Leaching and Solvent Extraction at Mary Kathleen Uranium Ltd

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. The reduction in average particle size has The aqueous feed to solvent extraction not produced any significant effect on leach usually contains 0.5 g/l OsOg at a flowrate of recoveries for sizes finer than 212 micrometres. 2400 lpm. The average grind sizing is 55 percent minus 75 micrometres.

The Aus I.M.M. North West Queensland Branch, Mill Operators' Conference, June 1978 G.O. RICHMOND 289

SOLVENT EXTRACTION The proportions ii» : Mopon 7/-A cffc Honanol 7$ Sand Clarification Horar hf.-iliruT Tffo The pregnant liquor which overflows Oil No. 1 thickener passes to on unclarified The polvpnt in contacted with the pregnant liquor storage tank 7.3 m diameter x clarified pregnant 1 iquor in a gprics of mixers 4.5 n high. T!J ? A in di.imoter and 4 ra high and containing n ralxer impollfrn which art- a typo of shrouded 1.8 m deep bed of sand rotained by layers of turbino and provide both pumping: and miyinR of graded gravel. Threo colurana are unod for the phases to be contacted. ThoEo unita arc clarification whilst the fourth is cither back driven by 11.0 kW raotoro with Beovea variable washing or on standby. The liquor is aplit opeod driv<>3 connected to 20 to 1 reduction equally between the three clnrifiers and pasnoo flenrboxes. downwardn through the bed. When the differential Three streams onter oach extractor mixer ; pressure between the top and bottom of the bed 1. Interstage nolvent phase, exceeds 70 kFa the column is removed from ?. interstage aqueous phase, service for baekwashing. The column is drained, ?. recycle solvent phase. filled with water, air agitated, and backflushed Fol.lowing contact in the mixer the aqueous with water. The bed is then reconsolidated, and solvent emulsion overflown to a fibre glatM finally refilled ready for use. lined settler of 9.7 m diameter by 4.S m. The solida removed by backflushing are Isophthalic resins are used in the fibre #lasn currently sent to tailings. linings. Wooden baffles are placed in this The clarified liquor is then sent to a mixer to evenly distribute flow patterns across clarified pregnant liquor storage tank which the settler. In the settler the solvent ^n<3 is 7.3 o diameter x 4.5 m high. aqueous phases disengage and aqueous air! solvent Extraction bands emerge. Typically solvent layers of

The extraction of the uranium from the 0.4 m are present with aqueous layers of 4.0 a pregnant liquor now takes place in four mixer The separated phases then overflow internal settler units. The pregnant liquor or aqueous weirs inside the settlers. The aqueous phase phase is contacted with a solvent or organic then passes to the next mixer whilst the solvent phase which flows c.ountercurrent' to the aqueous phases flows to an external weir. Each external phase. The solvent has three components : a weir has two compartments. The first compartment tertiary amine, Adogen J64 from Ashland contains the inlet flow from the internal weir Chemicals, which has the ability to selectively and an outlet for the recycle solvent line to extract uranium from aqueous solution; Nonanol, the mixer of that same vessel. Any solvent not a long chain alcohol which is added as a required for recycle overflows a fixed weir into modifier to prevent the formation of third a second compartment where it is transferred to phase amine salts and to improve phase the next stage of the process. separation, home heating oil which is used as a All transfer lines are fitted with valves carrier for the tertiary amine. for regulation of flows. Levels inside the settler can also be

The Aus. I. MM. North West Queensland Branch, Mill Operators' Conference. June 1978 290 G.O. RICHMOND

adjusted by the change of internal weir h.-iylits. disengaging fr&ni a solvent aqueous eifiultiioii ajjd This method is very useful for the oonti'o!; of the chanctta of ejitraimaent are considerably solvent depths in the settlers. reduced. Excess aqueous phiue is returned to Sight-glasses are provided on all vessels the first extractor. to give indication of the relative height; of Strip Circuit aqueous and solvent layers. > The clean loaded solvent ia new ready for Typical flowrates are 2400 lpm of aqleoua stripping. The extraction process can now be and 400 lpm of solvent, these flows giving an reversed as impurities have now been removeJ iri effective sixfold upgrading of the uraniun the raffinate. The loaded solvent is now concentration. The loaded solvent contains contacted with a 100 - 120 g/l solution of between } - 5 g/l U,0Q. Recycle solvent flow- ammonia sulphate in a four stage counters Ji-ni.t rates are of the order of 3200 lpm. Recycle of circuit with the pH being adjusted to 4.4 i:. two solvent ia necos3&ry to improve the ph&3e stages. Thi3 otripo ur/mium from the advent and ratios of solvent to aqueous in the mixer to loads the ammonia sulphate solution to produce a greater than 1.3/1. This ratio must be main- high grade 3trip liquor which io uuitable for tained to keep the mixer operating on a rolvent direct precipitation of ammonium diuranate at

continuous phase, ie.( an emulsion of aqi.ieous pH 7.b- droplets contained in a continuous solvent The strip circuit consists of four vessels phase. If the mixer "flips" or changes .o an each of the same design as the extractor mixer aqueous continuous phase the resultant e'nilsion settlers but smaller, the settlers being 4.5 ia will be very stable and uillnot disengag'.- diameter by 4.5 m. Plow between vessels uses quickly enough in the settler. the same system of internal and external weirs

The aqueous solution which has passed with the exception that no recycle is required through four extractors is now termed rsffinate for the first strip vessel. The remaining three -• and,this liquor is pumped to the tailings dam. strip vessels have aqueous recycle to allow the Haffinate may be recycled as wash water in the mixers to operate on aqueous continuous phase. wash circuit if necessary. The raffinal e The first vessel operates organic continuous in normally contains 0.003 g/l "x^a* order to reduce organic carryover to the precipitation section whilst the other atrip vessels Each mixer is fitted with a conductivity operate aqueous continuous because this allows mixer. easier pH control and measurement. The pH of the incoming pregnant li.juor is adjusted to 1.7. Too low a pH decreases the The pH is adjusted in the second and third extraction oo-effioient of the solvent whilst stages to 4.0 and 4.4 respectively by the addit- too high a pH can cause problems with slow ion of a 5 percent aqueous ammonia solution. settling emulsions. The ammonia concentration is measured by The loaded solvent then passes to a the temperature differential between the water scrubber tank 4-5 n diameter by 4.9 m. Here and aqueous ammonia solution. the solution is contacted with a fresh-water Chemistry flow of 100 lpm and a recycle aqueous .'low Uranium can be extracted from sulphate sufficient for scrubber mixer to operate on leach liquors as a sulphate anion by high mole- aqueous continuous phase. This removes the cular weight tri-alkyl tertiary amines. majority of entrained aqueous impurities from Formation of the amine sulphate and bi-aulphate the solvent as the solvent phase is no i salts and typical extraction reactions are

The Aus i.M.M. Nonh Wei.t Queensland Branch, Mill Operators' Conference. June 1978 G.D. RICHMOND shown ns followa t definition betwoPi) tho two pliaoofi nnii by t)io

4 (OIVT) difforono'? in the lovpJs of tho two internal overflow woir3. Effective weir heights include (R

The Aus. I.M.M. North West Queensland Branch, Mill Operators' Conference. June 1978 292 CO. RICHMOND

used in uranium solvent extraction plants. Feed Typically for an aqueous flcwriitu of 2400 1pm solutions should contain lea." than 50 ppa and a solvent flowrate of 4^0 1;>IB a :;olvtnt suspended solid3 but even at this level very fine recyclt of at least 2720 lpui vould !-•• required. quantities of crud will form with corresponding When the feed solution contains fine extra losses of solvent. suspended solid3 or high silica concentrations The rate of crud formation ia considerably the organic to aqueous ratio in the extract JI-B increased if certain impurities are present in should be increased to as high ae 2 to 1 to theliquor. At Mary Kathleen the most trouble- prevent the mixer from flipping to the wron^ some impurity ia dissolved silica and concen- phase. When a mixer flips after .» period ut trations of 3 &A normally lead to massive crud steady operation this usually indicates a fortoatlono. Under normal conditions dissolved deterioration in the quality of tho feed liquor. silica levels rarely exoeoci 1 g/l but when Other factors to affect crud formation are overloading occurs silica levols rapidly mixer speeds and high pi! levels. High mixer increase and the problem is complicated by the speeds can cause inereaa<.

The Aus. I.M.M. Noith West Queensland Branch, Mill Operators' Conference, June 1978 G.D. RICHMOND 293

TiUo procedure requires the crud to pans through hno bocorao nocfnr irv to koej1 'i clor.or control ."!i rainy ne four mtxero with tho résultant over certain vari'ibn1" to mnintain maxinxm formation of more crud on each pnop. plant orfioiency. In tho loarhinp »action th""1 emphasis in on holding correct pH and rodox PRECIPITATION values. In the solvent extraction mctjon it The aqueous Hquor overflowing tho firot is important to receive clear feed liquors, 1.o strip veonol is termed high grade strip liquor maintain correct phase continuity and to hold and io pumped directly to a. two atago correct pK rrvidingn in the rstrip circuit. precipitation circuit, Anhydroun ammonia is added to thene VOSGOIB to adjust the pH to 7.0 ACKHOWUiDOEHBHT and7.8 in the firct and aooond otngo reopeotively, The author would like to thos I: î-îary With the inorenno In pH tho uranium in oolution Kathleen tlraniuti Limitof] for peraiicr.Ion t.n prooipltntca nn amnoniura dluronnto (JHl )?.Vs0 , A 7 publinh thin paper.

early teothins problems tho leachins, liquid-solid aoparation and solvent Morritt, B.C., 1971 The Extr-ictivo extraction sections are now operating quite Metallurgy of Uranium (Colorado Sciioo] of aatisfactorily. With increased throughput it Minos Research Institute : Colorado).

The Aus. I.M. M. North West Queensland Branch. Mill Operators' Coherence, June 1978