Title of Invention

A PROCESS FOR THE PRODUCTION OF ZINC TEROXY CHROMATE

Abstract

A process for the production of zinc tetraoxy chromate which comprises : (i) treating 100-150 litre of chromate effluent with 5 to 15 kgs. of strong alkali and oxidising agent selected from compounds having peroxide group followed by boiling for a period of 3 to 10 minutes to obtain solution containing ferric hydroxide, (ii) removing ferric hydroxide by filtering to obtain filtrate, (iii) adding hydrated metal chloride and alkali in a concentration range of 40-50% to maintain the pH in the range of 8 to 10 the filtrate obtain in step (ii) stirring the solution for 15 minutes at room temperature to obtain precipitate, (iv) washing the precipitate so obtain and filtering by conventional methods and drying at a temperature in the range of 100-120°C for a period in the range of 1 to 4 hours to obtain a solid cake, (v) pulverizing the cake to particle size in the range of 100 to 250 mesh size to obtain zinc tetroxy chromate.

Full Text

This invention relates to a process for the production of zinc tetroxy chromate. The invention relates basically to the paint and pigment industry and useful for the iron and steel industry, especially in prevention of corrosion of structural steels, in aesthetic look of the structures and pipelines, carrying industrial/sea water. Corrosion of metal is bound to take place which could be minimised to a very low level but cannot be checked completely. Atmospheric corrosion contributes more than 60% of corrosion of metals and steels since the steel items are excessively exposed to different environments. The situation of steels in the marine atmosphere is very serious because of severity of the condition. In order to mitigate the corrosion, different preventive measures, namely metallic, inorganic, diffusion, organic coatings, etc., are in use. Out of these preventive measures, organic coatings are considered more promising and used widely because of ease of application, availability to the need of the situation and its aesthetic reasons. In order to meet the need of the industries, appropriate corrosion preventive pigments are used in the formulation. Widely used zinc tetroxy chromate is one of the pigments used in paint industry. Hitherto known process uses chromic oxide and zinc oxide for the preparation of zinc tetroxy chromate at non-specified parameters . The known process consists of reacting suspended zinc oxide in water with chromic oxide while stirring continuously. The reaction takes 12 to 14 hours to complete which may be represented by the following equation. (Equation Removed) However, the existing process is costly in the sense that it uses zinc oxide (ZnO) and chromic oxide (CrO ) 3 which have high cost. Optimisation of parameters are difficult to maintain due to the fact that (1) amount of ZnO and CrO needed for complete conversion to zinc 3 tetroxy chromate are not fixed, leading to wastage of material as well as carry over of unreacted ingredients and (2) also it takes 12 to 14 hour time to complete the reaction. It is generally observed that chromate effluent from passivation bath/tinplate processing tank, chromium + 6 electroplating bath contains hexavalent chromium (Cr ). + 3 It is reduced to trivalent chromium (Cr )before discharging into water since it leads to water pollution. This, therefore, incurs recurring expenditure. In view of the above facts, it is envisaged to develop a process to produce a value added (zinc tetroxy chromate) product out of the chromate effluent. The main object of the present invention is to provide a process for the production of zinc tetroxy chromate from chromate based effluent which overcomes above mentioned drawbacks. In the present invention, the effluent is treated + 6 with alkali and peroxides which convert Cr compounds, such as dichromates to chromates and ferrous to ferric which may be represented by the following equations : (Equation Removed) + + Further, soluble impurities like Fe are precipi- + 3 tated out as hydroxide of Fe . pH of the solution is maintained at a suitable alkaline pH for complete precipitation of the tetroxy chromate with the addition of more alkali. The yellow precipitate so formed is allowed to settle for 10-30 minutes, filtered, washed and o dried at 100-120 C for 4-6 hours. Finally, the product is pulverised and packed. Another novelty of the present invention is that level + of Cr in the final discharge is reduced to a very low level ( Accordingly, the present invention provides a process for the production of zinc tetraoxy chromate which comprises : (i) treating 100-150 litre of chromate effluent with 5 to 15 kgs. of strong alkali and oxidising agent selected from compounds having peroxide group followed by boiling for a period of 3 to 10 minutes to obtain solution containing ferric hydroxide, (ii) removing terric hydroxide by filtering to obtain filtrate, (iii) adding hydrated metal chloride and alkali in a concentration range of 40-50% to maintain the pH in the range of 8 to 10 the filtrate obtain in step (2) stirring the solution for 15 minutes at room temperature to obtain precipitate, (iv) washing the precipitate so obtain and filtering by conventional methods and drying at a temperature in the o range of 100-120 C for a period in the range of 1 to 4 hours to obtain a solid cake, (v) pulverizing the cake to particle size in the range of 100 to 250 mesh size and packing to obtain zinc tetroxy chromate. The amount of Filtered effluent used may ra-ge from 100 -150 lits, Metal chloride used in the range of 20 - 40 i Alkali in the range of 5 - 15 kg According to a feature of the invention, the chromate effluent containing Cr+6 may be selected from passivation bath, tinplate processing tank or chromium plating bath and may hsve composition in the range of 5-10 gm/litre. The effluent may be alkalised using caustic lye of 40 - 50%, sodium, potassxjm or lithium hydroxide and subsequently oxidized with sodium peroxide or hydrogen peroxide for conversion of dichromatea (if any) to chromates and Fe to Fe (i.e. hydroxides) respectively. Ferric hydroxide, so obtained, may be filtered cut using usual filtering unit. The filtrate may be treated with soluble metal chloride which may be selected from barium, zinc, lead, strontium and the like. The concentration of the chloride may be in the range of 20 - 40%. The resultant mixture may further be alkalized with alkali, such as potassium hydroxide, sodium hydroxide or lithium hydroxide and the like for maintaining the pH in the range of 8 to 10. By the process of present invention, very bright yellow pigment is obtained having .purity of 99% and the yield is 99.963%. The time taken for producing this compound is 3 to 4 hours only. The resultant effluent contains traces of sulphate and chloride, 2 ppm of Cr+ as given in Table-1. TABLE-1 (Table Removed) The following examples are given by way of illustration and should not be construed to limit the scope of invention. Example-1 100 ml of the effluent of Tinplate processing tank containing chromate in the range of 1.7-2% is treced with 0.5 ml of 6% hydrogen peroxide and, then, with 4 gms of potassium hydroxide to make it alkaline. The resultant solution is boiled for 5 minutes and cooled. Ferric hydroxide, so formed is filtered by usual procedure The solution treated with 20 gms of zinc chloride (hydrated) isstirred for 5-10 minutes. 7.5 gms of potassium hydroxide is, then, added and stirred to bring the pH to 8-9, yellow precipitate of zinc tetroxy chromate starts precipitati . The precipitate is filtered and cashed till no soluble impurities left out. It is dried at 110°C for 3 hours, pulverised and packed. Zinc tetroxy chromate of 99% purity is obtained with yellow coloration. Result The filtrate was analysed and found to contain Example-2 100 ml of the effluent as in Example-1 is treated with 0.5 ml of 6% hydrogen peroxide and, then, with 2 gms of potassium hydroxide to bring the solution to alkaline condition. The whole mixture is boiled for 5 minutes to convert any ferrous ions to ferric ions. After cooling, the solution is filtered to remove ferric hydroxide. The filtrate treated with 25 gms of zinc chloride (hydrated) of commercial grade. Then, 9 gms of potassium hydroxide (commercial grade) is added to bring the pH to 9. The solution is stirred for 15 minutes at room temperature. Yellow precipitate, so obtained, is filtered and washed till no soluble impurities is left out. It is dried at 120°C for 3 hours, pulverized and packed. Zinc tetroxychromate of 99,2% purity is .obtained. Result The filtrate was found to contain Example-3 250 ml of the chromate effluent from Tinplate processing tank is treated with 1.5 ml of 6% hydrogen peroxide and, then, with 5. gms of sodium hydroxide to make the solution alkaline. The whole mixture is boiled for about 5 minutes to convert any ferrous ions to ferric ions. After cooling, the solution is filtered to remove ferric hydroxide. Then, the filtrate is treated with 70 gms of zinc chloride (hydrated) • of commercial grade, and thereafter, 20 gms of sodium hydroxide is added.' The solution is, then, stirred for 10 minutes at room temperature. Yellow precipitation starts precipitating which is filtered and washed till no soluble impurities is left out. It is,' then dried at 110°C for 4 hours, pulverised and packed. The purity of tetroxy chromate is 99%. Result The filtrate was found to contain Advantages The following are the main advantages of the present invention. 1) By the process of the present invention, the cost of production of zinc tetroxy chromate is reduced to two and half times than the existing process. The existing process uses 2!n0 and CrO3 which are comparatively very costly whereas present invention uses ZnCl2 of commercial grade and caustic lye or alkali and the effluent, which are comparatively very cheaper. The present invention makes the process very simple and the parameters are easily optimised than the existing process. The present process involves reactions at controlled pH. The present invention reduces the reaction time by two to three times less than the existing process. This results in less energy consumption, less cost of production and increased productivity than the existing process. Hazardous chromate (Cr ) effluent which is discharged after treatment is utilized by the present invention in the preparation of a value added product. We claim : 1. A process for the production of zinc tetraoxy chromate which comprises : (i) treating 100-150 litre of chromate effluent with 5 to 15 kgs. of strong alkali and oxidising agent selected from compounds having peroxide group followed by boiling for a period of 3 to 10 minutes to obtain solution containing ferric hydroxide, (ii) removing grfeerric hydroxide by filtering to obtain filtrate, (iii) adding hydrated metal chloride and alkali in a concentration range of 40-50% to maintain the pH in the range of 8 to 10 the filtrate obtain in step (J2-H stirring the solution for 15 minutes at room temperature to obtain precipitate, (iv) washing the precipitate so obtain and filtering by conventional methods and drying at a temperature in the o range of 100-120 C for a period in the range of 1 to 4 hours to obtain a solid cake, (v) pulverizing the cake to particle size in the range of 100 to 250 mesh size and pe^kiag to obtain zinc tetroxy chromate. 2. A process as claimed in claim 1 wherein the amount of treated effluent ranges from 100-150 lits. the amount of metal chloride ranges from 2 0-40 kgs and the amount of alkali 5-15 kgs. 3. A process as claimed in claims 1-2 wherein the chromate effluent used is selected from passivation bath, tinplate processing tank, chromium plating bath, + 6 containing hexavalent (Cr ) in the range of 0.5 to 15%. 4. A process as claimed in claims 1 to 3 wherein the alkali used is selected from caustic lye, sodium hydroxide, potassium hydroxide, lithium hydroxide. 5. A process as claimed in claims 1-4 wherein oxidising agent used is selected from sodium peroxide, hydrogen peroxide. 6. A process as claimed in claims 1 to 5 wherein metal chloride used is selected from barium, zinc, lead, strontium the concentration of metal in the range of 20 to 40%. 7. A process for the production of zinc tetroxy chromate substantially as herein described with reference to the examples.

Documents:

1248-del-1995-abstract.pdf

1248-del-1995-claims.pdf

1248-del-1995-correspondence-others.pdf

1248-del-1995-correspondence-po.pdf

1248-del-1995-description (complete).pdf

1248-del-1995-form-1.pdf

1248-del-1995-form-2.pdf

1248-del-1995-form-4.pdf

1248-del-1995-form-9.pdf