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Leaching and Solvent Extraction at Mary Kathleen Uranium Ltd

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Leaching and Solvent Extraction at Mary Kathleen Uranium Ltd <f LEACHING AND SOLVENT EXTRACTION AT MARY KATHLEEN URANIUM LTD. By G. D. RICHMOND1 ABSTRACT nignificant ehJUitfon having occurred in f.n« Mary Kathleon Uranium Ltd. recoiHiioncod uranium extraction aoctionr,. Thono include : opomtionp in early 1976 following n twolvo tho uoo of a parallel cyclono waoh circuit for year period of care and maintenance. Several ooarno nolidn and n thickener waoh circuit, for sections of tho plant were modified or fine nolidn in the liquid-aolid eoparation completely changed for the second operation. section} tho roplacemor.' of the pregnant liquor The most important change was the replacement pre-coat filters with four 3.md clarifitrs; the of ion exchange with solvent extraction as the replacement of the ion exchange process with a means of purifying and upgrading uranium rich solvent extraction section; the use of anhydrous solutions. ammonia in the precipitation section in place of Most of the problems experienced in the caustic magnesia. solvent extraction system originate from the A flowsheet of the extraction section of leach liquor which has a strong tendency to Mary Kathleen is shown in Figure 1. fora stable emulsions. This has been countered by scne careful control of leaching conditions LEACHIKS and by closer observation of operations in the The leaching section consists of eight solvent extraction area. Host problems have 3.6 o diameter by 6.1 m deep vessels arranged in now been resolved and plant recoveries are quite two sets of parallel banks of four vessels in satisfactory. series. Feed to the leaching section is dirided equally between the two banks in a siaple pulp IHTRODWCTIOH splitter box. There is provision to add water Mary Kathleen Uranium Ltd. first commenced into this box to control leach temperature. Pulp the mining and milling of uranium ores in 1515a. flows from one vessel to the next via an overflow This operation ran successfully until 1963 uhen spout directly opposite the incoming feed. By- the lack of sales contracts forced the closure pass launders axe available so any vessel may be of the mine. The treatment plant was kept on emptied for maintenance. Each vessel has a a c*ire and maintenance basis until the end of conical bottom fitted with a dump valvo. 1974 when the recoamissioning commenced. Each vessel is agitated by two stage mixer Several alterations were made to the flowsheet turbine agitators driven by 57-3 kW motors and its various itemg of equipment before the through Palk triple reduction gear boxes. re-opening of the treatment plant, the most All components in contact with the leach pulp are rubber lined to resist abrasion and corrosion. PLANT MSTALLUBGIST Sulphuric acid, produced in another station MARY KATHLEBH URAHIBM LTD. of the treatment plan* , is added at *)&.'. percent The Aus. I.M.M. North West Queensland Branch, Mill Operators' Conference. June 1978 Ordinal US received at The Institute 15.5.78 ?8<5 266 G.O. RICHMOND FLOWSHEET FOR LEACHING, LS.S. AND SOLVENT EXTRACTION SECTIONS PULP FROM GRINDING —-a THICKENER^CYCLONE WASH CIRCUIT TO TAILING JUNCLAR1FIED PREGNANT LIQUOR TANK DAM SOLVENT EXTRACTION JSAND CLARIFICATION FIG.1- ORGANIC AaCLARIFIED PREGNANT V-^LLIQUOI R TANK •AQUEOUS MIXER EXTRACTORS «• BARREN SOLVENT ^-RAFFINATE TO I 1 I 1 j TAILING DAM .OADED SOLVENT —•-FRESH WATER «..~fSCRUBBER STRIPPERS AMMONIUM SULPHATE STRIP SOLATION ^-BARREN SaVENT \ ^ i UaO«TO WASHING PRECIPITATION DRYING AND DRUMMING Tft* Au«. I.M.M. North W«tt QuMniiand Branch. Mill Operators" Confaranca, June 1978 G.O. RICHMOND 287 to each voasol through automatic valvon OPEBATIOHAL i fitted with VKon diaphragms. Each automatic Several variabl'-n control the rate of valve has a m;»nual hy-pans system. Acid i3 leadline: and the ultimate loacli recover ion. added to the pulp by simple drop pipoa Redox potential extending one metre into the pulp. Redox Tho control of an adequate redox potential potential in the leach pulp in maintained by ia the key to maximum leach recoveries. Valurn the addition of pyrolunite (containing 50-54 abovo 400 mV mint be held otherwise a oubjjtan- percent Mnft-) in the grinding circuit. tial decrenoo in loaching rate w'll be Instruments in the leaching section oxporioneed. Data collected oo far Indicnted ,' currently meaauro pH in each venael nnd redox that there is little benefit in leach efficiency in the nooond and third vessels of both bnnkn. in incrnfining the redox potential abovo 5"0 oV. Frovigion hao been fflndo for the incluoion In fact hlRh valuon havo a marked of two extra lonchlng vessels one for each bank. dotriraontal effect nn thoy cauno ovorleachinff. These voaooln nro larger being 4.6 m diameter Thin produceo on oxnenn of olino difficult to and 5.2 m deep and aro flat bottomed veoools. handle in llquid-nolid separation and high levels They will become the final leach vessel in eaoh of dinoolved impuritien, noticeably in nilici, bank. potentially harmful to solvent extraction. The correct addition of pyrolusite hao been CHEMSTBY difficult to maintain due to long lag times Uranium is present in the ore in the between addition of pyrolunite and the measure- mineral uraninite finely disseminated in ment of redox potential. Also the control of allanite. The uraniuo ia present partly in the feedrate has been a problem as the particle size tetravalent state and this must be oxidised to is large and feedrates are low, the hexavalent state before dissolution occurs. £H Pyrolusite (approximately 50-54 percent MnOb) The leaching pH is controlled by the is added to the leach vessels for this purpose. addition of 98.5 percent sulphuric acid. The The pyrolusite upon dissolution in the leach pH is reduced stagewise to restrict acid kiquor, oxidises iron present, in solution, consumption and tho eet points currently used either dissolved from the ore or introduced by are 2.2, 2.0, 1.8 and 1.8 for the four vessels wear of grinding media, to produce ferric ions. respectively. Lowering set points below these This in turn oxidises the uranium from the values does not appear to improve leaching rate tetravalent to the soluble hexavalent state. recovery and is detrltnetal in that it increases The ferric ions are reduced to ferrous ions in aoid consumption and generates over-leached fine this reaction. material. Briefly the chemical reactions are : The redox measurement of each vessel is pB Fe + HsSO,-* Fe2* + StfT + H2 dependent. If the pH rises above the set point • the redox value decreases, probably due to a + ttiCb 2Fe* reduction in the dissolution rate of the 2H20 pyrolusite. Hegular cleaning of pE probe* and acid drop WCb (uranyl ion) lines is required because of rapid deposition of calcium sulphate. A more complete description of this process can be found in Merritt (1971). The Aus. I.M.M. North West Queansland Branch. Mill Operators' Conference. June 1976 288 G.D. RICHMOND Pulp Penalty LIQPID-SOLID SEPABATIOM The discharge density of the pulp from Liquid solid separation is carried out the grinding circuit ia controlled to about using- a five 3ta#e countercurrent washing 50 percent solids. For higher densities, cyclone circuit operating in parallel with a five temperatures become excessive and overleaching stage countercurrent thickener circuit. An occurs. Also, as densities approach 60 percent initial split of fine and coarse particles is solids reaction rates decrease with increasing made in the No. 1 cyclone and fine particles pulp viscosity. then pass through the thickener circuit whilst Low densities are detrimental because of the coarno particles are washed in the remaining- 1 • shorter particle rep dence time in the cyclones. Wash water enters the circuit tluough leach vessels; Ho. 5 cyclone feed and then pa33e& alternatively 2. a reduction in leach rate due to lower between cyclone and thickener in decreasing leach temperatures 1 tuid order. The overflow from No. 1 thickener passes 3. a low density feed to the liquid-solid to the solvent extraction section for separation circuit reduces the washing clarification. The wash water is acidified to efficiency. pH 2.0 in order to improve settling properties Temperature of the fine material. Raffinate from the solvent In the leach vessels, heat is released by extraction section can be used for wash water. the dilution of concentrated sulphuric acid in Its use is restricted because it contains Suum water. According to the quantity of water, solvent carry-over which adversely affects any present, or pulp density the temperature may natural rubber lined components of the circuit. range from near ambient upwards to as high as Also some impurities such as phosphate and silica 90°C. In practice the temperature is controlled in solution can cause problems in solvent in the range 45 - 60°C by water addition to the extraction if they are continuously recycled. pulp. This avoids the problems of overleaching Pulp densities of the thickener underflows whilst maintaining the leaching rate as high as are maintained batween 50-55 percent solids and possible. of the cyclones underflows between 65 - 70 Beeidence Time percent solid*. Residence time in the leach vessels is All thickeners, cyclones and auxiliary determined by the pulp density and mill tonnages. components are rubber lined. Typically for a 70 tph mill tonnage producing a Thickener underflow pulp is transferred §0 percent solids pulp the residence time in with VMR 6 Dorrco diaphragm pumpa with a Warman the leach vessels is four hours. Provided redox repulper between each stage. values are sufficient, ie., 400 mV and pH values Each cyclone is fed by a 6/4 Warman pump. are correct almost total leaching can occur The underflow from No. 5 cyclone and No. 5 within one hour. thickener are combined and pumped to the tailings Particle Size dam.
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