MANUFACTURE OF HIGH PURITY ZIRCONIA FROM LOW-SILICA ZIRCONIFEROUS MATERIALS

Abstract: Leaching of a low-silica zirconium concentrate, preferably derived from caustic-leached plasma-dissociated zircon, (95 % Zr02), with hot fairly concentrated sulphuric acid. This concentration must be such as to derive a boiling point of 175 - 250 DEG C for the acid, at which temperature the leaching operation is carried out. A preference for 80 - 88 % sulphuric acid, boiling at 200 - 250 DEG C is exercised. The preferred technique is to use 98 % sulphuric acid as a starting material, together with the zirconiferous material, the water of dilution being derived from the reaction with the zirconium concentrate. The concentrate passes into solution as the sulphate and zirconia is extracted therefrom by known methods. The above procedure represents an attractive economic option for the recovery of high purity zirconia, which has particular application in the electronics industry and the production of partially stabilised zirconia ceramics.

Description: MANUFACTURE OF HIGH PURITY ZIRCONIA FROM LOW-SILICA ZIRCONIFEROUS MATERIALS This invention ,relates to a process for obtaining high purity zirconia (ZrO,) from zirconiferous materials, and is particularly concerned with the production of zirconia from dissociated zircon by sulphuric acid leaching. Demand for high purity zirconia (99.9% Zr0 2 or better), is increasing, particularly for use in the production of partially stabilised zirconia (PSZ), ceramics and for various electronic applications. The most abundant and widely distributed zirconium mineral is zircon (ZrSi0 4 ) and many methods have been developed for the production of zirconia and other zirconium compounds from this mineral. The selection of a satisfactory process is very much dependant, not only upon the purity of the product required, but also on the nature of the waste products of the process and disposal problems which may be associated with them. In general, the presently used processes involve either chemical or thermal decomposition of zircon to produce a mixture of zirconia and silica (or compounds thereof) followed by chemical treatment of the decomposition products to produce pure zirconia. Chemical decomposition of zircon can be accomplished by sintering with alkali or alkaline earth oxides. Sodium hydroxide or sodium carbonate are usually used for this purpose and the reaction can be controlled to produce a product consisting of sodium zirconate and sodium silicate. This product is then leached with water to dissolve the sodium silicate and hydrolyse the sodium zirconate to crude hydrous zirconia, which is then further purified. For example, the crude zirconia may be dissolved in hydrochloric acid followed by sulphuric acid and reprecipitated as zirconium sulphate (e.g., Zr(S0 4 )-.4H 2 0) by adjustment of the sulphate/water ratio of the solution or as hydrated zirconium oxide by precipitation with ammonia. There is a considerable body of prior art (for example US Patent No. 2,387,046), relating to the use of sulphuric acid to refine impure zircon and treat low silicon zirconia-based ores. The prior art indicates that considerable difficulty may be encountered when the silica content of zirconia-containing ores is greater than 10%, particularly when it is present as ZrSiO, rather than free silica (see U.S. Patent No. 2,387,046). Consequently, processes have been developed which involve preliminary steps for breaking the ZrO j /SiO- bond and reducing the SiO- level, e.g. by fusion with caustic soda (U.S. 2 , 387 , 046 , U. S . 3 , 398 , 005 ) . Zircon can also be completely dissociated by heating ^ at temperatures in excess of 1800 β C, for example using a plasma arc furnace. When zircon sand is injected into the ς 5 plasma arc, it melts and dissociates into zirconia and silica. On rapid cooling, solidification occurs with the formation of extremely small zirconia crystallites in an amorphous silica matrix. Dissociation of zircon can also be accomplished by heating in a conventional electric 10 furnace (operating at about 2000°C) or in a so-called "advanced electric reactor" (AER) furnace operating at about 2400°C. Furnaces of this latter type are described in International Patent Publication No. WO 85/04158 and the references cited therein. 15 The dissociated zircon product, however produced, e " may then be chemically treated to separate the zirconia from the silica. There are two basic approaches: 20 (a) Leaching with caustic soda to dissolve the silica and leave the zirconia crystallites as a relatively pure product. (b) Treating with sulphuric acid to convert the 25 zirconia to zirconium sulphate which can be separated from the unattacked silica. Japanese Patent Publication No. 23975/1969 describes a process for producing high purity zirconia which involves 30 - dissociation of zircon by heating in an electric furnace (1900-2000°C) followed by rapid quenching crushing the dissociated material to 100-300 mesh particles treating the particles with concentrated sulphuric acid at 200° - 300° for 4-5 hours then 300° - 400° for 3-4 hours leaching the thus-treated material with water to extract the zirconia as zirconium sulphate and subsequent chemical treatment to produce hydrated zirconium oxysulphate which is then pyrolyzed to give zirconia. The overall yield of the process is 80-90% based on the zirconia content of the starting ore. The need for grinding of the dissociated ore and the long acid-treatment times constitute serious economic disadvantages for this process. In international Patent Application No. PCT/AU86/00018 there is described a process based on the finding that the efficiency of. sulphuric acid leaching of dissociated zircon can be greatly enhanced if proper attention is paid to the removal from the system of the water which is produced during leaching, according to the reaction Zr0 2 .Si0 2 + 2H 2 S0 4 -> Zr(S0 4 2 + 2H 2 0+ Si0 2 In this process, water is continuously removed from the reaction system, thus maintaining constant acid concentration and temperature. As a result, leaching kinetics are considerably improved and satisfactory leaching can be achieved in 2 to 3 hours. We have now found that the sulphuric acid leaching process can be applied to the efficient and economical treatment of low-silica-containing zirconiferous materials, including low-silica dissociated zircon, particularly caustic-leached dissociated zircon. A particular advantage of the process of the present invention is that it can be operated at significantly lower temperatures than the preferred temperatures of 300 β C and higher of the process of Application No. PCT/AU86/00018, with consequent energy savings. In addition, the constraints of operating at very high temperatures with highly concentrated (98%) sulphuric acid are avoided. According to the present invention, there is provide a process for the production of high-purity zirconia from low-silica-containing zirconiferous material, which comprises the steps of leaching said low-silica-containing zirconiferous material with sulphuric acid at a temperature in the range of from 17-5 β C to 250 β C, said sulphuric acid being at a concentration such that it boils at the leaching temperature, and thereafter recovering the resulting zirconium-containing product. The step of recovery of the high-purity zirconia produced during the leaching can be performed by methods known per se. including methods described in Application No. PCT/AU86/00018. Preferably, the starting material used in the process of this invention is caustic-leached, plasma- dissociated zircon containing, for example, approximately 95%. ^ r0 2 by we 9 nt * Preferably also, the process of the present invention is carried out at a temperature in the range of from 200°C to 250°C, using sulphuric acid at a corresponding concentration of from 80% to 88% by weight. , In a first embodiment, the process of the present invention may be performed by leaching the low-silica- containing zirconiferous starting material with sulphuric acid at a temperature within the range of 175 β C to 250°C, 0 initially commencing with concentrated (98%) sulphuric acid. Water produced as the leaching proceeds dilutes the sulphuric acid until the concentration of the acid is such that it boils at the leaching temperature (for example until the concentration is 88% at a leaching temperature : of 250 β C). t In an alternative, and preferred, embodiment, the leaching process is initiated and continued using sulphuric acid having a concentration such that it boils a the process temperature. . The process of the present invention is illustrated by the following non-limiting Examples: EXAMPLE I 67g samples of caustic-leached, plasma-dissociated zircon (95% Zr0 2 ) were digested with the equivalent of 330g of 98% sulphuric acid at varying concentrations and at varying temperatures (as shown in Table 1) in a 500 cm glass reactor fitted with a glass stirrer rotating continuously at 600 rev/min, a thermo-pocket containing a thermometer (or pyrometer) and an air-cooled reflux condenser. The reactor was heated by a heating mantle. The air-cooled condenser returned the refluxing sulphuric acid to the reactor, whilst allowing water produced during the reaction to escape . In this way, the refluxing sulphuric acid solution was maintained at the desired concentration, and the desired reaction temperature was maintained during the course of the reaction. The results for the series of leaching tests are given in Table 1. They clearly show that significant leaching rates can be obtained from the low-silica- containing starting material when the leaching is carried out in accordance with the present invention. TABLE 1: Leaching of Caustic-leached, plasma- dissociated zircon (95% Zr0 2 ) . Leaching Conditions Temp. Cone.of H,S0 4 Time % of r0 2 leached (by wt.) (based on 95% Zr0 2 ) 250°C 88% 1 hr 95% 225 β C 85% 2.5hr 94% 200°C 80% 4 hr 85% 175°C 73% 6 hr 35% 150°C 65% 4 hr 3% EXAMPLE 2 To demonstrate the effect of initially leaching with 98% H 2 S0 4 at 25 ° βc ' compared with leaching with 88% H 2 SO. at 250 β C, a 67g sample of caustic-leached, plasma-dissociated zircon (95% Zr0 2 ) was digested with 330g of 98% sulphuric acid at a constant temperature of 250°C for 2 hrs as described in Example 1. The percentage of ZrO, leached was 86% (compared with 95% ZrO, leached after 1 hr using 88% H 2 S0 4 - see Table 1, Example 1) . « EXAMPLE 3 To demonstrate the effect of using plasma- dissociated zircon (67% Zr0 2 ), compared with caustic- leached, plasma-dissociated zircon (95% Zr0 2 ) for lower temperature, lower H 2 S0 4 concentration leaches, a lOOg sample of plasma-dissociated zircon (67% Zr0 2 ) was digested with 368g of 88% w/w H 2 S0 4 (the equivalent of 33Og of 98% w/w H 2 S0 4 ) at a constant temperature of 250 β C for 4 hrs as described in Example 1. The percentage of Zr0 2 leached was 4% (compared with 95% Zr02 leached after 1 hr using caustic-leached, plasma-dissociated zircon (95% Zr _) - see Table 1, Example 1). EXAMPLE 4 To demonstrate that a lower acid to zirconia ratio can be used without reducing the leach efficiency, a 150g sample of caustic-leached, plasma-dissociated zircon (95% Zr0 2 ) was digested with 445g of 85% H 2 S0 4 (385g of 98% H 2 S0 4 plus 60g of H 2 0) at 225 β C for 2.5 hours as described in Example 1. The percentage of ZrO, leached was 94%.

Claims: CLAIMS 1. A process for the production of high-purity zirconia from low-silica-containing zirconiferous material, characterised in that it comprises the steps of leaching said low-silica-containing zirconiferous material with sulphuric acid at a temperature in the range of from 175°C to 250 β C, said sulphuric acid being at the concentration which boils at the leaching temperature, and thereafter recovering the resulting zirconium-containing product. 2. A process as claimed in Claim 1, characterised in that the starting material is caustic-leached, plasma- dissociated zircon. 3. A process as claimed in Claim 1 or Claim 2, characterised in that the leaching process is initiated and continued using sulphuric acid having a concentration, such that it boils at the process temperature. 4. A process as claimed in Claim 3, characterised in that leaching is carried out at a temperature in the range of from 200°C to 250 β C, using sulphuric acid at a concentration of from 80% to 88% by weight. 5. A process as claimed in Claim 1 or Claim 2, characterised in that the low-silica-containing zirconiferous starting material is leached with sulphuric acid at a temperature within the range of 175 β C to 250°C, initially commencing with concentrated (98%) sulphuric acid, so that water produced as the leaching proceeds dilutes the sulphuric acid until the concentration of the acid is such that it boils at the leaching temperature. 6. High-purity zirconia produced from low-silica-containing zirconiferous material by leaching said low-silica-containing zirconiferous material with sulphuric acid at a temperature in the range of from 175 β C to 250°C, said sulphuric acid being at the concentration which boils at the leaching temperature, and thereafter recovering the resulting zirconium-containing product.

Inventors: HOUCHIN MARTIN RICHARD, SINHA HARI NARAYAN, HOUCHIN, MARTIN, RICHARD, SINHA, HARI, NARAYAN
Applicants: COMMW SCIENT IND RES ORG, COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH OR
Application year: 1987
Main classification: C22B3
IPC codes: C01G25, C04B35, C22B34