Leaching of secondary zinc oxides using sulphuric acid

Lottering, Catherine (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Located in the south-western Spergebiet region in Namibia, Skorpion Zinc is one of the few mines in the world with an economically viable oxide zinc deposit. The ore is processed in a sulphuric acid atmospheric leaching circuit, followed by solvent extraction and electrowinning. Zinc casting is used to obtain the 99.995% SHG zinc final product. With Skorpion Zinc’s predicted mine closure date of 2017 approaching, the company is investigating various possibilities for maximizing zinc throughput. The possibility of using secondary zinc oxides, such as electric arc furnace dust (EAFD), galvanizing dross and flue dusts was investigated during the course of this project. This investigation was aimed at determining whether it would be possible to process these oxide sources in conjunction with the feed ore, to make up for the zinc deficit from the pit. To this end, the project had two main goals: determine whether it would be possible to process the various zinc oxide samples provided in Skorpion’s plant, and determine the maximum potential feed quantities of the different materials tested to the refinery, from both a production and profitability perspective. Optimum processing conditions for these materials were also briefly investigated. Leaching test work showed that it would be possible to obtain recoveries in excess of 93% on the EAF dust, 96.9% from the zinc dross and 98% from the flue dust samples in the Skorpion process, using the normal process conditions of 50°C, a pH of between 1.8 and 1.85 and a slurry feed containing 20% solids. Literature suggested that increasing temperature and decreasing pH would result in faster zinc recovery and potentially better zinc recovery, depending on whether or not equilibrium was attained. To determine the effect of these factors on the zinc recovery and kinetics, a full factorial experimental design for temperature and pH was developed, using temperatures ranging from 40°C to 70°C in increments of 10°C, with pH’s ranging from 1.2-2.1 in increments of 0.3. Results from these experiments showed that the system reached equilibrium by the end of the 120 minutes residence. Thus, there was little improvement in overall zinc recovery (less than 5% overall change in recovery across all experimental conditions). Kinetics of leaching from EAFD seemed unaffected by changes in pH and temperature. Contrary to expectations from literature, all samples seemed to experience slower kinetics at 70°C than at lower temperatures. This may be due to the fact that all reactions occurring in the system were exothermic, and increasing the temperature to this point limited the reaction kinetics. There was little consistency in the pH and temperature trends for the rest of the samples. It was therefore not possible to draw conclusions about the relationship between leaching kinetics, temperature and pH. However, it was clear from these results that it would be possible to process the different samples in the Skorpion refinery, from a technical point of view. Using the data obtained from the experimental work, a mass balance was performed over the Skorpion refinery. Blends of the different secondaries with the Skorpion ore were tested in ratios of 10, 20, 30, 40 and 50% oxide in this mass balance to determine which secondary source could provide the greatest zinc return, without causing impurity accumulation in the refinery. The results of this mass balance indicated that, under the normal Skorpion operating conditions, the flue dust would allow for maximal zinc production per annum. Base case conditions, using only Skorpion ore, allowed for a maximum production of 151 kt/y. EAF dust provided a maximum zinc production of 220 kt/y, using a blend of 20% EAFD with 80% ore. This particular scenario was limited by the maximum feed throughput for the Skorpion refinery (limited to 230 t/h) and experienced no impurity accumulation. Higher ratios of this particular oxide experienced accumulation of copper, magnesium and manganese. Zinc dross zinc production was limited by the amount of nickel in this sample. The maximum zinc production for this particular oxide was at 10% zinc dross, 90% ore blend and resulted in a total zinc production of approximately 183 kt/y. The scenario which resulted in the greatest zinc production involved using zinc fume oxide in a 50:50 blend with Skorpion ore. Although the throughput was limited to a total of 152 t/h feed by the amount of copper contained in the source, the large amount of zinc contained in this sample meant that a total of 495 kt/y of zinc production was theoretically possible from this scenario. The results of the different mass balance scenarios were used to construct a simplistic financial feasibility model. Considering only zinc contained in the oxide, the profit obtained from the zinc for each of the scenarios was determined. A certain %LME (London Metal Exchange) zinc price for the zinc contained in the oxide sources was assumed to determine the oxide purchase price. The profit for the oxide supplier was calculated, based on this price, and assuming that the supplier would be responsible for covering the cost of transport to site. Using an assumed oxide price of 20% of LME zinc price for zinc in the oxide, all three samples resulted in a profit for Skorpion, with the total profit increasing as the blending ratios increased. Scaled profit per ton for the zinc produced from each was as follows: EAFD – 28.1, zinc fume – 37 and zinc dross – 32.5. However, at this purchase price, the suppliers experienced losses of 3.2 on the EAFD and 6.8 on the zinc dross, while the zinc fume oxide supplier made a profit of 10.8. Under these conditions, only the zinc fume oxide could feasibly be processed, as it was the only sample which provided a profitable situation for both parties. Increasing the assumed oxide price to 30% increased the suppliers’ profits to 6.6, 20.5 and 3 for the EAFD, zinc fume oxide and zinc dross suppliers respectively. However, this lowered the Skorpion profit to 17.5, 27.1 and 22.5 for the EAFD, zinc fume oxide and dross respectively. This translated to a decrease in profit of 38%, 27% and 31% respectively. Thus, with the highest Skorpion and supplier profit of the three, the zinc fume sample was still the best option for processing.

AFRKAANSE OPSOMMING: Skorpion Zinc, geleë in suid westerlike Spergebiet streek in Namibië, is een van die min myne in die wêreld wat ‘n ekonomies lewensvatbare sinkoksied bron besit. Hier word die erts verwerk deur dit in ‘n atmosferiese swaelsuur logingsproses op te los. Daarna word dit verder verwerk deur oplosmiddel ekstraksie, elektroplatering en gieting van die sink om die finale produk van 99.995% suiwer SHG sink te maak. Met die Skorpion mynsluitingsdatum van 2017 wat nader kom, begin die maatskappy ondersoek instel na moontlike maniere om die sink produksie te vermeerder. Die moontlikheid om sekondêre sinkoksied bronne soos staaloond afval, metaalskuim en skoorsteen stof te gebruik is tydens hierdie projek ondersoek. Die hoofdoel van hierdie projek was om te bevestig of dit moontlik sou wees om hierdie sekondêre sink bronne saam met die Skorpion erts as voer te gebruik om die sink produksie te vermeerder. Om hierdie doelwit te bereik het die projek uit twee primêre ondersoek bestaan: bepaal of dit moontlik sou wees om die sekondêre sink bronne in die Skorpion aanleg te behandel, en bepaal die maksimum hoeveelhede van die verskillende tipes materiaal wat in die aanleg hanteer kan word – uit beide ʼn produksie en ʼn finansiële oogpunt. Optimale bedryfstoestande vir hierdie materiaal is ook kortliks ondersoek. Logingstoetse het getoon dat dit moontlik sal wees om herwinnings van meer as 93% vanaf staaloond afval, 96.9% vanaf metaalskuim en 98% vanaf skoorsteen stof te behaal deur van Skorpion se standaard bedryfstoestande van 50°C, ʼn pH van 1.8 tot 1.85 en 20% vastestowwe in die voer gebruik te maak. Die literatuurstudie het aangetoon dat ʼn verhoging in temperatuur en verlaging in pH vinniger logingskinetika en hoër sink herwinning sou teweegbring, indien ewewig nie bereik word nie. Om die effek van temperatuur en pH op die logingsproses te bepaal was ʼn volledige eksperimentele skedule opgestel. Temperatuur was volgens hierdie skedule in inkremente van 10°C verhoog vanaf 40°C na 70°C. Die pH was in inkremente van 0.3 verander van 1.2 na 2.1. Resultate van hierdie toetse het bewys dat die proses chemiese ewewig bereik binne die 120 minute eksperimentele tydperk. Dus was daar baie min verandering in die sink herwinning (minder as 5% verandering oor die hele stel toetse). Kinetika van die loging van staaloond afval het nie gelyk asof dit beïnvloed word deur verandering in temperatuur en pH nie. Al die bronne het stadiger kinetika teen 70°C ervaar, teenoor laer temperature – moontlik as gevolg van die feit dat die reaksies in die stelsel eksotermies was. Daar was nie ʼn duidelike verhouding tussen die temperatuur en pH vir die res van die monsters nie. Dus was dit onmoontlik om die verhouding tussen temperatuur, pH en loging vas te stel. Hierdie toetse het wel gewys dat dit uit ʼn tegniese oogpunt moontlik is om die drie verskillende bronne in die Skorpion proses te behandel. Die resultate van die logingstoetse was gebruik om ‘n massabalans uit te voer. Dit massabalans het verskillende mengsels van Skorpion erts en sekondêre sink oksied (mengsels van 10, 20, 30, 40 en 50% oksied) gebruik om te bereken wat die maksimum sink produksie sou wees onder verskillende omstandighede, sonder om enige opeenhoping van onsuiwerhede in die proses te veroorsaak. Hierdie massabalans resultate het gewys dat die skoorsteen stof die hoogste produksie sou lewer teen die standaard bedryfstoestande in die Skorpion aanleg. Die basis, met net Skorpion erts, het 151 kt/jaar sink opbrengs gelewer. Die maksimum staaloond afval sink produksie van 220 kt/jaar was bereken deur ʼn 20% staaloond afval en 80% erts mengsel te gebruik. In hierdie geval was die produksie beperk deur die aanleg se voer deurset (beperk tot 230 t/uur) en daar was geen opeenhoping van onsuiwerhede in die proses nie. Hoër staaloond afval persentasies het opeenhoping van koper, magnesium en mangaan veroorsaak. Die hoeveelheid nikkel in die metaalskuim het die sink produksie vanaf hierdie bron beperk. Teen ʼn 10% metaalskuim, 90% erts mengsel was die maksimum sink produksie vir hierdie bron 183 kt/jaar. Skoorsteen stof in ʼn 50:50 mengsel met erts het die maksimum sink produksie veroorsaak. Hierdie situasie was beperk deur die hoeveelheid koper in die skoorsteen stof (152 t/uur voer), maar die hoë sink konsentrasie in hierdie bron het veroorsaak dat sink teen 495kt/jaar geproduseer kan word. Resultate van die massabalans was gebruik om ʼn basiese finansiële studie te doen. Die wins vanaf die sink in die oksied was bepaal vir elkeen van die mengsels in die massabalans. Om die prys vir die oksied te bepaal was ʼn aanname van ʼn sekere persentasie van die LME sink prys gebruik vir die sink in die oksied. Die verskaffer se wins was bereken, met die aanname dat die verskaffer sou verantwoordelik wees vir die vervoer van die oksied na Skorpion se aanleg. Teen 20% van die LME sink prys, het Skorpion ʼn wins gemaak met al drie oksied bronne: staaloond afval – 28.1, skoorsteen stof – 37 en metaal skuim – 32.5. Die staaloond afval en metaalskuim verskaffers het egter elk een ʼn verlies van 3.2 en 6.8 gely, terwyl die skoorsteen stof verskaffer ‘n wins van 10.8 getoon het. Onder hierdie omstandighede was die skoorsteen stof die enigste lonende opsie, aangesien dit die enigste bron was wat vir beide partye ʼn wins getoon het. ʼn Verhoging van die sink prys in die oksied na 30% het ʼn wins vir al drie verskaffers veroorsaak: 6.6 vir die staaloond afval 20.5 vir die skoorsteen stof en 3 vir die metaalskuim. Aan die ander kant het hierdie verandering die Skorpion wins verlaag na 17.5 vir staaloond afval, 27.1 vir skoorsteen stof en 22.5 vir die metaalskuim. Hierdie is ‘n verlaging van 38%, 27% en 31% onderskeidelik. Dus is die skoorsteen stof bron nog steeds die beste, aangesien dit die hoogste wins vir Skorpion voorsien.

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