Platinum group metals behaviour during iron precipitation and goethite seeding in nickel sulphate solution

Coetzee, Rje (2016-03)

Thesis (MEng)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Platinum producers’ base metal refineries leach converter matte to solubilise base metal sulphides and produce a platinum group metal residue. Nickel, cobalt and some of the iron present in the matte are solubilised during atmospheric sulphate leaching. The pregnant leach solution also contains dissolved other precious metals (OPMs, ruthenium, rhodium and iridium). The solution passes through holding tanks prior to being fed to heat exchangers and crystallisers for the production of nickel sulphate hexahydrate as by-product. An iron sludge precipitates during long residence times in tanks and on contact with hot surfaces. This sludge entrains OPMs and could potentially be used as a mechanism for recovery of dissolved OPMs. The behaviour of OPMs needed to be established during impurity precipitation in the nickel sulphate solution. Goethite material was also seeded, as a means to provide a growth surface during precipitation. The solution was heated to 70 °C or 90 °C and the pH was adjusted to pH 2.5 and pH 4.0, with caustic soda (NaOH). The total metal concentration was varied between the total metals of the first stage leach, 65 g/ℓ, and that of upgraded solution, 95 g/ℓ. Goethite seeding (10 g/ℓ, 50 g/ℓ and no seed) and iron valence (ferrous and ferric) were varied in six hour experiments. It was found that the OPMs could be selectively precipitated via neutralisation, with pH and temperature being crucial factors. At pH 2.5, 89 % ruthenium and rhodium precipitated and 14 % iridium precipitated. All ruthenium and rhodium precipitated at pH 4.0 and iridium precipitation increased to 88 %. OPM precipitation was 34 % faster at high temperature experiments. The order of readability to precipitate was ruthenium, followed by rhodium and then iridium. OPM precipitation was also possible without iron in synthetic solutions. Different iron oxide phases formed. Spherical, aggregated crystals between 50 nm – 100 nm in size were observed with elemental compositions agreeing well with that of synthetic ferrihydrate and schwertmannite. Sulphate inclusion was more prominent during the rapid precipitation in ferric runs. Ruthenium and rhodium were finely dispersed within the iron oxide precipitates of lower densities. Iridium precipitated in a clearly distinguishable phase consisting of iridium (50 – 80 wt.%), chloride and oxygen. The observation of the acicular rod crystals that are associated with goethite, the absence of spherical crystals, the uniform density observed with back-scatter images and the observed increase of overall particle size strongly suggest that seeding did induce the targeted growth of goethite in nickel sulphate solution with ferric iron. Goethite seeding did not increase the rate of iron precipitation or OPM recovery.

AFRIKAANSE OPSOMMING: Platinum produseerders se basismetaal raffinaderye loog onedel metale vanuit omskakelaar mat, om sodoende ‘n platinum groep metaal residu te produseer. Nikkel, kobalt en yster word gedurdende atmosferiese sulfaat loging opgelos. Hierdie logingsoplossing bevat ook sogenaamde ander waardevolle metale, OPMe, wat rutenium, rodium en iridium insluit. Die logingsoplossing word in tenke gestoor en daarna gevoer na hitteruilers en kristalliseerders om nikkel heksahidraat kristalle as ‘n by-produk te produseer. Yster presipiteer wat ‘n slyk newe-produk vorm gedurende die lang residensie tyd in tenke en ook wanneer die oplossing in kontak kom met warm oppervlaktes. OPMe word in hierdie slyk waargeneem en dus word die verwydering van slyk oorweeg as ‘n metode om OPMe te ontgin. Dit word verlang om vas te stel hoe OPMe optree gedurende yster presipitasie. Goethiet saad materiaal wat potensieel as ‘n middel kan optree om geordende kristal groei te bevorder, is ondersoek. Die oplossing was verhit na 70 °C en 90 °C en the pH was aangepas met bytsoda (NaOH) na pH 2.5 en pH 4.0. Die totale metaal konsentrasie in die oplossing was gevarieer tussen die konsentrasie van tipiese logingsoplossings by 65 g/ℓ, sowel as die van opgegradeerde oplossing by 95 g/ℓ. Goethiet saad (10 g/ℓ, 50 g/ℓ en geen saad) en yster valensie (Fe(II) en Fe(III)) was ook gevarieer in ses uur experimentele lopies. OPMe het selektief gepresipiteer as die logingsoplossing met ‘n neutralisasie middel behandel word. pH en temperatuur was die belangrikste faktore. 89 % rodium en ruthenium presipitasie en 14 % iridium presipitasie is waargeneem by pH 2.5. As die pH verhoog word na 4.0 sal alle rutenium en rodium presipiteer en 88 % iridium sal presipiteer. OPM presipitasie is 34 % vinniger by hoë temperatuur eksperimente. Rutenium het die maklikste gepresipiteer, gevolg deur rodium en laastens iridium. OPM presipitasie het ook plaasgevind in die afwesigheid van yster in oplossing. Verskillende ysteroksied soliede fases het gevorm. Sferiese, aangepakte, 50 nm – 100 nm kristalle was waargeneem, wat ooreenstem met sintetiese weergawes van ferrihidraat en schwertmanniet. Sulfaat inkorporasie was meer prominent gedurende die spoedige presipitasie in Fe(III) lopies. Rutenium en rodium was fyn versprei in die ysteroksied fases. Iridium het as ‘n aparte solied gepresipiteer, wat primêr uit iridium (50 – 80 massa%), chloried en suurstof bestaan. Resultate stel voor dat saad materiaal suksesvol geteikende presipitasie teweeg bring. Slegs langwerpige, silindriese goethiet kristalle en ‘n uniforme partikeldigtheid word waargeneem. Die partikelgrootte het ook effektief gegroei. Die byvoeging van goethiet saad het alhoewel nie die tempo van yster presipitasie of OPM ontginning verhoog nie.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98814
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