Pyrometallurgical recovery of cobalt from waste reverbaratory furnace slag by DC plasma-ARC furnace technology

Banda, Wezi (2001-12)

Thesis (MScEng.)--University of Stellenbosch, 2001.

Thesis

ENGLISH ABSTRACT: Slag cleaning has become a common practice at many smelters in the nonferrous industry to maximize recovery of valuable metals. However, during the carbothermic reduction of nonferrous slag to recover cobalt, in particular, iron is recovered predominantly. High iron levels present a problem for the subsequent treatment of the alloy as it may increase the solids loading to the filter and lead to increased reagent consumption during leaching. Finding an appropriate slag modifier that would selectively improve the recovery of cobalt against that of iron to the metallic alloy can solve this problem. In the present study the effects of lime (CaO), rutile (Ti02), and fluorspar (CaF2) on the recovery of cobalt from waste nonferrous slag have been investigated under reducing conditions at 1500°C. The selective recovery of cobalt compared to the recovery of iron at different levels of flux additions is discussed in this study, to show the selectivity of these fluxes. It is also shown in the study that the recovery of cobalt does not only depend on the oxygen partial pressure and temperature but on the slag composition as well. The slags used in the experiments were a) synthetic slag prepared from chemically pure reagents and its composition was derived from that of the industrial nonferrous slag composition typical of reverbaratory furnace and b) actual slag obtained from an old slag dump situated on the Zambian Copperbelt region. The investigation has shown that Ti02 addition leads to the most selective cobalt recoveries in all cases. On the other hand, both CaO and CaF2 lead to higher overall cobalt recoveries. The effect of Ti02 on the slag chemistry leads to the formation of iron titanate compounds in the slag unlike CaO, which displaces "FeO" from the fayalitic slag and thus increases the activity of FeO in the molten slag, which in tum affects the iron recoveries to the alloy product. CaF2 on the other hand, affects the fluidity of the slag leading to improved recoveries by improved settling of metals through the slag to the alloy product. A 44V/I100A DC-plasma arc furnace was used to reduce slag to recover cobalt at about 13.5kV A power input. The major part of the study was conducted in a tube furnace and the findings were applied to the extraction of cobalt from slag using the plasma-arc furnace. It was found that the synthetic slag experiments could be used as a guide to understand the behaviour of cobalt during the carbothermic recovery of cobalt from silica saturated fayalitic slags. Cobalt recoveries were higher in the DC furnace than the corresponding reduction experiments carried out in the tube furnace. However, significant amounts of silicon and carbon were detected in the metallic alloy product of the DC plasma-arc furnace.

AFRIKAANSE OPSOMMING: Slak suiwering is besig om standaard praktyk te word by vele smelters om die herwinning van waardevolle metale uit slak te maksimeer. Tydens die karbotermiese reduksie van slakke vanaf kopersmelters om kobalt te herwin, word beduidende hoeveelhede yster ook herwin. Hoë ystervlakke in die herwinde legering veroorsaak probleme met die stroom-af verwerking van die legering aangesien dit lei tot verhoogde reagensgebruik tydens loging en 'n verhoogde vastestof las of die filterstelsel. Die identifisering van 'n geskikte slak modifiseerder, wat die selektiwiteit van herwinning van kobalt relatief tot yster verhoog, sal hierdie probleem verminder. In hierdie tesis word die rol van kalk (CaO), rutiel (Ti02) en vloeispaat (CaF2) toevoeging tot die selektiewe herwinning van kobalt uit afvalslakke ondersoek. Die karbotermiese reduksie van die slak vind by 1500 °C plaas. Dit word getoon dat bo-en-behalwe die suurstofpotensiaal en die bedryfstemperatuur, die slakchemie 'n beduidende rol speel. Die slakke gebruik in hierdie studie is: a) 'n sintetiese slak gemaak van chemies-suiwer rou-materiale (waarvan die samestelling afgelei is van die samestelling van tipiese reverbereeroond slakke), en b) monsters van die ware slak verkry vanaf 'n slakhoop van die Zambiese Kopergordel. Die studie het getoon dat Ti02 toevoeging gelei het tot die hoogste selektiwiteit in alle gevalle. Daarteenoor het CaF2 en CaO gelei tot hoër algehele herwinnings van kobalt. Ti02 toevoeging lei tot die vorming van ystertitanaat komplekse in die slak, teenoor CaO wat die "FeO" uit die fajaliet struktuur verplaas en die aktiwiteit van FeO in die slak verhoog en dus gevolglik herwinning van yster tot die legering beïnvloed. CaF2 verhoog egter die vloeibaarheid van die slak wat lei tot verhoogde herwinning deurdat die legering druppels makliker uitsak deur die slak. 'n 44VIII OOAGelykstroom-plasmaboogoond is gebruik om die kobalt te herwin uit die slak teen ongeveer 13.5 kVA Die grootste gedeelte van die studie is gedoen in 'n buisoond en die bevindings was dan toegepas op die ekstraksie van kobalt uit slak in die plasmaboogoond. Dit was gevind dat die sintetiese slak eksperimente gebruik kan word as 'n voorlopige gids om die gedrag van kobalt in silika-versadigde slakke tydens karbotermiese reduksie te verstaan. Kobalt herwinnings was oor die algemeen hoër il) die plasmaboogoond as vir die ooreenstemmende reaksies in die buisoond. Daar het egter beduidende hoeveelhede silikon en koolstof In die legering opgelos tydens die plasmaboogoond eksperimente.

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