Thesis (PhD (Process Engineering))--University of Stellenbosch, 2010. / ENGLISH ABSTRACT: Previous work has shown that microwave heating of mineral ores induces fractures around grain boundaries due to the differences in absorption of microwaves and the resulting differential thermal expansion among the various mineral phases in the ore particles. As a consequence, this reduces the energy required in subsequent grinding and enhances liberation of valuable minerals. In this study, first, the influences of different variables on bulk strength reduction of microwave treated ores have been investigated. Nine different binary ore models were constructed by randomly disseminating 10 vol.% microwave absorbing minerals in transparent matrices. Computational simulations of heating, thermal damage and unconfined compressive strength (UCS) tests on the conceptual binary ores have been undertaken by using finite-difference modelling techniques. The influence of thermo-mechanical properties of minerals on strength reduction of microwave treated ores was examined. It was shown that in general the thermal properties of the microwave absorbing mineral and the mechanical properties of the transparent matrix have the most significant effect on the strength reduction. Binary ores containing a microwave absorbing mineral that has a high thermal expansion coefficient in a strong transparent matrix achieved higher reductions in strength. The influence of absorbent phase grain size on strength reduction of ores was also quantified. It was shown that for the same energy inputs and mineral types, the reductions in strength were much higher in coarse-grained ores. It has also been shown that for the same mineralogy and treatment condition, ores with poorly disseminated heated phase achieved much higher strength reduction. The effect of microwave treatment on the mechanical state of an ore sample was also examined. It was demonstrated that unconfined compressive strength is less sensitive to microwave-induced micro-fractures and found to be a poor descriptor of liberation behaviour. A new method of characterizing damage in microwave treated ore using a continuum approach was developed. The method measures the damage around the grain boundary regions during the heating process. Using the method, it was possible to elucidate in detail the influences of power density, mineralogy, ore texture on microwave treatment of ore. It was shown that the amount of grain boundary damage incurred at a specific power density and energy input is dependent both on the ore mineralogy and its texture. The energy inputs that were required for significant (> 10%) grain boundary damage in the ores range from 0.09 to 7.06 kWh/t depending on the power density applied, the ore mineralogy and its texture. It was also shown that for a given mineralogy and ore texture there is a power density level below which no further increase in grain boundary damage is possible by increasing exposure time. The effect of pulse repetition frequency on grain boundary damage was also elucidated using the method. It was found that high pulse repetition frequencies (³ 50 Hz) resulted in an amount of grain boundary damage that was indistinguishable from that caused by continuous wave operation for a fixed energy input. It has also been shown that for a fixed microwave energy input the best result would be obtained by using the lowest possible pulse repetition frequency and highest peak pulse power. The effect of microwave treatment of ores at different treatment conditions on the extent of damage and crack pattern was also investigated in detail using bondedparticle model (BPM). It has been shown that the amount of micro-cracks and also the cracks pattern in an ore sample after microwave treatment significantly depend on its mineralogy, microwave treatment condition (power density) and absorbent phase grain size. It has also been shown that a minimum power density is required to localize damage around the grain boundary in an ore sample. This minimum power density was found to strongly depend on the ore mineralogy and its texture. Initial simulation test work concerning the effect of microwave treatment on liberation of minerals is also presented. It has been shown that microwave irradiation considerably changed the fracture pattern of an ore in simulated single particle crushing. The fracture pattern of the ore treated at high power density (Pd = 0.1 kW /mm3abs for 1 ms) was along the grain boundary and the absorbent mineral was intact. In the ore treated at lower power density for the same energy input (Pd = 1 W/mm3abs for 0.1 s) both intergranular and transgranular fractures were observed. However, in all cases the fracture patterns were preferentially localized around the grain boundary compared to that of the untreated ore. / AFRIKAANSE OPSOMMING: Vorige studies het getoon dat mikrogolfverhitting van mineraalertse tot breuke om die ertskorrelgrense aanleiding gee, omdat die verskillende mineraalfases in die ertsdeeltjies die mikrogolwe verskillend absorbeer, en dus ook verskillend uitsit. Korrelgrensbreuke verminder die vereiste energie vir latere slypwerk, en verhoog die vrystelling van waardevolle minerale. Hierdie studie het eerstens die uitwerking van verskillende veranderlikes op die algehele sterktevermindering van mikrogolfbehandelde ertse ondersoek. Hiervoor is nege verskillende binêre ertsmodelle vervaardig deur mikrogolfabsorberende minerale met ʼn volumepersentasie van 10% lukraak in deursigtige matrikse te versprei. Met behulp van eindigeverskilmodelleringstegnieke is berekeningsimulasies van verhitting, warmteskade en onbegrensde druksterkte (“unconfined compressive strength”) op die konseptuele binêre ertse uitgevoer. Die invloed van termomeganiese mineraaleienskappe op die sterktevermindering van mikrogolfbehandelde ertse is eerste onder die loep geneem. Daar is bevind dat die warmte-eienskappe van die mikrogolfabsorberende mineraal, en die meganiese eienskappe van die deursigtige matriks, die beduidendste uitwerking op sterktevermindering het. Binêre ertse wat ʼn mikrogolfabsorberende mineraal bevat met ʼn hoë warmte-uitsettingskoëffisiënt in ʼn sterk deursigtige matriks, het groter sterkteverminderings getoon. Die invloed van korrelgrootte in die absorbeerfase op die sterktevermindering van ertse is volgende versyfer. Die studie het getoon dat, op grond van dieselfde energie-insette en mineraalsoorte, grofkorrelrige ertse groter sterktevermindering ondergaan het. Eweneens is bewys dat, met dieselfde mineralogie en behandelingsomstandighede, ertse met ʼn swak verspreide verhittingsfase ook groter sterktevermindering ervaar. Die uitwerking van mikrogolfbehandeling op die meganiese toestand van ʼn ertsmonster is boonop ondersoek. Die studie het getoon dat onbegrensde druksterkte minder gevoelig vir mikrogolfgeïnduseerde mikrobreuke is, en as ʼn swak aanwyser van vrystellingsgedrag beskou word. ʼn Nuwe metode om skade by mikrogolfbehandelde ertse te tipeer is gevolglik met behulp van ʼn kontinuumbenadering ontwikkel. Dié metode meet die skade rondom die korrelgrens gedurende die verhittingsproses. Deur middel van voormelde metode was dit dus moontlik om die invloed van kragdigtheid, mineralogie en ertstekstuur op die mikrogolfbehandeling van erts deeglik te ondersoek. Daar is bevind dat die mate van korrelgrensskade by ʼn bepaalde kragdigtheid en energie-inset, van sowel die ertsmineralogie as ertstekstuur afhang. Na gelang van die toegepaste kragdigtheid, die ertsmineralogie en ertstekstuur, het die vereiste energie-insette vir beduidende (>10%) korrelgrensskade van 0,09 tot 7,06 kWh/t gewissel. Dit het voorts geblyk dat enige bepaalde mineralogie en ertstekstuur oor ʼn minimum kragdigtheidsvlak beskik, onder welke vlak geen verlenging in blootstellingstyd enige verdere korrelgrensskade kan veroorsaak nie. Die uitwerking van pulsherhaalfrekwensie op korrelgrensskade is ook met behulp van bogenoemde metode verklaar. Die studie het getoon dat, op grond van ʼn vaste energie-inset, hoë pulsherhaalfrekwensies (≥50 Hz) en gelykgolfwerking presies dieselfde hoeveelheid korrelgrensskade tot gevolg het. Volgende is daar met behulp van ʼn gebondedeeltjiemodel (“bonded-particle model”) noukeurig ondersoek ingestel na die uitwerking van verskillende mikrogolfbehandelingsomstandighede op die hoeveelheid skade en die kraakpatroon by ertse. Die studie het getoon dat die hoeveelheid mikrokrake sowel as die kraakpatroon in ʼn mikrogolfbehandelde ertsmonster in ʼn groot mate van die betrokke erts se mineralogie, mikrogolfbehandelingsomstandighede (kragdigtheid) en korrelgrootte in die absorbeerfase afhang. Daar is ook bevind dat ʼn minimum kragdigtheid nodig is om skade tot die gebied om die korrelgrens te beperk, welke minimum kragdigtheid oënskynlik grotendeels deur die ertsmineralogie en -tekstuur bepaal word. Die studie bevat ook die resultate van aanvangsimulasietoetse oor die uitwerking van mikrogolfbehandeling op mineraalvrystelling. Die toetse het getoon dat mikrogolfbestraling ʼn beduidende verandering tot gevolg het in die ertsbreekpatroon met gesimuleerde enkeldeeltjievergruising. Die breekpatroon van die erts wat by hoë kragdigtheid (Pd = 0,1 kW/mm3abs vir 1 ms) behandel is, het ál langs die korrelgrens gestrek, terwyl die absorberende mineraal nog ongeskonde was. In die erts wat by laer kragdigtheid dog dieselfde energie-inset behandel is (Pd = 1 W/mm3abs vir 0,1 s), is sowel tussenkorrel- as oorkorrelbreuke opgemerk. In teenstelling met die onbehandelde erts, was die breekpatrone by die behandelde erts egter in alle gevalle steeds merendeels rondom die korrelgrens geleë.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2011. / ENGLISH ABSTRACT: The extraction of mineral values from ore requires liberation of the minerals followed by separation. Liberation is achieved by size reduction operations which are inefficient processes typically accounting for up to 70% of the energy consumption in a mineral concentrator (Tromans, 2008). As the grade of ores reserves declines, future viability of mineral operations will be determined by the costs of comminution. Recent work has shown that microwave treatment of secondary crusher product at specific microwave energy consumption of the order of 1 kWh/t reduces the work index of the ore and increases grade and recovery in batch flotation tests. Improved liberation at coarse sizes was also demonstrated (Kingman, 2006). Based on these findings work is ongoing to commercialise the technology. The objective of this study is to develop a modelling framework to determine the value proposition of microwave treatment of ore. It was noted that various models exist in literature for modelling of mineral processing flotation flow sheets, but these models do not incorporate the feed ore liberation property as an input variable in their calculations. Thus, a fundamentally derived property based model was identified as appropriate for flow sheet modelling of microwave treated ore, as it utilised liberation as an indirect variable in calculating the flotation rate constant through the use of contact angle to describe particle surface hydrophobicity. The model was successfully incorporated into the flotation flow sheet units developed in HSC Chemistry and used with Mineral Liberation Analyser (MLA) data to investigate the effects of changes in feed ore liberation on rougher cell flotation recovery. Different liberation scenarios based around modification of porphyry copper flotation feed were created. A sensitivity analysis of the various feed stream liberation scenarios was carried out to test the ability of the model to effectively model the differences in downstream processing of microwave treated and untreated ores. For a single flotation cell of size 85 m3 with a solids feed flow rate of 890 tph, it was observed that below a certain size (120 μm in the case of the porphyry copper ore) changes in flotation feed liberation had no significant effect on value mineral recovery. Significant differences in value mineral recovery were observed only at coarser sizes above 120 μm. The results indicated that improvement in recovery of value minerals due to improved liberation from applying microwave technology has size limits and is significantly dependent on the feed grind size. Feed grind size sensitivity analysis was then carried on the same single cell flow sheet utilising feeds with the same mineralogy but with different grind sizes. The results indicated that maximum benefits from the application of microwave technology would be best obtained by utilising coarse grinding at sizes between P70 = 200 μm and P70 = 300 μm for the porphyry copper ores considered in this study. Coarse grinding appears to be the best way to exploit improved liberation in downstream processing of microwave treated ores. Investigations similar to those carried out on the single cell flotation flow sheet were then carried out on a continuous plant rougher flotation flow sheet. The flow sheet consisted of nine rougher cells in series each with a volume of 85 m3 with a solids feed flow rate of 890 tph into the bank of rougher cells. The results indicated that there was no significant difference in final rougher bank overall cumulative recovery at fine grind sizes below a P70 grind size of 120 μm with improvements in feed ore liberation. Feed grind size sensitivity analysis showed a significant variation in cumulative recovery at coarse grind sizes of above P70 = 129 μm .This variation was attributed to improvements in flotation feed ore value mineral liberation from locked composite particles to the maximum possible theoretical liberation scenario of fully liberated value mineral particles. A 7.2 percentage point improvement in cumulative value mineral overall recovery and a 2 to 3 percentage point improvement in enrichment ratio was also observed above the P70 = 250 μm grind sizes after improving the flotation feed ore value mineral particle liberation of a typical flotation plant feed to a maximum. The increases in grade and cumulative recovery at coarse sizes were attributed to improvements to the flotation plant feed ore value mineral particle liberation. From the results, it was concluded that microwave technology application will offer greater benefits in downstream processing of coarse ground ores. / AFRIKAANSE OPSOMMING: Die ekstraksie van mineraalwaardes uit erts vereis bevryding van die minerale gevolg deur skeiding. Bevryding word bereik deur verkleiningsprosedures wat ondoeltreffende prosesse is en wat gewoonlik vir tot 70% van die energieverbruik in ʼn mineraalkonsentreerder verantwoordelik is (Tromans, 2008). Algaande die graad van ertsreserwes afneem, sal toekomstige lewensvatbaarheid van mineraalprosesse bepaal word deur die koste van vergruising. Onlangse werk het getoon dat mikrogolfbehandeling van sekondêre vergruiserproduk by spesifieke mikrogolf-energieverbruik van ongeveer 1 kWh/t die werkindeks van die erts verminder en die graad en opbrengs in lotflottasietoetse verhoog. Verbeterde bevryding by growwer groottes is ook aangetoon (Kingman, 2006). Werk gaan voort op grond van hierdie bevindinge ten einde die tegnologie te kommersialiseer. Die doel van hierdie navorsing is om ʼn modelleringsraamwerk te ontwikkel om die waardeproposisie van mikrogolfbehandeling van erts te bepaal. Daar is in die literatuur afgekom op verskeie modelle vir die modellering van vloeidiagramme vir flottasie van mineraalverwerking, maar hierdie modelle inkorporeer nie die voerertsbevrydingseienskap as ʼn insetveranderlike in hulle berekeninge nie. ʼn Fundamentele afgeleide eienskapgebaseerde model is geïdentifiseer as geskik vir vloeidiagrammodellering van mikrogolfbehandelde erts, aangesien dit bevryding as ʼn indirekte veranderlike by die berekening van die flotteertempokonstante aangewend het deur die gebruik van kontakhoek om hidrofobisiteit van die deeltjieoppervlak te beskryf. Die model is suksesvol in eenhede van die flottasievloeidiagram wat in HSC Chemistry ontwikkel is, geïnkorporeer en tesame met data van die mineraalbevrydingsontleder (MBO) gebruik om die gevolge van veranderinge in voerertsbevryding op die opbrengs van voorskeiselflottasie te ondersoek. Verskillende bevrydingscenario’s is geskep wat óm die modifisering van porfierkoperflotteringstoevoer heen gebaseer is. ʼn Sensitiwiteitsontleding van die verskillende voerstroombevrydingscenario’s is uitgevoer om die vermoë van die model om die verskille in stroomaf-verwerking van mikrogolfbehandelde en onbehandelde ertse te toets, doeltreffend te modelleer. In die geval van ʼn enkele flottasiesel van 85 m3 groot met ʼn vastestof-toevoervloeitempo van 890 tph, is waargeneem dat veranderinge in flottasietoevoer-bevryding benede ʼn sekere grootte (120 μm in die geval van die porfierkopererts) geen beduidende uitwerking op die opbrengs van die waardemineraal gehad het nie. Beduidende verskille in die opbrengs van die waardemineraal is slegs by growwer groottes bo 120 μm waargeneem. Die resultate het daarop gedui dat verbetering in die opbrengs van waardeminerale as gevolg van verbeterde bevryding ná die toepassing van mikrogolftegnologie beperkinge ten opsigte van grootte het en opvallend afhanklik is van die toevoermaalgrootte. Sensitiwiteitstoetsing van toevoermaalgrootte is daarna op dieselfde enkele selvloeidiagram wat voerders met dieselfde mineralogie gebruik uitgevoer, maar met verskillende maalgroottes. Die resultate het daarop gedui dat maksimum voordele van die toepassing van mikrogolftegnologie die beste verkry sou word deur gebruik van growwe maling by groottes tussen P70 = 200 μm en P70 = 300 μm vir die porfierkoperertse wat in hierdie navorsing in oorweging geneem is. Growwe maling skyn die beste manier te wees om verbeterde bevryding in stroomaf-verwerking van mikrogolfbehandelde ertse te eksploiteer. Ondersoeke soortgelyk aan dié wat op die vloeidiagram van die enkelselflottasie uitgevoer is, is toe op ʼn deurlopende vloeidiagram van die aanlegvoorskeierflottasie uitgevoer. Die vloeidiagram het bestaan uit nege voorskeiselle in serie elk met ʼn volume van 85 m3 met ʼn vastestof-toevoervloeitempo van 890 tph in die ry voorskeiselle. Die resultate het daarop gedui dat daar geen aanmerklike verskil in algemene kumulatiewe opbrengs van die finale voorskeiry by fyn maalgroottes benede ʼn P70-maalgrootte van 120 μm met verbeteringe in voerertsbevryding was nie. Sensitiwiteitsontleding van voermaalgrootte het ʼn beduidende variasie in kumulatiewe opbrengs by growwe maalgroottes van bo P70 = 129 μm getoon. Hierdie variasie is toegeskryf aan verbeteringe in waardemineraalbevryding van flottasietoevoererts uit geslote saamgestelde deeltjies tot die maksimum moontlike teoretiese bevrydingscenario van ten volle bevryde waardemineraaldeeltjies. ʼn Persentasiepuntverbetering van 7.2 in die kumulatiewe algemene opbrengs van waardemineraal en ʼn persentasiepuntverbetering van 2 tot 3 in die verrykingsratio is ook bo die P70 = 250 μmmaalgroottes waargeneem ná verbetering van die bevryding van die waardemineraaldeeltjies van die flottasietoevoererts van ʼn tipiese flottasieaanlegtoevoer tot die maksimum. Die toenames in graad en kumulatiewe opbrengs by growwe groottes is toegeskryf aan verbeteringe in die bevryding van die waardemineraaldeeltjies van die flottasietoevoererts. Op grond van die resultate is daar tot die gevolgtrekking gekom dat toepassing van mikrogolftegnologie groter voordele in stroomaf-verwerking van grofgemaalde ertse sal bied.

Thesis (MScEng (Process Engineering))--University of Stellenbosch, 2006. / As easy to mine high grade ore bodies are being depleted, many mining industries are experiencing an increasing need to process lower grade ores, and thus the high costs involved in the mineral recovery from these ores (of which comminution energy costs are a large component) are of major concern. It has been estimated that up to 70% of the total energy consumption in mineral processing is used up by comminution processes, which characteristically may have efficiencies of less than 0.1% in terms of the transfer of electrical energy into particle breakage. In many cases, very fine grinding is required to liberate the valuable inclusions in such low grade ores, which also leads to slimes losses of valuable minerals due to the inefficiencies of recovery methods in the ultra-fine size ranges. For many years the use of thermal pretreatment has been suggested as a way to decrease the costs of size reduction, and improve the liberation of valuable minerals in ores to aid later beneficiation technologies, but it was not until exploration into the use of microwaves to selectively heat only some of the minerals in ores, that this form of treatment became economically viable. A low grade copper ore from Palabora was subjected to microwave treatment and then tested for ore strength in a laboratory rod mill, using the developing cumulative size distributions of the rod mill products with time to quantitatively determine the effects of microwave treatment on ore strength. It was seen that after microwave treatment the ore responded more readily to milling, producing a finer grind than for untreated ore at every measured time interval of milling. From this data, comminution models were created to describe the grinding of this ore in various flowsheet simulations. An investigation was also performed to determine the effect of the application of microwave treatment on the liberation of minerals, due to the preferential breakage induced along grain boundaries during the selective thermal expansion of certain mineral inclusions in ores during microwave treatment. To ensure consistency between results for microwave treated and untreated material, it was decided to use the same grinding time for both when preparing ore for the next stage of testing. A grinding time was chosen which would produce an 80% passing size of 800 μm for the microwave treated ore. This time was determined from the previous grinding tests and was found to be approximately 16 minutes. After particle size classification of the mill products through sieving, a size range suitable for gravity separation processes was chosen for sink-float testing, with the aim of investigating whether microwave treatment had liberated enough gangue material at large particle sizes to offer the possibility of removing this hard gangue material early on in the process, before costly fine grinding is required. XRF analysis of the products showed little difference in recoveries of gangue material to the floats between treated and untreated material, and that while most of the copper reported to the sinks, that some of the copper was always entrained in the floats. These losses of valuable minerals to the gravity tailings will lead to overall losses in copper mineral recovery from the plant. QEMSCAN® analysis showed that there was a significant increase in mineral liberation in the size ranges associated with flotation as a result of the microwave treatment. An increase in liberation of the copper minerals which are easily recovered by flotation (i.e. chalcopyrite, cubanite, bornite, chalcocite and digenite) of 8.4% over that of the untreated ore was seen. This indicates that significant increases in copper recovery are possible after microwave treatment, and also that less fine grinding is then required to extract the valuable minerals from the ore, which leads to a reduction in loss of these valuable minerals to slimes. Palabora Mining Company supplied enough data on their plant operations from 1989 to enable models to be built to describe the operation of the mills and classifiers used in their comminution circuit. This data, together with the work performed to compare the performance of microwave treated and untreated Palabora ore in both milling and liberation (which allowed for basic recovery models to be built), allowed flowsheet simulations of the plant operations. Simulations of the plant after the addition of microwave pretreatment of the ore showed that the total energy used in comminuting the ore (including that of the microwave treatment) to the correct size distribution for mineral recovery by flotation were reduced by 19% from that required for untreated ore, and was mainly due to reductions in the circulating loads over the mills. By exploiting the greater milling capacity allowed for by these lower circulating loads, it was shown that it was theoretically possible to obtain increases of up to 46% in maximum throughput after microwave treatment, while retaining the same final grind size in the feed sent to flotation as is required for untreated ore. The addition of gravity separation processes to remove liberated gangue material from the comminution circuit early on, led to further savings in energy and also grinding media, and also decreased the requirements for flotation reagents and smelter fuel later on in the flowsheet. Unfortunately, the losses of entrained copper to the gravity separation tailings were such that overall economic losses were incurred by the operation. It was concluded that when dealing with low grade ores, only the implementation of very efficient and mineral specific separation technologies could make the removal of gangue material at large particle sizes (i.e. > 1 mm) viable. Economic analyses based on the simulations of the plant under various operating conditions showed potential increases in plant profitability after the addition of microwave pretreatment of the ore before milling, and were reported using net present value (NPV) calculations for the plant over a 10 year period with monetary values discounted at 20%. When operating under the same conditions and throughput as in the 1989 data provided by Palabora Mining Company, an increase in the NPV of the plant of 23% over that for the reported operation was seen after the addition of microwave pretreatment, and an increase of 72% in NPV given a 10% increase in throughput which is made possible by microwave pretreatment of the ore. In real money terms, after 10 years of operation the increase in NPV of the plant with the addition of microwave pretreatment of the ore was seen to be around R259 million (under the conditions reported for the plant operation in 1989), and around R795 million if the 10% increase in throughput which is only made possible by microwave pretreatment is realized. Current conditions at Palabora are very different from those supplied by the plant for the operation in 1989, however, as the mining operation has since been moved underground resulting in the throughput of the plant being greatly reduced, with the consequence that the plant is currently operating at a loss. Palabora mining company posted a net loss of R158 million over the 6 months leading up to June 2004, while an economic analysis of the proposed addition of microwave pretreatment of the ore at an increased throughput of 10% made possible by this treatment, indicated that a loss of only R138 million would have been incurred over the same 6 month period had this been implemented. Thus, while benefits from the introduction of microwave pretreatment of the ore before milling can still be seen under the operating conditions of the plant during the time period investigated, these alone would not have be able to bring the plant to profitable operation.