and any alkali metal salt of these acids, either individually or in combination. In yet another embodiment of the present invention, the amount of organic ligand used may be in the range of 0.1 - 1.0 mole per mole of chromium. In still another embodiment of the present invention, while adding the reducing agent and the organic ligands the temperature of the reaction mixture may be preferably maintained in the range of 85-100°C. In yet another embodiment of the present invention, the pH of the slurry after ageing may be in the range of 2.5-3.0. In still another embodiment of the present invention, the known method used for ascertaining the absence of chromium(VI) in the reaction mixture may be such as diphenyl carbazide test, UV-Visible spectrum test. In yet another embodiment of the present invention, the separation of solids may be carried out by cloth filtration, paper filtration, centrifugation. In still yet another embodiment of the present invention, the conventional drying method used may be such as drum drying, spray drying. The present invention is described below in detail. A silica salt, corresponding to 0.1 - 1.5 moles of silica per mole of chromium is mixed with that of hexavalent chromium and 15-30 moles of water per mole of chromium is added to the reaction mixture with continuous stirring. Sulphuric acid calculated as 1.6-4.0moles per mole of chromium is then added to the reaction mixture while continuing the stirring. 0.1 - 0.2 mole of a reducing agent per mole of chromium and 0.1 - 1.0 mole of organic ligand such as dicarboxylic and hydroxy dicarboxylic acid per mole of chromium are added to the above reaction mixture either simultaneously or in different successions, in one or more instalments, with continuous stirring over a period of preferably 1 hr. Stirring is continued for preferably 1 hr even after the complete addition of the above agents. The resultant mixture is then allowed to settle. After a period of 8-12 hrs, pH of the slurry is adjusted in the range of 2.5-3.0 by known method. Solids from the resultant slurry are then separated by known method and the separated reaction mixture is dried by conventional method for 3-60 sec at a temperature in the range of 130-260°C to get the chrome-silica compound. The novelty of the present invention lies in selecting the starting materials and in providing a process to prepare a chrome-silica compound with additional complexation provided by organic ligands, thereby ensuring, unlike conventional silica based tanning agents, absence of tearability of the leathers on ageing, and facilitating higher uptake of both chromium and silica, resulting not only in the reduction of pollution problem but also in raising the shrinkage temperature of leather above 105°C (at 1.0% metal oxide offer), with an advantage of added properties namely softness, suppleness, stretch and smoothness in the leathers * tanned with the prepared chrome-silica tanning salt. The invention is described in detail in the following examples, which are provided by way of illustration only and therefore should not be construed to limit the scope of the present invention. Example 1 105 gms of sodium meta silicate (Na2SiO3.5H2O) was stirred with 303 gms of sodium dichromate in a flask fitted with a stirrer. 550 ml of water was added to the above mass with continuous stirring. 188 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 41 gms of molasses was added to 41 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 20 minutes with continuous stirring. The temperature of the resulting mixture was noted to be 95°C. A mixture of 40 gms of phthalic acid and 55 gms of sodium tartrate was added to this reaction mixture over a period of 10 minutes. 50 gms of molasses was added to 50 ml of water taken in a beaker and the mixture was added to the above mass over a period of 30 minutes. A mixture of 43 gms of phthalic acid and 60 gms of sodium tartrate was added to the above mass over a period of 10 minutes. The temperature of the reaction mixture was noted to be 90°C. The stirring was continued for another 2 hrs after the complete addition of the organic ligands. A 10% solution of the above mass was subjected to diphenyl carbazide test whereby no chromium(VI) could be detected. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.7. This mixture was then filtered through a cloth and the filtrate was spray dried at a temperature of 260°C with contact time of 3 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 1.0% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 109°C, exhibiting an exhaustion of 91 and 90% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 2 189 gms of sodium meta silicate (Na2SiO3.9H2O) was stirred with 147.1 gms of potassium dichromate in a flask fitted with a stirrer. 480 ml of water was added to the above mass with continuous stirring. 129 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 43.2 gms of molasses was added to 44 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 45-60 minutes with continuous stirring. Simultaneously, a mixture of 8.3 gms of phthalic acid and 7.3 gms of adipic acid was added to the reaction mixture over a period of 20 minutes. The temperature of the reaction mixture was noted to be 98°C. The stirring was continued for another 2 hrs after the complete addition of organic ligands. A 10% solution of the above mass was subjected to UV-Visible spectrum test whereby no chromium(VI) could be detected. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.2. 10 gms of sodium carbonate was added to the mixture over a period of 15 min and the stirring was continued for 30 min. The pH was found to be 2.5. The resulting mixture was then filtered through a cloth and the filtrate was spray dried at 260°C with contact time of 4 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. * This product was used for tanning pickled goat skins at an offer of 1.25% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 112°C, exhibiting an exhaustion of 90 and 89% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 3 151 gms of sodium aluminium silicate was stirred with 126 gms of ammonium dichromate in a flask fitted with a stirrer. 520 ml of water was added to the above mass with continuous stirring. 144 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 24 gms of molasses was added to 24 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 20 minutes with continuous stirring. Simultaneously, 50 gms of oxalic acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 88°C. 20 gms of molasses was added to 20 ml of water taken in a beaker and the mixture was added to the above mass over a period of 20 minutes. Simultaneously, 44.5 gms of oxalic acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 100°C.The stirring was continued for another 1 hr after complete addition. Then 3 gms of sodium sulphite was added to the mixture and the stirring was continued for another 45 minutes. A 10% solution of the above mass was subjected to diphenyl carbazide test whereby no chromium(VI) could be detected. The reaction mixture thus formed was aged for 8 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.0. 13 gms of sodium carbonate was then added to the mixture over a period of 20 minutes with continuous stirring. The stirring was continued for another 30 minutes and the pH was found to be 2.6. The resulting mixture was then filtered through a cloth and the filtrate was drum dried at 130°C with contact time of 50 sec. The resulting product of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 0.9% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 115°C, exhibiting an exhaustion of 89 and 86% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 4 276 gms of sodium ortho silicate (Na4SiO4) was stirred with 149 gms of sodium dichromate in a flask fitted with a stirrer. 500 ml of water was added to the above mass with continuous stirring. 214 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 25 gms of molasses was added to 25 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 20 minutes with continuous stirring. The temperature of the resulting mixture was noted to be 95°C. A mixture of 30 gms of maleic acid and 20 gms of lactic acid was added to this reaction mixture over a period of 10 minutes. 20 gms of molasses was added to 20 ml of water taken in a beaker and the mixture was added to the above mass over a period of 30 minutes. A mixture of 28 gms of maleic acid and 25 gms of lactic acid was added to the above mass over a period of 10 minutes. The temperature of the reaction mixture was noted to be 89°C. The stirring was continued for another 2 hrs after the complete addition of the organic ligands. A 10% solution of the above mass was subjected to diphenyl carbazide test whereby no chromium(VI) could be detected. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.7. This mixture was then filtered through a cloth and the filtrate was spray dried at a temperature of 260°C with contact time of 3 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 1.5% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 107°C, exhibiting an exhaustion of 89 and 92% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 5 183 gms of zirconium ortho silicate (ZrSiO4) was stirred with 149 gms of sodium dichromate in a flask fitted with a stirrer. 270 ml of water was added to the above mass with continuous stirring. 170 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 25 gms of molasses was added to 25 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 20 minutes with continuous stirring. Simultaneously, a mixture of 20 gms of phthalic acid and 22 gms of tartaric acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 94°C. 20 gms of molasses was added to 20 ml of water taken in a beaker and the mixture was added to the above mass over a period of 20 minutes. Simultaneously, a mixture of 22 gms of phthalic acid and 20 gms of tartaric acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 91°C.The stirring was continued for another 1 hr after complete addition. Then 8 gms of sodium bisulphite was added to the mixture and the stirring was continued for another 45 minutes. A 10% solution of the above mass was subjected to UV-Visible spectrum test whereby no chromium(VI) could be detected. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.7. This mixture was then filtered through a cloth and the filtrate was spray dried at a temperature of 260°C with contact time of 3 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 1.3% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 109°C, exhibiting an exhaustion of 90 and 91% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 6 16 gms of potassium meta silicate (K2SiO3) was stirred with 149 gms of sodium dichromate in a flask fitted with a stirrer. 270 ml of water was added to the above mass with continuous stirring. 86 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 45 gms of molasses was added to 44 ml of water taken in a beaker and the mixture was added drop by drop b,y a dropping funnel to the above mass over a period of 45-60 minutes with continuous stirring. Simultaneously, 27 gms of sodium oxalate was added to the reaction mixture over a period of 20 minutes. The temperature of the reaction mixture was noted to be 96°C. The stirring was continued for another 2 hrs after the complete addition of organic ligands. A 10% solution of the above mass was subjected to diphenyl carbazide test whereby no chromium (VI) could be detected. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.7. This mixture was then filtered through a cloth and the filtrate was spray dried at a temperature of 260°C with contact time of 3 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 1.5% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 107°C, exhibiting an exhaustion of 89 and 92% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. Example 7 45 gms of lithium meta silicate (Li2SiO3) was stirred with 149 gms of sodium dichromate in a flask fitted with a stirrer. 360 ml of water was added to the above mass with continuous stirring. 105 ml of 98% sulphuric acid was added slowly to the above reaction mixture. 20 gms of molasses was added to 20 ml of water taken in a beaker and the mixture was added drop by drop by a dropping funnel to the above mass over a period of 20 minutes with continuous stirring. Simultaneously, a mixture of 23.8 gms of adipic acid and 10 gms of citric acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 86°C. 20 gms of molasses was added to 20 ml of water taken in a beaker and the mixture was added to the above mass over a period of 20 minutes. Simultaneously, a mixture of 20 gms of adipic acid and 11 gms of citric acid was added to the reaction mixture over a period of 10 minutes. The temperature of the reaction mixture was noted to be 90°C.The stirring was continued for another 1 hr after complete addition. A 10% solution of the above mass was subjected to diphenyl carbazide test * whereby chromium(VI) was detected by the formation of slight pink color. Then 7 gms of sodium bisulphite was added to the mixture and the stirring was continued for another 45 minutes. The reaction mixture thus formed was aged for 12 hrs. The pH of the 1:1 aqueous solution of the above mixture was then checked and was found to be 2.7. This mixture was then filtered through a cloth and the filtrate was spray dried at a temperature of 260°C with contact time of 3 sec. The resulting powder of the chromium-silica compound was stored in a plastic container. This product was used for tanning pickled goat skins at an offer of 1.5% metal oxide on pelt weight and the resultant leathers were found to have a shrinkage temperature of 110°C, exhibiting an exhaustion of 88 and 90% for silica and chromium respectively. The tanned leathers were subsequently post tanned to garments using conventional procedure. The resultant leathers were found to be more soft, supple, stretch and smooth with strength characteristics that are comparable to that of conventional chrome tanned leathers. The advantages of the present invention are the following. 1. The chromium-silica compound of the present invention exhibits good solubility in water and is a potential self tanning agent. 2. The present invention provides a simple process to prepare a matrix of chromium and silica with additional complexing through organic ligands, thereby ensuring that the leathers manufactured by using this product do not tear on ageing and exhibit shrinkage temperature more than 105°C. 3. Using the chrome-silica compound as a self-tanning agent produces fuller leathers with no drawn grain appearance. 4. The product exhibits around 85-95% exhaustion of both chromium and silica and produces leathers with shrinkage temperature above 105°C. 5. Shelf life of the product is about 15-24 months. 6. The leathers tanned by the chromium-silica compound of the present invention, when treated with lesser amounts of post tanning chemicals compared to that used in conventional post tanning, exhibit excellent softness, suppleness, stretch, smoothness, fluffiness and strength. We claim 1. An improved process for the preparation of a tanning agent, which comprises: vii) mixing a salt of silica and salt of monovalent cation of hexavalent chromium such as herein described in aqueous medium to have silica to chromium ratio in the range of 0.1-1.5 moles, viii) adding to the said reaction mixture, as formed in step (i), sulphuric acid, so as to have 1.6-4.0 moles of sulphuric acid per mole of chromium, ix) adding 0.1-0.2 moles of conventional reducing agent per mole of chromium, such as herein described, and conventional organic ligands preferably dicarboxylic acid and hydroxy dicarboxylic acid ranging from 0.1-1.0 moles per mole of chromium to the reaction mixture as formed in step(ii), x) ascertaining the absence of chromium(VI) in the reaction mixture, as formed in step (iii), by known method, and adding additional reducing agent, if necessary, till complete reduction of chromium(VI), xi) ageing the resulting mixture, as formed in step (iv), for 8-12 hrs followed by the adjustment of pH of the slurry in the range of 2.5-3.0 by conventional method, xii) removing the solids from the resulting slurry by known methods such as filtration, centrifugation, dried by conventional method to get the tanning agent in powder form. 2. A process, as claimed in claim 1, wherein the salt of monovalent cation of hexavalent chromium used is selected from sodium dichromate, potassium dichromate, ammonium dichromate. 3. A process, as claimed in claims 1 & 2, wherein the silica salt used is selected from sodium meta silicate, sodium aluminium silicate, potassium meta silicate, lithium meta silicate, sodium ortho silicate, zirconium ortho silicate. 4. A process, as claimed in claims 1 to 3, wherein the amount of water added to the reaction mixture is in the range of 15-30 moles of water per mole of chromium. 5. A process, as claimed in claims 1 to 4, wherein the reducing agent used is selected from molasses, glucose, sodium sulphite, sodium bisulphite, either individually or in combination. 6. A process, as claimed in claims 1 to 5, wherein the dicarboxylic and hydroxy dicarboxylic acids used as organic ligand is selected from phthalic acid, oxalic acid, adipic acid, citric acid, maleic acid, tartaric acid, lactic acid, and any alkali metal salt of these acids, either individually or in combination. 7. A process, as claimed in claims 1 to 6, wherein while adding the reducing agent and the organic ligands the temperature of the reaction mixture is preferably maintained in the range of 85-100°C. 8. A process, as claimed in claims 1 to 7, wherein the pH of the slurry after ageing is in the range of 2.5-2.8. 9. A process, as claimed in claims 1 to 8, wherein the separation of solids is carried out by cloth filtration, paper filtration, centrifugation. 10. A process, as claimed in claims 1 to 9, wherein the conventional drying method used is drum drying, spray drying and is effected at 130 to 260 deg.C for 3 to 60 sec. 11. An improved process for the preparation of a tanning agent, substantially as herein described with reference to the examples.

Documents:

770-del-2000-abstract.pdf

770-del-2000-claims.pdf

770-del-2000-correspondence-others.pdf

770-del-2000-correspondence-po.pdf

770-del-2000-description (complete).pdf

770-del-2000-form-1.pdf

770-del-2000-form-19.pdf

770-del-2000-form-2.pdf