Title of Invention

A PROCESS FOR PRODUCTION OF ZINC OXIDE BASED VARISTOR FROM ZINC ORE

Abstract

A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine comprising the steps of (I) leaching zinc calcine with nitric acid or hydrochloric acid; (ii) adding a base to the acid extract to precipitate and separate out lead and iron as hydroxides at 8 - 10 pH; (iii) lowering the pH of the said extract obtained at the end of step ii, to 6 - 7.5 to precipitate zinc hydroxide and optionally washing it with a polar solvent several times to make it sulphate ion free; (iv) dissolving the zinc hydroxide in nitric or hydrochloric acid to pH less than 2-6, and optionally adding sulphide source to precipitate and separate out sulphides of metals such as copper, cadmium, and silver; (v) mixing soluble compounds of antimony, bismuth chromium, cobalt and manganese and preferably at least one soluble metal compound as accessory additive selected from any element of the groups IA, IIA, IIIB and rare earths and organic complexing agent(s) with the extract obtained after step iv, at pH < 6 and evaporating water to form zinc oxide varistor precursor gel; (vi) drying said varistor precursor gel at temperatures less than 200 °C, to form varistor precursor powder; (vii) calcining said varistor precursor powder at 400 - 700 °C to obtain varistor powder and (viii) compacting varistor powder and sintering the varistor compacts at 900 - 1350°C for 1 - 4 hours, in controlled atmosphere with a controlled heating and cooling in range of 1-20° C/min to obtain zinc oxide based varistor.

Full Text

FORM 2 THE PATENTS ACT 1970 COMPLETE SPECIFICATION (See Section 10) TITLE A Process for Production of Zinc Oxide Based Varistor from Zinc Ore APPLICANT Department of Atomic Energy, Government of India, Anushakti Bhavan, Chhatrapati Shivaji Maharaj Marg Mumbai - 400 039 The folio wing specification particularly describes the nature of the invention and the manner in which it is to be performed:- Field of Invention This invention relates to the production of zinc oxide varistor which is used as a lightning arrester or surge absorber for protecting electric power lines or electronic components and circuits. This invention particularly relates to the production of zinc oxide varistor from zinc calcine. Background and Prior art ZnO varistor is a highly non-ohmic multiphase polycrystalline ceramic device, primarily composed of ZnO with other minor additive oxides. The varistor is usually made of zinc oxide (typically 80 - 90 %) and additives or dopants such as bismuth oxide, cobalt oxide, manganese oxide, chromium oxide etc. Additive oxides play vital role in the microstructural and electrical properties of zinc oxide varistors. The properties of ZnO based varistor are similar to a pair of back to back Zenar diodes but with markedly enhanced energy handling capacity [M.Matsuoka, Jpn.J Appl. Phys. 10, (1971) p- 736]. The excellent non-linear characteristics and surge protection capacity of ZnO varistor has resulted in its extensive applications as surge protection devices in electronic circuits and high voltage transmission systems. The voltage-current characteristics of varistor can be represented by a power law I = KVa where a is the non-linear coefficient, V is the voltage across the sample, I is the current flowing through it and K is a constant. 2 The extensive use of varistor in high voltage transmission systems has demanded varistor with high breakdown field. High breakdown field can be achieved with smaller grain size which in turn calls for fine starting powders and/or sintering at lower temperature where grain growth is inhibited. The main application of ZnO varislors is in lightning arresters to protect electric power lines against lightning surges. The second major application is surge protection in relatively low voltage circuits. They are often called surge absorbers. The third application is voltage stabilisation, especially for high voltages above 10 kV and at low currents, less than 1 mA. Preparation of zinc oxide varistor has been described in following patents: US Patents: 5,707,583 (January 13, 1998), Yodogawa, M., describes a method for preparing the zinc oxide based varistor in which a zinc oxide based varistor is prepared in the form of a sintered body which itself has a voltage-dependent non-linear resistance; firing is done in an atmosphere containing at least 60% of oxygen at or above the temperature at which additives form a liquid phase, and thereafter the sintered body is heat treated in an atmosphere containing at least 10% of oxygen at a temperature of 600 to 1000°C. It preferably contains as additives, at least one rare earth element selected from the group consisting of La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium); Co (cobalt); and at least one Group Illb element selected from the group consisting of B (boron), Al (aluminum), Ga (gallium) and In (indium). The sintering of the varistor samples is carried 3 out at a temperature above 1300°C (1300 - 1500 °C) for 1 - 10 hours. The rare earth elements form reaction products to generate a liquid phase to thereby promote liquid phase sintering and the high oxygen partial pressure promotes grain growth. Due to grain growth of ZnO, zinc oxide based varistor according to this invention has a large ZnO grain size, hence a smaller number of grains in the direction of applied voltage. Accordingly, the zinc oxide based varistor has a low varistor breakdown voltage. The zinc oxide based varistor of the invention desirably have a mean grain size of about 25 to 200 urn, a varistor breakdown voltage of 60 to 700 V/cm having a non-linearity coefficient (a, measured between 1mA and 10mA) of 15 -40. The zinc oxide base varistor of the invention is used in electric and electronic equipment for low voltage operation, especially those to be mounted on automobiles. US Patent No: 5,594,406 (January 14, 1997), Koyama, et al., describes a process for production of zinc oxide varistor in which a varistor is formed by diffusing lead borosilicate-type glass, into a surface of a fired or sintered zinc oxide substrate, i.e., "varistor element," during formation of an electrode on the surface of the substrate. Typically, an electrode paste or material, comprising a mixture or lead borosilicate-type glass frit and Ag powder, is applied to the substrate and provides the lead borosilicate-type glass for diffusing into the substrate. The improvement is that the lead borosilicate-type glass frit for the electrode paste or material comprises a mixture of PbO, B203, SiC>2 and at least one metal oxide selected from the group consisting of cobalt oxide, magnesium oxide, yttrium oxide, antimony oxide, manganese oxide, tellurium oxide, lanthanum oxide, cerium oxide, praseodymium oxide, neodymium oxide, samarium oxide, europium oxide, gadolinium oxide, terbium oxide, dysprosium oxide, holmium oxide, erbium oxide, thulium 4 oxide, ytterbium oxide and lutetium oxide. The value of non-linearity coefficient (a, measured between 50 A and 1 mA) is less than 7.5. Us Patent No: 5,592,140 (January 7, 1997), Tokunaga, et al., describes a manufacturing method for varistor formed of bismuth and antimony in which the varistor element contains zinc-oxide as a main constituent and at least bismuth and antimony as accessory constituents. The content of bismuth in the form of Bi203 is in a range from about 0.1 to 4.0 mol % and the content of antimony in the form of Sb2C>3 constitutes a mol-ratio of SbzCV Bi203 less than or equal to about 1.0. These materials are mixed thoroughly and are pressed into a compact. After coating both sides of the compact with Ag or Ag—Pd paste, the compact and its electrodes are sintered simultaneously at a temperature of about 800 to 960°C. The sintered density of the varistor samples was 95 % T.D. The value of non-linear coefficient (a, measured between 1 mA and 25A) is in the range of 17.4 -35. In the above-mentioned patents ZnO varistors have been prepared through conventional route starting with high purity ZnO or zinc compound as the starting material. A comparative study of the above-mentioned patents is given in Table No 1 5 Table 1 SR No Patent No. Electrical Characteristics of Varistor Remarks 1 US 5707583 a : 15-40 measured between 1mA and 10 mA Breakdown Field 60 -700V/cm The varistor composition is based on Zn-Pr system; the sintering temperature is higher (1300 - 1500 °C) which yields varistor having large ZnO grains which is suitable for low voltage applications; a is measured in the low current region; the process can not produce varistor of high breakdown field 2 US 5594406 Low value of a (-7.5) measured between 1mA and 50 A Boro-silicate type of glass containing specific metal ions is diffused into ZnO substrate for producing low voltage varistor with poor non-linearity coefficient 3 US 5592140 a: 17.4-35 measured between 1mA and 25 A The varistor composition is based on Zn-Bi-Sb system; the process produces varistor samples having sintered density of 95 %T.D. using high purity zinc oxide and/or compound as a starting material. Composition of Zinc Oxide varistor Commercially manufactured ZnO varistors usually contain basic additives (Bi203, Sb203, MnO, Q2O3 and CoO) along with some accessory additives to control grain sizes, ZnO resistivity, and ZnO stability. Bismuth oxide is known as the varistor former as it forms the basic microstructure of varistor i.e. insulating grain boundary formation. The formation of bismuth oxide phases in the final varistor microstructure folly depends on the sintering temperature and the rate of 6 heating and cooling. Bismuth oxide also helps in densiiication through liquid phase sintering. Antimony oxide also plays an important role in the control of microstructure. It produces a spinel phase (Zn7Sb20i2). The spinel particles are generally located at the grain boundary which impede the growth of zinc oxide grains. Cobalt oxide is mainly added to enhance the electrical characteristics of varistor in terms of non-linearity coefficient. Manganese oxide does not have a significant influence on the microstructure but increases the value of non-linearity coefficient. Chromium oxide stabilises the spinel phase and improves the breakdown voltage of the varistor. Tri-valent oxides (AI2O3, Ga2O3) act as donor and thus decreases the resistivity of zinc oxide grains. They increase the breakdown voltage and increase the leakage current. They also decrease the pore volume and grain growth. The content of trivalent oxides should be optimum to obtain the best non-linear V-I characteristics with low leakage current. Mono-valent oxide dopants act as acceptors hence increase the resistivity of zinc oxide grain. The increase of ZnO resistivity decreases the non-linearity and decrease in leakage current. The basic microstructure of zinc oxide varistor can also be obtained by replacing bismuth oxide with rare earth and alkaline earth oxides (BaO, SrO, Tb407 or Pr6On). However the non-linearity coefficient of the varistor formed by replacing bismuth oxide has been found to be lower than that of ZnO -Bi203 based varistor. T. K. Gupta and Miller A. C, [J Mater Res 3 (1988) p 745] report that the amphoteric dopants such as Na, K are found 7 to minimise the interstitial Zn, originating from non-stoichiometric nature of ZnO and stabilise the varistor properties. Kazuo Eda, has summarised the roles of the additives in varistors (IEEE Electrical Insulation Magazine, 1989 Vol. 5, No. 6, p 28) as given in Table 2. Table 2 Roles of Additives in ZnO Varistors Role Additives Isolation among ZnO grains and supplying required elements to grain boundaries (02, Co, Mn, Zn, etc.) Bi, Pr > Ba, Sr, Pb, U Improvement of stability Sb, glass, Ag, B > Ni, Cr Improvement of non-linear coefficient in a high current region (formation of donors in ZnO) Al, Ga, > F, Cr Grain growth suppression Sb, Si Grain growth promotion Be > Ti > Sn The effectivity is roughly indicated by the symbol >. M.Matsuoka (JpnJ Appl. Phys. 10, (1971) p- 736 ) has reported preparation of ZnO varistor by standard ceramic techniques and measured the non-linear electrical behaviour of ZnO based varistors containing varying amounts of metal oxide additives and showed the composition having 1 mol % Sb203, 0.5 mol % each of Bi203, Cr203, MnO, CoO and rest -ZnO, yields the highest non-linear coefficient. The effect of antimony oxide on the electrical properties has been studied by Kazuo Eda, (IEEE Electrical Insulation Magazine, 1989 Vol. 5, No. 6, p 28) and it has been shown that with 1 mol% antimony oxide to the ZnO system having 1 mol% each of bismuth oxide and cobalt oxide and 0.5 mol % MnO yielded better electrical properties. 8 Process for making zinc oxide based varistors: The properties of the varistor are influenced not only by the composition but also by the processing methods. The commercial varistors are usually fabricated by the conventional powder metallurgical technique that involves mixing, milling, compaction and sintering. The problem associated with the conventional method is the poor homogeneity with respect to the distribution of the additives. On the other hand, the chemical route provides finer varistor powders with molecular level of homogeneity with respect to distribution of dopants. These powders require lower sintering temperature, leading to finer ZnO grain size in the final microstructure that results in superior electrical characteristics, which are ideally suited for high voltage applications. Complexing Agents: To prepare ultrafine varistor powder various complexing agents have been used. Sonder et al. (Am. Ceramic Soc. Bull. 64(4), 665 (1985) ), describe a process in which all the constituent oxides of the varistor are first dissolved in nitric acid; granular urea is added to it and the solution is heated to melt the urea. The mass is dehydrated at 250 °C and calcined at 675 °C to give oxide powder. The powder is compacted and then sintered in air at 1100 °C. Hishita et. al, J Am Ceram Soc. 72 (1989) 338 describe an amine process for the preparation of ZnO varistor. ZnO varistors with good electrical properties prepared by sol-gel processing have been described by Hohenberger G and Tamandl G, J Mater Res., 7(1992) p-546 and Lauf R. J. and Bond W.D., Am. Ceram. Soc. Bull., 63 (1984) p - 284. G Hohenberger and G Tomandi describe a process for making zinc varistor in which a concentrated solution (1) containing zinc acetate, cobalt acetate, manganese acetate and 9 boric acid is mixed with a solution (2) containing antimony acetate, chromium nitrate, bismuth nitrate and aluminium nitrate prepared in excess of ethylene glycol Both the solutions (1 & 2) were mixed to form a gel. The gel is freeze dried and then calcined at 450°C for 15 hours to remove the organics. The powder prepared by R J Lauf and W D Bond by sol gel technique was found to be active and homogenous. The varistor was fabricated using this powder by hot pressing at temperatures below 800°C to attain high density. However the hot pressed samples could not display non-linear properties. The other complexing agents useful for the preparation of varistor powder include oxalic acid, EDTA, poly-alcohol (gylcerol, ethylene glycol), PVA(poly vinyl alcohol), polyacrylic acids etc. Our co-pending application 778/MUM/2001 relates to a process for preparation of high purity zinc oxide powder from zinc calcine comprising the steps of (i) dissolving zinc calcine by leaching with nitric or hydrochloric acid at 3.0 - 5.0 moles of acid per lOOg of said calcine at 50-2500C to extract acid leachable; (ii) adding a base to the acid extract to precipitate out lead and iron as hydroxide at pH 8 - 10 (iii) lowering pH of the basic extract, remaining after removing the said precipitates at 2 - 6 (iv) adding a sulphide source to precipitate out sulphides of metals such as copper, cadmium and silver; (v) ammoniation of the extract remaining after removing sulphide precipitates, to precipitate out pure zinc hydroxide; (vi) washing zinc hydroxide precipitate obtained in step (v), with polar solvent at pH 5-8, to remove soluble salts and other ions, (vii) calcining the washed and dried zinc hydroxide obtained at step (vi) at 250 - 650°C to get high purity zinc oxide powder. Object: The principal object of the present invention is to provide a method for preparing a zinc oxide based varistor having high non-linearity index (alpha in the range of 25 - 45), high breakdown field (4000 -5000 V/cm) and low leakage current, directly from calcined zinc ore (sphalerite, ZnS). Another object of the invention is to simultaneously generate oxides of the different accessory materials used in a varistor from their soluble salts during the process of making zinc oxide thereby bringing about homogenous distribution of dopants in the system leading to superior electrical characteristics. Our approach was to prepare zinc oxide varistor powder by incorporating steps to remove impurities and add additives during preparation of zinc oxide through zinc salt starting from zinc ore and conventional powder metallurgy techniques thereafter. 10 Summary of Invention Accordingly the present invention is related to a process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine comprising the steps of (i) leaching zinc calcine with nitric acid or hydrochloric acid; (ii) adding a base to the acid extract to precipitate and separate out lead and iron as hydroxides at 8 - 10 pH; (iii) lowering the pH of the said extract obtained at the end of step ii, to 6 - 7.5 to precipitate zinc hydroxide and optionally washing it with a polar solvent several times to make it sulphate ion free; (iv) dissolving the zinc hydroxide in nitric or hydrochloric acid to pH less-than 6, and optionally adding sulphide source to precipitate and separate out sulphides of metals such as copper, cadmium, and silver; (v) mixing soluble compounds of antimony, bismuth chromium, cobalt and manganese and preferably at least one soluble metal compound as accessory additive selected from any element of the groups I A, II A, IIIB and rare earths and organic complexing agent(s) with the extract obtained after step iv, at pH (viii) compacting varistor powder and sintering the varistor compacts at 900- 1350 °C for 1-4 hours, in controlled atmosphere with a controlled heating and cooling to obtain zinc oxide based varistor. Embodiments of the invention: Zinc oxide content of the varistor prepared by the process of present invention is more than 85 mole %; preferably 90 - 97 mole %. t1 It will be seen from the summary of invention, that the process has lot of flexibility; certain amounts of impurities can be tolerated making purification steps optional. However, impurities like iron and lead have to be removed. Of different zinc ores, calcined sphalerite of Indian origin is preferred in the process of the present invention. The leaching of zinc calcine is carried out with hydrochloric or nitric acid, at 3.0 - 5.0 moles per lOOg of said zinc calcine, at 50 - 250 °C preferably at 80 -150 °C to extract acid leachables. After removing the lead and iron hydroxide precipitate on alkalising the acid extract, pH of the extract may be lowered by heating the leach extract up to 100°C if the base used for alkalising is volatile as an alternative to acid treatment. In case of acid treatment, use of the same acid used for leaching is preferred. This will not introduce another ion impurity. In this process, impurities of metals such as copper, cadmium, silver etc. may be removed by sulphiding with a sulphide source selected from the group of compounds such as ammonium sulphide, hydrogen sulphide, sodium sulphide, poly-sulphides. Of the different additives, there are no reports on properties electrical character of zinc varistor with neodymium as dopant. This invention clearly shows its advantages as shown in Examples 5 -7. The quantities as well as the preferred range of different additives for the preparation of zinc oxide varistor are as follows: Additive salt in terms of its oxide Range of amount added (mole %) Preferred range (mole %) Bismuth oxide 0.1 -5.0 0.5-1.0 Antimony oxide 0.1 -2.0 0.1 - 1.0 Chromium oxide 0.01 - 1.0 0.1-0.5 Cobalt oxide 0.1 -1.0 0.1-0.5 Manganese oxide 0.1-1.0 0.1-0.5 Rare earth oxide* 0.001 - 2.0 0.1-1.0 Illb group elements oxide 0.001- 1.0 0.01 - 0.5 la group elements 0.001 - 1.0 0.01 - 0.5 Ha group elements 0.001 - 2.0 0.01 - 1.0 *At least one rare earth element selected from the group consisting of La (lanthanum), Ce (cerium), Pr (praseodymium), Nd (neodymium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium), Dy (dysprosium), Ho(holmium), Er (erbium), Tm (thulium), Yb (ytterbium), and Lu (lutetium). In another embodiment of the process of invention organic complexing agent is selected from a group of poly-functional hydroxy-acids such as citric acid, tartaric acid, lactic acid, glycolic acid and malic acid; of these citric acid is preferred. In another embodiment of the process of invention at least 2 moles of complexing agent is used per 3 moles of zinc oxide equivalents of zinc content of acid leachables of zinc calcine taken. In another embodiment of the process of invention the zinc oxide varistor precursor powder is calcined preferably at 400 - 650° C. The zinc oxide varistor green compact is sintered preferably at 1050- 1250 °C under static air. 13 Examples: The invention will now be illustrated with the help of Examples. The examples are by way of illustration only and in no way restrict the scope of the invention. Example 1. Chemicals used: Zinc calcine (from Hindusthan Zinc Ltd., Udaipur) showed following composition: Zn - 59 - 61 % by wt. Zn (Acid Soluble ) - 54 - 55 % by wt. Fe -- 9 - 10 % by wt. Pb -- 1 - 2 % by wt. Cd- 0.2-0.3% by wt. Cu- 0.1 - 0.16 % by wt. Ag - 60 - 90 ppm Ni - 25 - 40 ppm Co -- 25 - 40 ppm S -- 2 - 3 % by wt.; Acid insolubles — 2 - 3 % by wt. Manganese acetate, antimony oxide, chromic oxide, cobalt nitrate, bismuth nitrate, aluminium nitrate; nitric acid and citric acid were lab reagent grade chemicals. Process: Zinc calcine (40 g) was leached with 176 ml nitric acid (1:1) at 100°C. The leached solution was filtered using Wattman #41 filter paper. The filtrate was made alkaline (pH of 8 -10) by adding ammonium hydroxide (1:1) (288 ml) when the iron and lead hydroxide precipitated. The precipitate was filtered out. The filtrate was acidified by adding (1:1) nitric acid, when zinc hydroxide precipitated in the pH range 7.5 - 6. 14 The zinc hydroxide precipitate was filtered out and washed with distilled water for several times until zinc hydroxide became sulphate free, as confirmed by barium chloride test. The zinc hydroxide was then dissolved with 1:1 nitric acid (120 ml) to obtain a zinc nitrate solution of pH 4. To the zinc nitrate solution (NH4)2S (conc.)( 0.5ml) was added and the precipitated copper and cadmium sulphides were filtered out to obtain 1 litre of purified 0.3 Molar [0.2948(M)j concentration zinc nitrate solution at pH 3.5. To 658 ml of this solution, aqueous solutions of Manganese acetate (0.2479 g), Cobalt nitrate (0.2921 g), Bismuth nitrate (0.9930 g), Chromium nitrate [prepared by dissolving Chromic oxide (0.2011 g) in (1:1) nitric acid], Antimony citrate [prepared by dissolving Antimony Oxide (0.5849 g) in 20 g citric acid] and Citric acid (30 g) were added. In addition, Aluminium nitrate (0.2096 g) was also added. The water was evaporated from the solution until a viscous liquid (gel) was obtained. The gel was dried in vacuum oven at The dried gel was calcined at 650°C for 2 hours under air to form the varistor powder. The powder was then compacted into disks of 10 mm diameter and 4 mm thickness by a uniaxial hydraulic press at 283 Mpa pressure. Zinc stearate was used for die lubrication. The green compacts were then sintered under static air in the furnace, raising the temperature at a heating rate of 5°C/min. to 1150 °C maintaining the temperature for 1 hour, then cooling it to 950 °C at the rate of 8 °C/min, then further cooling it to 800°C at the rate of 3°C/min. and then allowing the furnace to cool. The zinc varistor pellets thus obtained (16 g), were examined for Composition, Microstructural characteristics and Electrical characteristics as follows: 15 Electrical characteristics: The pellets were thinned down to about 2 mm by grinding emery paper. Silver coating was applied using conductive silver paste on both faces of the varistor disks, which were subsequently baked at 700°C for 15 min. Variation of current as a function of applied voltage was measured at room temperature using a DC powder supply in a current range upto ImA/cm2. Measurement in the high current range beyond ImA/cm" was carried out by using a pulse generator. The non-linearity coefficient (a) was calculated in the range between 1 mA/cm and 1 A/cm2 using the following equation: a =(dJ/J)/(dE/E)=d(logJ)/d(logE) = (logJ2 - logJi)/ (logE2 - logEi) where E2 and E2 are electric fields at current densities J2 and Ji respectively. (IEEE Trans. Elec. Insul. Vol. 23 ,1988, p -279). Microstructural characterisation: Microstructural characterisation of the sintered pellets was carried out using both a light optical microscope (LOM) and a scanning electron microscope(SEM). The pellets were lapped with the help of diamond paste and etched with 1% HC1 solution. The grain size was measured using linear intercept method. The densities of the pellets were measured by the Archimedes technique as well as by dimensions. The varistor powder produced through Urea route as described by Sonder et al. used the similar composition as compared to the composition given in Table 3. The comparison of 16 the properties of the varistor prepared through example 1 and urea route are given in Table 4. Composition of varistor powder: The elemental composition of the varistor powder thus prepared was determined by XRF method and by standard chemical analysis. The results of the examination of the product are given in Tables 3 and 4. Table 3 Composition of varistor powders of Examples 1-4 Oxide mol % ZnO Sb203 Bi203 Cr203 CoO MnO A1203 Examples 1-4 Urea route* 97 1 0.5 0.5 0.5 0.5 0.025 * Sonder et al., Am. Ceram. Soc. Bull. 64 (1985) 665 Table 4 Characteristics of the varistor samples Sintered at 1150°C for 1 hour Process a E (V/cm) Density (gm/cc) Example 1 Example 2 Example 3 Example 4 45 23.2 20.4 20 4137 4606 3237 4686 99 % T.D 94% T.D. 94%T.D. 96%T.D Urea route* 21 2800 94 % T.D. * Sonder et al., Am. Ceram. Soc. Bull. 64 (1985) p- 665 17 Example 2 In this example the varistor samples were prepared according to the experimental procedure described in example 1, but without last purification step of zinc nitrate solution, viz., "To the zinc nitrate solution (NH4)2S (Conc.)( 0.5ml) was added and the precipitated copper and cadmium sulphides were filtered out". Thus after washing of the zinc hydroxide precipitate free of sulphate ions, it was dissolved in nitric acid (1:1) to make a zinc nitrate solution of pH of 3.5. That is copper, cadmium and silver present in the zinc solution was not removed by using ammonium sulphide precipitation step. The final product was examined as in Example 1 and the results are given in Tables 3 and 4. Example 3. In this example the varistor samples were prepared according to the experimental procedure described in example 1, but without the purification (by washing) step of zinc hydroxide precipitate, viz., "The zinc hydroxide precipitate was filtered out and washed with distilled water for several times until zinc hydroxide became sulphate free, as confirmed by barium chloride test". Thus the unwashed zinc hydroxide was dissolved with nitric acid (1:1)(120 ml) to obtain a zinc nitrate solution of pH 3.5. The solution was then subjected to "To the zinc nitrate solution (NH4)2S (Conc.)( 0.5ml) was added and the precipitated copper and cadmium sulphides were filtered out" 18 The final product was examined as in Example 1 and the results are given in Tables 3 and 4. Example 4 In this example the varistor samples were prepared according to the experimental procedure described in example 1, but without the purification (by washing) step of zinc hydroxide precipitate, viz., "The zinc hydroxide precipitate was filtered out and washed with distilled water for several times until zinc hydroxide became sulphate free, as confirmed by barium chloride test." and without last purification step of zinc nitrate solution, viz., "To the zinc nitrate solution (NtL^S (conc.)( 0.5ml) was added and the precipitated copper and cadmium sulphides were filtered out" Thus the unwashed zinc hydroxide was dissolved in nitric acid (1:1) ( 120 ml) to obtain a zinc nitrate solution of pH 3.5. Both the steps: intermediate washing of the zinc hydroxide to remove sulphate ions and the ammonium sulphide precipitation step to remove copper, cadmium and silver present in the zinc solution, were not carried out. The final product was examined as in Example 1 and the results are given in Tables 3 and 4. It will be seen from the Table 4 that among the four examples, varistor powder of Example 1 [prepared using intermediate washing of the zinc hydroxide (carried out after stage 1 refining to remove sulphate ions) and stage 2 refining (to remove mainly copper and cadmium from the zinc bearing solution)] yields the better electrical properties. 19 Examples 5-7: In these examples variation in accessory additive is studied. The process of Example 1 was followed replacing aluminium nitrate with neodymium nitrate in amounts as shown in Table 5 and 6. The electrical characteristics of the varistor samples of examples 5,6 and 7 are shown in Table 6. Table 5 Composition of varistor powder in example 5, 6, and 7 Example No. Additive ZnO Sb203 Bi203 Cr203 CoO MnO Nd203 5 mol % 96.75 1.0 0.5 0.5 0.5 0.5 0.25 6 mol % 96.625 1.0 0.5 0.5 0.5 0.5 0.37 7 mol % 96.50 1.0 0.5 0.5 0.5 0.5 0.50 Table 6 Electrical characteristics of the varistor samples Example Neodymium Nitrate Wt. (mg) Neodymium Oxide Mole % a Breakdown Voltage (V/cm) Density % T.D. 5 439.2 0.25 20 6660 95 6 658.1 0.37 34 5110 94 7 878.7 0.50 50 5000 98 20 We Claim 1. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine comprising the steps of (I) leaching zinc calcine with nitric acid or hydrochloric acid; (ii) adding a base to the acid extract to precipitate and separate out lead and iron as hydroxides at 8 - 10 pH; (iii) lowering the pH of the said extract obtained at the end of step ii, to 6 - 7.5 to precipitate zinc hydroxide and optionally washing it with a polar solvent several times to make it sulphate ion free; (iv) dissolving the zinc hydroxide in nitric or hydrochloric acid to pH less than 2-6, and optionally adding sulphide source to precipitate and separate out sulphides of metals such as copper, cadmium, and silver; (v) mixing soluble compounds of antimony, bismuth chromium, cobalt and manganese and preferably at least one soluble metal compound as accessory additive selected from any element of the groups IA, IIA, IIIB and rare earths and organic complexing agent(s) with the extract obtained after step iv, at pH 2. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in claim 1, wherein zinc calcine is calcined sphalerite of Indian origin. 22 3. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-2, wherein leaching of zinc calcine is carried out with hydrochloric or nitric acid (1:1), at 3.0 - 5.0 moles per lOOg of said zinc calcine, at 50 - 250 °C to extract acid leachables. 4. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-3, wherein acid leaching of the said zinc calcine is carried out at 80 - 150 °C. 5. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any of claims 1 - 4, wherein, lowering pH of the alkaline extract, remaining after removing the said lead and iron hydroxide precipitate, to less than 6, in step iii, is carried out by heating the said extract up to 100 °C when base used in step (ii) is volatile. 6. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-5 wherein, sulphide source used for precipitating metal ions is selected from the group of compounds such as ammonium sulphide, hydrogen sulphide, sodium sulphide, poly-sulphides. 7. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-6, wherein said zinc oxide base varistor contains neodymium oxide as an additive selected from the group of rare earth 23 8. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-7 wherein, Jhe^ amount of bismuth compound added is equivalent to give 0.1 mole % - 5.0 mole % of bismuth oxide. 9. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-8 wherein, the amount of bismuth compound added is equivalent to give 0.5 mole % - 1.0 mole % of bismuth oxide. 10. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-9 wherein, the amount of antimony compound added is equivalent to give 0.1 mole % - 2.0 mole % of antimony oxide. 11. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-10 wherein, the amount of chromium compound added is equivalent to give 0.01 mole % - 1.0 mole % of chromium oxide. 12. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1- 11 wherein, the amount of cobalt compound added is equivalent to give 0.1 mole % - 1.0 mole % of cobalt oxide. 13. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-12 wherein, the amount of manganese compound added is equivalent to give 0.1 mole % - 1.0 mole % manganese oxide. 24 14. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-13, wherein the amount of rare earth compound added as an accessory additive is equivalent to give 0.001 mole % - 2.0 mole % of rare earth oxide . 15. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-14 wherein, the amount of compounds of group Illb elements such as boron, aluminium, gallium or indium added is equivalent to give 0.001 mole % - 1.0 mole % of oxides of the said elements of the group Illb. 16. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-15 wherein, the amount of compounds of group la elements such as sodium, potassium, rubidium or caesium added is equivalent to give 0.001 mole % -1.0 mole % of oxides of the said elements of the group la 17. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-16 wherein, the amount of compounds of group Ila elements such as magnesium, calcium, strontium and barium added is equivalent to give 0.001 mole % - 2.0 mole % of oxides of the said elements of the group Ila 18. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-17 wherein, organic complexing agent is 25 selected from a group of poly-functional hydroxy-acids such as citric acid, tartaric acid, lactic acid, glycolic acid and malic acid; 19. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-18 wherein, poly-fuctional hydroxy-acid used as a complexing agent is citric acid. 20. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-19 wherein, at least 2 moles of complexing agent is used per 3 moles of zinc oxide equivalents of acid leachables of zinc calcine taken. 21. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-20 wherein, zinc oxide content of the varistor is more than 85 mole %. 22. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-21 wherein, zinc oxide content of the varistor is preferably 90 - 97 mole %. 23. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-22 wherein, the zinc oxide varistor precursor powder is calcined preferably at 400 - 650° C. 26 24. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine as claimed in any claim 1-23 wherein, the zinc oxide varistor green compact is sintered preferably at 1050- 1250° C under static air. 25. A process for preparation of zinc oxide based varistor in the form of sintered body from zinc calcine substantially as herein described in the text and in the examples. Dated this 7TH day of AUGUST 2001 S Majumdar & Co. Agents for the Applicants 27

Documents:

777-mum-2001-cancelled pages(08-01-2005).pdf

777-mum-2001-claims(granted)-(08-01-2001).doc

777-mum-2001-claims(granted)-(08-01-2001).pdf

777-mum-2001-correspondence(05-03-2007).pdf

777-mum-2001-correspondence(ipo)-(21-10-2004).pdf

777-mum-2001-form 1(09-08-2001).pdf

777-mum-2001-form 19(30-10-2005).pdf

777-mum-2001-form 2(granted)-(08-01-2001).doc

777-mum-2001-form 2(granted)-(08-01-2001).pdf

777-mum-2001-form 3(09-08-2001).pdf

777-mum-2001-power of attorney(05-05-2004).pdf

777-mum-2001-power of attorney(09-08-2001).pdf