Title of Invention

"AN IMPROVED PROCESS FOR PREPARATION OF OPTICAL WINDOW GRADE ZINC SULPHIDE"

Abstract

This invention relates to an improved process for the preparation of optical window grade zinc sulphide. The process comprises the steps of preparing stock solution by reacting zinc oxide with 2M H2 SO4 followed by addition of zinc granules and decanting the same. The metallic impurities are oxidised by adding of H2O2 into said solution. Stock solution of zinc sulphate (ZnSO4) is reacted with pure FhS gas and the resultant zinc sulphide powder is fired.

Full Text

AN IMPROVED PROCESS FOR PREPARATION OF OPTICAL WINDOW GRADE ZINC SULPHIDE This invention relates to a process for preparation of optical window grade zinc sulphide. Zinc sulphide is an important material for fabrication of IR transmitting windows/domes used in areas like remote sensing. This is because of its excellent thermo-mechanical properties coupled with its very good transmission in 2-12µ range of the infra-red (IR). In one of the process known in the art, zinc sulphide is prepared by reacting zinc salt solution with Hydrogen sulphide gas, followed by firing at around 700 degree Celsius in an inert atmosphere of Nitrogen gas. The disadvantage of the above process is that the uncontrolled pH conditions lead to poor yield and irregular sized particles. In the other process known in the art, zinc sulphide is prepared by reacting zinc salt solution with thioacetamide solution and subsequent firing at around 700 degree celsius. The disadvantage of the above process is that yield of optical window grade zinc sulphide obtained is less than 10%. In yet another process known in the art, zinc sulphide is prepared by reacting zinc salt solution with solution of sodium sulphide and subsequent firing at around 700 degree celsius. The above process has the disadvantage that tho alkaline conditions lead to formation of undesired hydrdxides which during the process of firing at 700 degree Celsius, i are partially converted to zinc oxide, which is an impurity. The other disadvantage of the above process is that the sodium ions create contamination. The above processes known in the art have the shortcomings that the pH is not controlled during the precipation reaction. The pH uncontrolled reaction leads to an unsatisfactory yield of zinc sulphide powder due to reversible nature of reaction caused by simultaneous generation of sulphuric acid (lSO) alongwith zinc sulphide (ZnS) ZnSO4 + H2S ↔ ZnS + H2SO4 The reversible reaction occurs when pH falls below 2. i ' The pH-uncontrolled reaction during the precipitation reaction also leads to the disadvantage that the zinc sulphide powder obtained has irregular physical properties. Another shortcoming of • the known processes is that firing of zinc sulphide powder is carried out at temperature of around 700 degree celsius which adversely affects the IR transmission capability of the resultant zinc sulphide powder in the following ways: (a) The grain size of zinc sulphide is a critical parameter which controls the transparency of zinc sulphide powder in the Infra-red (IR) region. The firing at lower temperature of 700 degree celsius leads to smaller grain size which results in loss of IR transmission. (b) Another critical parameter which vitally affects the IR transmission capability of zinc sulphide powder is the phase composition of zinc sulphide powder. for Optimum IR transmission capability, the α -ZnS phase in the ZnS powder should be less than 10 percent while the ß-ZnS phase should be more than 90%. Firing at temperature above 1000 degree Celsius leadn to conversion of ß-ZnS to α-ZnS thereby reducing Hie percent phase composition of ß-ZnS to below 90%. Thus firing at lower temperature around 700 derp ee Celsius as well as heating beyond 1000 degree celisuss both have adverse affect on the IR transmiMSion capability of ZnS due to unfavourable grain size and unfavourable phase composition respectively. '1 The. improved performance feature of ZnS are obtained by firing ZnS powder within the optimum range of temperature as disclosed in the present invention. The primary object of the present invention is to propose an improved process for preparation of zinc sulphide Which leads to an improved 70-80% yield of optical window grade zinc sulphide/ Another object of the present invention is to propose An improved process for the preparation of zinc sulphide Which leads to optical window grade zinc sulphide of improved optical transmission quality, as a result of filling which enables achievement of an optimum phase composition in the zinc sulphide powder. Whlch leads to zinc sulphide powder which compacted by hot isostatic pressing at 1200 degree co for .bout 3 hours yields pellets/windows showing tranemission in the IR range.According to this invention there is provided an improved process far preparation of optical window grade zinc sulphide comprising in the steps of s (a) preparing stock solution by reacting Zinc: Oxide with 2M H SO followed by addition of zinc granules and 2 4 decanting, (b) oxidising metallic impurities by addition of H 0. (c) reacting stock solution of Zinc Sulphates (ZrtSG ) 4 with pure H S gas. (d) firing the resultant Zinc sulphide powder, In accordance with this invention pH of the reaction of zinc sulphate (ZnSO ) with hydrogen sulphide 4 gas (H S) is controlled by controlled rate of addition of ammonia solution to the reaction mixture during the course of precipitation reaction. The pH is controlled by addition of 5-10% ammonia solution,, As the fall of pH be; law the value of 2,, re verse 53 the direction of the reaction leading to lower yield of sine sulphide powder,, the value of pH of the reaction is maintained at 2.4 + 0.1. In the proposed process, the firing of zinc sulphide powder is carried out at temperature between 700 950 C,, preferably within the temperature range of 9OO-950 C. The firing within this range of temperature leads to larger grain growth size which reduces IR transi.ffiisBi.on losses and leads to optical window grade nine sulphide of improved IR transmission capability, The yield obtained is about 7O to 80"/« which is quite high, considering the fact that the first lot of the p r e c i. p i. t a t. e is r e. j e c t e d „ T h e z i n c s u 1 p h i d e p o w d e r obtained is hot isostatically pressed at 1200 degree celsius for 3 hours, under 18 Ksi pressure in the purified Nitrogen gas environment,, to produce a window showing about 65—70% transmission in the infrared (IR) region The steps of the present invention consists in dissolving chemically pure zinc oxide in 2M H SO 2 4 (sulphuric acid) at. room temperature till the solution is saturated. The solution obtained by step (a) above is filtered AR grade zinc (Zn) granules is added to the solution and allowing it to remain undisturbed. The solution is decanted after 7 days,, The solution obtained is referred to as 'stock solution The stock solution obtained by step (b) above is mixed with double distilled water in the volume ratio of 1:7 so as to obtain 0.25 M ZnSO (Zinc sulphate) 4 sa1ution for precipitation „ A few drops of 6% H202 (Hydrogen peroxide) is added to the solution obtained by step (c) above, to oxidise unwanted meta11ic impurities 1ike iron „ 1 to 5% preferably 1% ammonia solution is added with constant stirring to the solution obtained by step (d) , till faint white colour precipitate is obtained,, The precipitate obtained in step (e) is filtered the filterate ZnSO taken into a precipitate vessel 4 fitted with 3 inlet joints (i.) for addition of dilute ammonia solution during the course of precipitation (ii) for inlet/outlet tube for hydrogen sulphide gas and ( iii ) for inserting pH e1ectrode . H2S gas is purified separately by successively bubbling through (i) 5N HCL (ii) saturated Ba(OH2) so1ution and ( iii ) doub1ed is ti11ed water The purified H2 S gas obtained by step (g) above is passed to the ZnSO solution in the precipitation 4 vessel. The pH of the solution starts falling from the original valve of about 5/7 As soon as the value of pH falls to the value of about 3.0 the supply of H S gas is stopped. The precipitate referred to herein after as 'first precipitate' is filtered off and rejected. The filterate obtained from step (h) in the same precipitation vessel and again purified H2S gas is allowed to bubble through the solution. The pH of the solution again begins to fall. However dilute ammonia solution 6--.1.07.,, preferably 87. is simultaneously added at such a rate that the pH of the solution remains steady at a value of 2.4+0.1. The precipitation is continued for half an hour at the room temperature with vigorous stirring „ The precipitate is filtered and washed with double disti11ed water ti11 it is fre e from SO 4-2 ions . "I"his is fo11owed with drying of the precipitateat about 110 C The dried precipitate obtained by step (k) is ground to a fine powder which is then taken in a quarts tube place d i n a furnace. The powder of step (1) is heated to a temperature range of 700 to 950 C preferably within the temperature range of 9OO--950 C and allowing JOLAR 1 Nitrogen gas which has been further purified by passing through alkaline pyrogallol, silica gel and molecular sieves to flow over it The heating of the powder in the pure nitrogen environment is continued example about. 6 hours and then furnace is put. off. The quartz tube from the furnace is taken out while cont.inus.ing the flow of nitrogen over the powder and coo1ing it toroomterriperature. The proposed invention is further illustrated wih the following example which is intended to be a typical examp1e with out in any way intending t o 1ienit th e scope of the present invention. EXAMPLE Chemically pure Zinc Oxide is dissolved in 2M H2SO4 solution till saturated. The filtered solution (ZnSo4 solution) ) is then left in contact with AR grade Zinc granules for a week. The solution is then ready for use as a stock solution (2M) of Zinc sulphate. To prepare 80g of Zinc sulphide powder, about 3 litre (0.25M) ZnSO4 solution is prepared from the above stock solution and a few drops of 6% H2O2 solution are added to oxidise the unwanted metallic impurities like iron, and then dilute Ammonia solution (1%) is added till a faint persisting precipitate is formed. The precipitate is filtered off and the filterate is taken into a precipitation vessel fitted with 3 inlet joints as described in the above description of the process. Hydrogen sulphide gas purified as per step (g) of the process is bubbled through ZnSO4 solution in the precipitation vessel. The pH of the solution starts falling from the original value of about 5.7. As soon as the pH value reaches 3.0, the H2S gas supply is stopped. The precipitate obtained (first precipitate) is filtered off and rejected. In the filtrate, the H2S gas is again bubbled. A dilute Ammonia solution (8%) is also simultaneously added at such a rate that the pH remains steady at a value of 2.4 + 0.1. The precipitation is continued for half an hour at the room temperature with vigorous stirring. The precipitate obtained is filtered, washed with double distilled water till free from S04~2 ions and dried at 110 degree celsius. The dried precipitate is ground fine and the powder obtained is taken in a quartz tube placed in a furnace arid treated in pure nitrogen environment at 800 degree celsius for 6 hours as per the above step (m) of the process and the furnace is then put off. The quartz tube is then taken out while the Nitrogen gas is still flowing over the powder and cooled to room temperature. The powder thus obtained is ready for use in window fabrication. The sample powder of zinc sulphide obtained by the above process was tested for transmission behaviour. The powder when hot isostatically pressed at 1200 degree celsius for 3 hours under 18 KSi pressure of Argon, produced n window showing about 65-70% transmission in IR region. It is to be understood that the above description of the process for preparation of optical window grade zinc sulphide is susceptible to considerable modifications, changes and adaptations by the persons skilled in the art. Such modifications are intended to be within the scope of the present invention which is set forth as per the following claims: I CLAIM: 1. An improved process for preparation of optical window grade zinc sulphide comprising in the steps erf : (a) preparing stock solution by reacting Zinc Oxide with 2M H SO followed by addition of sine granules and 2 4 decanting . (b) oxidising metallic impurities by addition of H O. (c) reacting stock solution of Zinc Sulphate (ZnSO ) 4 with pure H S gas., (d) firing the resultant Zinc sulphide powder, 2. A process as claimed in claim 1 where the said step of firing of the zinc sulphide powder is carried out at a temperature of 700 to 950 C in an inert, atmosphere of p referab1y nitrogen 3. A process as claimed in claim 2 wherein said firing is carried out preferably at a temperature of 9OO to 95O C in an inert atmosphere of preferably nitrogen. 4. A process as c laimed in claim 1 wherein the concentration of ZnSO solution taken for reacting with 4 purified H S gas in the range of O.1M to 0.8M. 5. A process as claimed in claim 1 wherein the pH of the reaction mixture of step (c) is controlled by addition of ammonia solution 5-10% 6. A process as; claimed in claim 5 wherein the pH is controlled by addition of ammonia solution preferably 8%. 7. A process as claimed in claim 5 & 6 wherein the pH is maintained at 2.4 + O.1. 8. A process as claimed in claim i wherein said zinc granu1es is of AR grade. 9. An improved process for preparation of optical window grade zinc sulphide substantially as herein described and i11ustrated

Documents:

52-del-1997-abstract.pdf

52-del-1997-claims.pdf

52-DEL-1997-Correspondence Others-(13-09-2011).pdf

52-DEL-1997-Correspondence-Others-(16-12-2009).pdf

52-del-1997-correspondence-others.pdf

52-del-1997-correspondence-po.pdf

52-del-1997-description (complete).pdf

52-del-1997-form-1.pdf

52-DEL-1997-Form-15-(13-09-2011).pdf

52-DEL-1997-Form-15-(16-12-2009).pdf

52-del-1997-form-19.pdf

52-del-1997-form-2.pdf

52-del-1997-form-3.pdf

52-DEL-1997-GPA-(13-09-2011).pdf

52-del-1997-gpa.pdf