Stripping rare earth elements and iron from D2EHPA during zinc solvent extraction

Alberts, Estelle (Stellenbosch : Stellenbosch University, 2011-12)

Thesis

ENGLISH ABSTRACT: At Skorpion Zinc mine, in south-west Namibia, zinc oxide ore is refined through sulphuric acid leaching, solvent extraction, electrowinning and casting of the final 99.995 % Zn metal. Over the last four years, the rare earth element concentrations, with particular reference to Y, Yb, Er and Sc, have significantly increased in the circulating electrolyte and the zinc-stripped organic phase streams in the electrowinning and solvent extraction processes. This project had two main objectives: firstly, the effect(s) of rare earths on the zinc solvent extraction and electrowinning processes were to be determined; based on these results, the second objective was to find a suitable method for removing rare earth elements from the organic phase during zinc solvent extraction. The investigation into the effect of the rare earths on zinc electrowinning showed that an increase of 100 mg/l in the electrolyte Y concentration caused a decrease of 6 % in current efficiency. The elemental order of decreasing current efficiency was found to be: Y > Yb > Er > Sc. In the zinc solvent extraction process, it was found that an increase in the total organic rare earth elements and iron concentration from 3100 to 6250 mg/l resulted in doubled viscosity and an increase in phase disengagement time from 100 to 700 seconds. The organic zinc loading capacity after two extraction stages was reduced by 1 – 3 g/l depending on the pregnant leach solution used. The detrimental effect of rare earth elements on solvent extraction and electroplating of zinc therefore justified the development of a rare earth element removal process. Stripping of low concentrations of rare earth elements from 40% D2EHPA diluted in kerosene to produce a clean organic for zinc extraction was investigated using bench-scale experiments in a glass jacketed mixing cylinder. For the rare earths, the best stripping agent was found to be H2SO4, followed by HCl and then HNO3. Hydrochloric acid achieved better Fe stripping than sulphuric acid. Acid concentration was tested in the range of 1 to 7 M, organic-to-aqueous ratio for the range of 0.25 to 6.0 and temperatures between 30 and 55 °C. More than 80% stripping of yttrium and erbium could be achieved at an optimum hydrochloric acid concentration of 5 M and more than 90% rare earth element (specifically Y, Er, Yb) stripping from the organic phase could be achieved with 5 M sulphuric acid. Stripping was improved by reducing the organic-to-aqueous ratio to as low as 0.5 and increasing the temperature. Stripping increased with increasing temperature in an S-shaped curve, flattening off at 50°C. The effect of O:A ratio was more significant than the effect of temperature on rare earth stripping. The results showed good repeatability, and were not limited by the rare earth concentration, agitation rate or equilibrium time in the range of set points used in the experiments. Statistical models were compiled to fit the experimental data obtained for Y, Yb, Er and Fe when stripped with sulphuric and hydrochloric acid respectively. All models showed dependence on the acid concentration and squared-concentration and interaction effects between the O:A ratio and temperature and stripping agent concentration were significant. The models were compiled for the experimental data obtained from stripping synthetically prepared organic and then tested on results obtained when stripping the plant organic phase. The following three process solutions were discussed for implementation on a plant scale for the removal of rare earths from the organic phase during zinc solvent extraction: Sulphuric acid stripping mixer settler or stripping column, improvement of available HCl stripping section and replacement of the organic inventory. The possibility of an oxalic acid precipitation process to obtain value from the rare earths as by-product was also discussed. It was concluded that the current process that uses HCl to strip off iron and rare earths would be the best practically and financially feasible process. Value can be gained from the rare earths if a rare earth element - oxalic acid precipitation section that is financially feasible can be established.

AFRIKAANSE OPSOMMING: By Skorpion Zinc myn, in suidwes Namibië, word sinkoksied erts gesuiwer deur middel van swaelsuur-loging, oplosmiddel ekstraksie, elektroplatering en gieting van die finale 99.995 % Zn metaal. Oor die afgelope vier jaar het die seldsame aardmetale konsentrasie, spesifiek Y, Yb, Er en Sc, noemenswaardig in die sirkulerende elektroliet en sink-gestroopte organiese fase toegeneem. Hierdie projek het twee hoofdoelstellings gehad: eerstens moes die effek van seldsame aardmetale op die sink oplosmiddel ekstraksie en elektroplatering prosesse bepaal word; gebaseer op hierdie resultate, was die tweede doelstelling om ‘n geskikte metode vir die verwydering van seldsame aardmetale vanaf die organiese fase gedurende sink oplosmiddel ekstraksie te vind. Die ondersoek na die effek van seldsame aardmetale op sink elektroplatering het gewys dat ‘n verhoging van 100 mg/l in die elektroliet Y konsentrasie ‘n verlaging van 6 % in kragdoeltreffendheid veroorsaak het. Die element-orde van verminderende kragdoeltreffendheid was Y > Yb > Er > Sc. Vir die sink oplosmiddel ekstraksie proses, is gevind dat ‘n verhoging in die totale organiese seldsame aardmetaal- en yster konsentrasie van 3100 tot 6250 mg/l ‘n verdubbelde viskositeit en ‘n verlenging in faseskeidingstyd van 100 tot 700 sekondes tot gevolg gehad het. Die organiese sink ladingskapasiteit na twee ekstraksie stappe is met 1 – 3 g/l verminder afhangende van die logings oplossing wat gebruik is. Die nadelige effek van seldsame aardmetale op oplosmiddel ekstraksie en sink elektroplatering het die ontwikkeling van ‘n seldsame aardmetale verwyderingsproses regverdig. Die verwydering van lae konsentrasies seldsame aardmetale vanaf die D2EHPA-keroseen organiese fase om ‘n skoon organiese fase vir sink-ekstraksie te verkry is ondersoek deur banktoetsskaal eksperimente. Vir die seldsame aardmetale is bevind dat H2SO4 die beste stropingsagent is, gevolg deur HCl en dan HNO3. Soutsuur het beter yster verwydering as swaelsuur bewerkstellig. Suurkonsentrasies van 1 tot 7 M, O:A verhoudings van 0.25 tot 6 en temperature tussen 30 en 55°C is getoets. Meer as 80% verwydering van yttrium en erbium kon bereik word met ‘n optimum HCl konsentrasie van 5 M en meer as 90% seldsame aardmetale (spesifiek Y, Er en Yb) verwydering vanaf die organiese fase met 5 M swaelsuur. Seldsame aardmetale en yster verwydering kon verbeter word deur die organies-tot-waterfase (O:A) verhouding te verminder tot so laag as 0.5 en deur die temperatuur te verhoog. Stroping het in ‘n S-kurwe verhoog soos die temperatuur verhoog het, en het afgeplat teen 50°C. Die effek van die O:A verhouding was belangriker as die effek van temperatuur op stroping. Die resultate het goeie herhaalbaarheid gewys, en is nie deur massaoordrag beperk nie. Statistiese modelle is saamgestel om die eksperimentele data wat vir Y, Yb, Er en Fe verkry is vir stroping met swaelsuur en soutsuur te pas. Al die modelle het afhanklikheid van die suurkonsentrasie en kwadratiese suurkonsentrasie gewys en interaksie effekte tussen die O:A verhouding, temperatuur en suurkonsentrasie was belangrik. Die modelle is saamgestel vir die eksperimentele data wat verkry is vanaf stroping van ‘n sintetiese organiese fase en is toe getoets op resultate wat verkry is vanaf stroping van die aanleg se organiese fase. Die volgende drie proses-oplossings is oorweeg vir implementering op ‘n aanlegskaal vir die verwydering van seldsame aardmetale vanaf die organiese fase gedurende sink oplosmiddel ekstraksie: Swaelsuur stroping menger-afskeidingstenk of stropingskolom, verbetering van die bestaande HCl aanleg en vervanging van die organiese inventaris. ‘n Moontlike oksaalsuur presipitasie proses om waarde vanaf die seldsame aardmetale as by-produk te verkry is ook bespreek. Daar is tot die gevolgtrekking gekom dat die huidige proses wat HCl gebruik om yster en seldsame aardmetale van die organiese fase af te stroop, die beste praktiese en finansieel vatbare proses huidiglik is. Waarde kan vanaf die seldsame aardmetale verkry word as ‘n seldsame aardmetale – oksaalsuur presipitasie afdeling wat finansieel haalbaar is daargestel kan word.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/17999
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