Characterisation of the uranium leaching behaviour of low grade Vaal River ores

Lottering, Maria Johanna (2007-12)

Thesis (MScIng)--Stellenbosch University, 2007.

Some digitised pages may appear illegible due to the condition of the original hard copy.


ENGLISH ABSTRACT: The uranium leaching behaviour of ore from three different mines in the Vaal River region, namely Kopanang, Great Noligwa and Moab Khotsong, was investigated. The aim of the work was to characterise the uranium leaching behaviour of the different ores. It involved a full mineralogical evaluation of the ore as well as determining optimum operating conditions for maximum extraction of uranium, relating the extractions to mineralogical features of the ores. The major bulk minerals in the three Vaal River ores have been identified and consist primarily of quartz (70-80 %), with lesser amount~ of muscovite (8-11%). Moab Khotsong and Noligwa ore samples were found to be very similar, and Kopanang is different from the two ores. Kopanang ore has less pyrite, quartz, and chlorite than the other two ores, but contains more pyrophyllite. Chlorite is an acid consumer and a Fe2+/Fe3+ producer in uranium leaching. The difference in mineralogy of the three ores definitely reflected in the reagent profiles during acid leaching of uranium, specifically acid consumption. With regards to uranium occurrence, bulk uranium analysis showed that 80-90 % of the uranium in the ores is contained as uraninite, 8-19 % as brannerite, and the balance as traces of coffinite and uranium phosphates. Uranium grain sizes were found to be very small, with 50 % of the particles passing 19.4, 21.3 and 23.2 IJm for Kopanang, Noligwa and Moab Khotsong respectively. The degree of liberation of the uranium-bearing minerals was low, between 11 and 45 %, and expectedly increased as particle size decreased. However, between 87 and 93 % of the uraninite particles and 71 to 86 % of the brannerite particles have more than 10 % of their surfaces exposed, and even higher proportions have more than 5 % of their surfaces exposed. The primary experiments were designed within practically attainable boundaries, although special experiments were performed outside the boundaries during subsequent mechanistic studies. H2S04 addition varied between 10 and 25 kg/t, temperature varied between 40 and 60°C, and Mn02 addition varied between 2 and 4 kg/t (100 percent Mn02). Mn02 was added as pyrolusite, 1.5 hours after addition of acid. A relative pulp density of 1.55 was used. Leaching times of 24 and 48 hours were investigated. The ore was milled to 80 % -75 IJm, even though coarser grinding was also investigated. It was found that 60-90 % dissolution could be achieved as leaching conditions were varied within the operating window. Acid addition had the greatest influence on final uranium extractions. Practically useful results, within the operating window attainable in a plant, can be summarised as follows: • Uranium dissolutions are 80-85 % for Great Noligwa and Moab Khotsong ores, and 85-90 % for Kopanang ore • Required residence time = 24 hrs • To achieve the above dissolutions, Kopanang ore requires at least 11 kg/t acid, while acid requirements for Great Noligwa and Moab Khotsong ores are 14 and 16 kg/t respectively • MnOz addition can be kept at a minimum, because sufficient Fe can be leached from the ore (meaning only enough MnOz for conversion of ferrous to ferric needs to be added) • The chlorite dissolution reaction consumes acid, generates ferric and ferrous ions, but also generates dissolved silica. Therefore, the kinetics of this reaction must be understood in order to control it to the lowest extent possible, without impairing the availability of dissolved iron. An understanding of the factors responsible for the leaching behaviour of uranium-bearing ores is critical in achieving optimal uranium recoveries. This is particularly important in light of the fact that dissolutions higher than 90 % are very difficult to achieve under the normal operating conditions employed on the South African acid leaching plants. While solubility limitations were initially suspected, it was found that the reason for the existence of the upper limit for dissolution was most likely due to the presence of aerially locked uranium minerals (uranium minerals that is not exposed to the leaching environment). However, residue analysis showed that most of the uraninite dissolved and that the major fraction of unleached uranium existed as brannerite which is in fact exposed to the leaching environment. Therefore, the slow leaching kinetics / intrinsic inertness of brannerite was considered as the limiting factor for not achieving optimum recoveries. To achieve optimal extraction, considering the mineralogical characteristics of the ores (in terms of a plant's flow/operational perspective), a diagnostic leaching approach was followed. A mineralogyleachability explanation is presented to rationalise the difficulty in exceeding 90 % dissolution from low grade uranium ores on the basis of a novel diagnostic leaching method. More specifically, to determine the interrelationship between mineralogy, mineral liberation and the leaching behaviour of uranium, a methodology was developed for unlocking uranium by a combination of chemical (drastic leaching of minerals associated with the residual uranium) and physical (fine grinding to increase area exposure and liberation) methods. Diagnostic leaching results indicated that to improve uranium dissolution beyond 90 %, uneconomical conditions (residence time between 48 and 72 hours, constant pH = 1 or Eh = 700 mV) need to be considered and it will still not necessarily be possible to increase beyond 95%. The maximum obtained is 98% using nitric acid digestion at evaluated temperatures (900C). Mineralogical analysis indication that it is possible to leaching brannerite but, the leaching kinetics thereof is very slow. Based on the diagnostic leaching tests the following is recommended for the tree ores tested: Sulphuric acid leaching must be used for treating Kopanang ore. If the brannerite concentration of Noligwa and Moab Khotsong ore is < 20 % sulphuric acid leaching is recommended but if the brannerite concentration> 20 % other leaching methods will be recommended (Le. pressure leaching or using a different leaching reagent). A non-linear decision tree model was developed for modelling of the experimental data and is presented in this thesis. In this context it would be more realistic to determine a range for expected recovery rather than trying to determine an exact value. Therefore, a classification tree model was used. Using this method, just evaluating the tree shows that: For uranium dissolution higher than 70% a residence time longer that 17 hours is required including an acid addition higher that 11.35 kg/t for Noligwa and Moab Khotsong ore while lower acid concentration can be tolerated for Kopanang ore. It is proven that that model can with 86% accuracy separate that into the various classes. Through cross validation it is also proven to be a representative model with an average success rate of 84 % for classification of data with a standard deviation of 2.8%. This model can be used to predict the expected dissolution range based only on the operating parameters within the Vaal River context. Since the model is based on laboratory experimental data it can not necessarily be used for plant operation/optimisation purposes. It is recommended to develop an equivalent model using pilot plant data to develop a more accurate model which can also be used for optimisation purposes. Although it is not part of the original scope of the project (based on the information gained though out the project), a proposal of an empirical model simulator is also presented. Since development of the simulator is still in the developing stages, this thesis will only include the algorithm as well as a basic model predictor which can be used for future research. The ideal is to develop a simulator has the ability to predict uranium leaching behaviour and reagent consumption based on mineralogy. Gold extractions obtained by forward leaching (direct cyanide leaching of gold) were compared with those obtained by reverse leaching (sulphuric acid leaching of uranium followed by cyanide leaching of gold) for three different Vaal River ores, to quantify the benefits of reverse leaching option. Reverse leaching of gold recovery by between 3 and 4 percentage points, improving total gold recovery to 98 %. A gold benefit of between 0.4 and 0.6 g/t was measured. The exact financial gain is dependant on the gold price and other economic factors, but an estimated benefit for treating an average of 240000 t ore/month is in the order of R 14000 OOO/month. The reverse leaching operation for the recovery of both gold and uranium is therefore a financially justified process route, because recovery of uranium will ensure that the costs are lower than the revenue increment.

AFRIKAANSE OPSOMMING: Die logingsgedrag van uraan erts vanaf drie verskillende myne in the Vaalrivier omgewing, naamlik Kopanang, Noligwa en Moab Khotsong is ondersoek. Die doel van die studie is die karakteriseering van die uraan logingsgedrag en behels 'n indiepte mineralogiese evalueering van die verskillende ertse, die bepaling van die optimale bedryfskondisies vir die ekstraksie van uraan as ook die bepaling van 'n verband tussen ekstraksie en mineralogiese eienskappe van die erts. Die grootmaat minerale van die drie Vaalrivier ertse is geidentifiseer en bestaan hoofsaaklik uit kwarts (70 - 80 %), met 'n laer konsentrasie muskoviet (8 - 11 %). Daar is gevind dat die Moab Khotsong en Noligwa erts monsters tot 'n groot mate ooreenstem, terwyl Kopanang erts daarvan verskil. Kopanang erts bevat minder piriet, kwarts en chloriet maar meer pirofilliet in vergelyking met die ander twee ertse. Chloriet is 'n suur verbruiker en 'n Fe2+/Fe3 + verskaffer in uraan logings prosesse. Dit blyk uit die resulte dat die verskil in die mineralogie van die drie ertse definitief reflekteer op die reagense profiele tydens suur loging van uraan, veral suur verbruik. Die grootmaatanalise, in terme van die aanwesigheid van uraan, toon dat 80 - 90 % van die uraan in die erts voorkom as uraniniet, 8 - 19 % as branneriet en die balans kom voor as koffiniet en uraanfosfate. Daar is gevind dat die uraan korrelgrootte baie klein is, met 50 % van die partikels kleiner as 19.4, 21.3 en 23.2 IJm vir Kopanang, Noligwa en Moab Khotsong respektiewelik. Die graad van bevryding van die uraan bevattende minerale was laag (tussen 11 en 45 %) en neem na verwagting toe soos partikel grootte afneem. Nietemin, tussen 87 en 93 % van die uraniniet partikels en 71 _ 86 % van die branneriet partikels het meer as 10 % van die mineraal oppervlakte blootgestel aan die logings omgewing met selfs hoer proporsies met meer as 5 % blootgestelde oppervlak area. Die primere eksperimente was ontwerp binne die praktiese grense haalbaar op 'n aanleg. Spesiale eksperimente was wei ook uitgevoer buite die grense, vir opeenvolgende meganistiese studies. H 2 S0 4 byvoeging was gevarieer tussen 10 en 25 kg/t, temperatuur gevarieer tussen 40 en 60°C en Mn02 byvoeging is gevarieer tussen 2 en 4 kg/t (100 persent Mn02)' Mn02 is toegevoeg as pirolusiet 1.5 uur na die toevoeging van die suur. 'n Relatiewe pulp digtheid van 1.55 was gebruik. 'n Loging residensie tyd van 24 en 48 uur was ondersoek. Die erts was gemaal tot 80 % -75 IJm alhoewel growwer maling wei ook ondersoek is. Daar is gevind dat 60 - 90 % oplossing wei bereikbaar is binne die bedryfsgense. Dit blyk dat die suur byvoeging die grootste invloed het op die finale uraan ekstraksie. Praktiese nuttige informasie, binne die bedryfsgense van 'n aanleg, kan as volg opgesom word: • Uraan oplossings tussen 80 - 85 % vir Great Noligwa en Moab Khotsong erts en tussen 85 - 90 % vir Kopanang erts. • Vereiste residensietyd = 24 uur • Om begenoemde oplossings te bereik, benodig Kopanang erts ten minste 11 kg/t suur, terwyl die suur vereistes vir Noligwa en Moab Khotsong erts 14 en 16 kg/t is onderskeidelik • Mn02 toevoeging kan tot 'n minimum beperk word aangesien voldoende Fe geloog word vanaf die erts (dus sleg genoeg Mn02 vir die omskakeling van Fe2+ na Fe3+ word benodig) • Die chloriet oplossings reaksie verbruik suur en genereer Fe3 + en Fe2+ as ook silika in oplossing. Dit is dus belangrik om die kinetika van die reaksie te verstaan om sodoende die reaksie tot 'n groot mate te onderdruk sonder om die beskikbare yster in oplossing te beinvloed 'n Deeglike begrip van die faktore verantwoordelik vir die logingskarakteristieke van uraan bevattende ertse is krities om 'n optimale uraan opbrengs te bereik. Dit is veral belangrik aangesien oplossings hoer as 90 % moeilik is om te bereik onder die normale beheer kondisies wat gebruik word op Suid Afrikaanse suurlogingsaanlegte. Terwyl oplosbaarheidsbeperkings oorspronklik verdink was, is daar gevind dat die bestaan van 'n hoer limiet vir uraan oplosbaarheid waarskynlik 'n gevolg is van die teenwoordigheid van area geslote uraan minerale (uraan minerale wat nie aan die logings area blootgestel is nie). Nietemin, residu analiese toon dat die meeste van die uraniniet opgelos het en dat die grootste fraksie van ongeloogde uraan voorkom as brannerite, wat wei bloot gestel is aan die logings omgewing. Gebaseer op die bevindings word die stadige loging kinetika / intrisieke traagheid van brannerite gereken as die beperkende faktor vir optimum uraan herwinnig. Om optimale ekstraksie te bereik, gelet op die mineralogiese karakteristieke van die erts (in terme van 'n aanleg se vloei / beheer perspektief), was 'n diagnostieke logingsbenadering gevolg. 'n Mineralogiese-uitloging verduideliking word bespreek om die beperking van oplossings hoer as 90 % te bespreek vir lae graad uraan ertse wat gebaseer is op 'n stap vir stap diagnostiese metode. Meer spesifiek, om die interaksie tussen die mineralogie, mineraal bevryding en logingsgedrag van uraan te bepaal is 'n metodologie ontwikkel om uraan minerale te bevry deur 'n kombinasie van chemise (drastiese loging van minerale geassosieer met die residu uraan) en fisiese (fyn maling om die area van blootstelling en bevryding van uraan minerale te vergroot) metodes. Die diagnostiese resultate wys daarop dat om uraan oplossing te verhoog bo 90 %, onekonomiese kondisies (residensie tyd tussen 48 en 72 uur, konstante pH =1 of Eh = 700 mV) oorweeg moet word en dit nie noodwendig oplossings hoer as 95 % bewerkstellig nie. Die maksimum ekstrasie wat bereik was is 98 % tydens salpetersuur vertering by hoe temperature (90°C). Mineralogiese analieses wys daarop dat dit wei moontlik is om branneriet te loog maar dat die loging kinetika baie stadig is. Gebaseer op die diagnostieke logings resultate word die volgende aanbeveel: Swawelsuur loging vir die behandeling van Kopanang erts. Indien die konsentrasie van branneriet in Noligwa and Moab Khotsong erts laer is as 20 % sal swawelsuur loging aanbeveel word maar indien dit verhoog (>20 %) moet ander logings tegnieke oorweeg word (bv. drukloging of die gebruik van ander logings reagense) 'n Nie linieere besluit boom model was ontwikkel vir die modelering van die eksperimentele data en word weergegee in die tesis. Aangesien daar meer waarde Ie in die bepaling van 'n verwagte uraan oplossings in 'n bepaalde gebied, eerder as om die presiese waarde te probeer bepaal is dit as 'n klassifikasie model hanteer. Deur hierdie metode te volg kan daar, gebaseer op die ontwikkelde boom die volgende afleidings gemaak word: Om oplossing bo 70% te bereik is 'n residensie tyd van langer as 17 uur benodig as ook 'n suur toevoeging van meer as 11.35 kg/t vir Noligwa en moab Khotsong erts terwyl minder suur benodig word vir Kopanang erts. Daar is getoon dat die model met 86 % akkuraatheid die data kan skei in die verskillende klasse. Kruis valideering van die model toon dat dit verteenwoordigend is en gemiddeld 84 % sukses behaal in die klassifiseering van data met 'n standard afwyking van 2.8 %. Hierdie model kan gebruik word om die verwagte oplossings gebied te voorspel gebaseer op die beheer veranderlikes binne the Vaalriver konteks. Aangesien die model gebaseer is op laboratorium gegenereerde data kan dit nie noodwendig gebruik word vir aanleg beheer doeleindes of optimiseering nie. Daar word dus aanbeveel om dieselfde model te ontwikkel vir proef aanleg data om 'n meer akkurate model te ontwikkel wat gebruik kan word vir optimiseerings doeleindes. Alhoewel dit nie deel was van die oorspronklike omvang van die projek nie (gebaseer op die informasie versamel deur die verloop van die projek) word 'n voorstel van 'n empiriese model simulator bespreek in die tesis. Aangesien die ontwikkeling van die simulator nog in die ontwikkelings fase is sal die tesis slegs die algoritme as ook 'n basiese model voorspeller simulator insluit wat vir toekomstige navorsing gebruik kan word. Die ideaal is om 'n simulator te ontwikkel wat die vermoee het om die uraan logingsgedrag te voorspel as ook die reagense verbruik gebaseer alleenlik op mineralogie. Goud ekstraksies verkrygbaar deur voorwaartse loging (direkte sian ide loging van goud) is vergelyk met ekstraksies verkry deur terugwaartse loging (swawelsuur loging van uraan gevolg deur sianide loging van goud) vir die drie verskillende Vaal rivier ertse. Die hoof doel is die kwantifiseering van die voordele ten opsigte van terugwaartse loging. Terugwaartse loging van gold verhoog die goud opbrengs tussen 3 en 4 persentasie punte wat lei tot 'n totale gold herwinning van 98 %. 'n Goud wins van tussen 0.4 en 0.6 g/t is bepaal. Die presiese finansieele wins is sterk afhanklik van die goud prys en ander ekonomieses faktore, maar 'n geskatte wins vir behandeling van gemiddeld 240000 t erts/maand is in die orde van R 14000 OOO/maand. Daarom is die terugwaartse logingsmetode vir die herwinnig van beide goud en uraan 'n finansieel geregverdigde proses roete.

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