Thesis (MEng)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: In Base Metal Refineries (BMRs), the copper sulphate leach solution produced during the final
pressure leaching stage contains impurities such as selenium and tellurium, as well as other
precious metals (OPMs, namely Rh, Ru, and Ir). Se and Te are removed by precipitation with
sulphur dioxide prior to electrowinning of Cu. While a small percentage of the dissolved OPMs
precipitate with the Se and Te, the largest portion remains in solution and is recycled to the first
stage leach. If a larger portion of the OPMs in solution can be recovered in the Se/Te
precipitation stage, OPM losses and the OPM inventory of the plant can be reduced. The aim of
this project was to determine operating conditions that would allow maximum OPM recovery
with minimal Cu and Ni co-precipitation in the Se/Te removal section of a BMR.
The effects that the operating temperature, pressure, stirring rate, reagent type, and reagent
quantity have on the metal precipitation behavior and precipitate characteristics were
determined experimentally. Thio-urea and sulphurous acid were evaluated as precipitation
reagents for temperatures of 80°C and 160°C, stirring rates of 250 rpm and 500 rpm, and
pressures equal to ambient pressure and 7 bar. 200 % and 320 % excess thio-urea and 720 %
and 960 % excess sulphur dioxide were used.
The precipitation of OPMs with sulphur dioxide was generally poor; the maximum percentage
Rh, Ru, and Ir precipitated were 35 %, 18 %, and 20 %, respectively. It was, however, found that
the OPM precipitation increased as the reagent amount was increased. Increasing the
temperature further increased Rh and Ir precipitation but affected the Ru precipitation
negatively. Thio-urea precipitated virtually all of the Rh contained in the solution irrespective of
the values of the process variables studied. As was the case with sulphur dioxide, increasing the
amount of thio-urea added resulted in increased Ru and Ir precipitation, while higher
temperatures favored Ir precipitation but not Ru precipitation. The maximum percentage Ru
and Ir precipitation achieved with thio-urea were 87 % and 60 %, respectively. Complete Se precipitation was observed at all process conditions, while Te precipitation
increased as the operating temperature and the reagent quantity were increased. Maximum Te
precipitation of 98 % and 90 % were achieved when using 320 % excess thio-urea and 960 %
sulphurous acid quantities, respectively, at 160°C and a stirring rate of 250 rpm. Increasing the
reagent quantity and temperature did, however, also result in increased copper and nickel
precipitation. The statistical analysis of the results allowed regression models to be fitted to predict the
percentage metal precipitation as a function of the investigated process variables. These models
were used to define an objective function to determine the optimal operating conditions. A
temperature of 80oC, a pressure of 7 bar, and 200 % excess thio-urea were proposed as the
optimum operating conditions that would yield 98 % Rh, 75 % Ru, and 48 % Ir precipitation
with less than 5 % Cu and Ni co-precipitation. Experimental validation tests confirmed the
model predicted values and proved repeatability of the experimental data. / AFRIKAANSE OPSOMMING: Die kopersulfaat logingsoplossing wat tydens die finale drukloging stadium in Basis Metaal
Raffinaderye (BMRe) produseer word, bevat onsuiwerhede soos selenium en tellurium sowel as
ander edelmetale (AEMe, naamlik Rh, Ru, en Ir). Se en Te word voor Cu elektrowinning
verwyder deur middel van presipitasie met swaweldioksied. Alhoewel ʼn klein persentasie van
die opgeloste AEMe saam met die Se en Te presipiteer, bly die grootste gedeelte in oplossing en
word gevolglik na die eerste loging stadium hersirkuleer. AEM verliese en die AEM inventaris
van die aanleg kan verminder word indien ʼn groter gedeelte van die AEMe in die Se/Te
presipitasie stadium herwin kan word. Die doel van hierdie projek was om bedryfstoestande te
bepaal om maksimum AEM herwinning met minimale Cu en Ni kopresipitasie in die Se/Te
verwyderingseksie van ʼn BMR te behaal.
Die effekte wat bedryfstoestande soos temperatuur, druk, roerder tempo, tipe reagens, en
hoeveelheid reagens op die metaal presipitasiegedrag en presipitaat eienskappe het, is
eksperimenteel bepaal. Tio-ureum en swaweligsuur is evalueer as presipitasie reagense vir
temperature van 80°C en 160°C, roerder tempo’s van 250 rpm en 500 rpm, en drukke gelyk aan
omgewingsdruk en 7 bar. 200 % en 320 % oormaat tio-ureum en 720 % en 960 % oormaat
swaweldioksied is gebruik. Die presipitasie van AEMe met swaweldioksied was swak in die algemeen; die maksimum
persentasie Rh, Ru, en Ir presipitasie wat behaal is, is 35 %, 18 %, en 20 %, onderskeidelik. Daar
is egter gevind dat die AEM presipitasie toeneem indien die hoeveelheid reagens toeneem. ʼn
Toename in die temperatuur het verder tot ʼn toename in Rh en Ir presipitasie gelei, maar dit het
Ru presipitasie negatief affekteer. Tio-ureum het basies al die Rh in oplossing laat presipiteer,
ongeag die waardes van die ander prosesveranderlikes wat ondersoek is. Soos wat die geval vir
swaweldioksied was, het ʼn toename in die hoeveelheid tio-ureum ʼn toename in die Ru en Ir
presipitasie tot gevolg gehad, terwyl hoër temperature Ir presipitasie bevoordeel en Ru
presipitasie benadeel het. Die maksimum persentasie Ru en Ir presipitasie wat met tio-ureum
behaal is, is 87 % en 60 %, onderskeidelik.
Volledige Se presipitasie is by alle proses toestande waargeneem, terwyl Te presipitasie
toegeneem het soos wat die temperatuur en die hoeveelheid reagens toegeneem het. Maksimum
Te presipitasie van 98 % en 90 % is behaal toe 320 % oormaat tio-ureum en 960 % oormaat
swaweligsuur, onderskeidelik, by 160°C en ʼn roerder tempo van 250 rpm gebruik is. ʼn
Toename in die hoeveelheid reagens en die temperatuur het egter ook meer koper en nikkel
presipitasie tot gevolg gehad. Die statistiese analise van die resultate het dit moontlik gemaak om regressie modelle te pas om
die persentasie metaal presipitasie as ʼn funksie van die ondersoekte veranderlikes te voorspel.
Hierdie modelle is gebruik om ʼn doelfunksie te definieer ten einde die optimale
bedryfstoestande te bepaal. ʼn Temperatuur van 80°C, ʼn druk van 7 bar, en 200 % oormaat tioureum
is voorgestel as die optimale bedryfstoestande wat 98 % Rh, 75 % Ru, en 48 % Ir
presipitasie met minder as 5 % Cu en Ni kopresipitasie tot gevolg sal hê. Eksperimentele
geldigheidsbepalingtoetse het die waardes wat deur die modelle voorspel is bevestig en die
herhaalbaarheid van die eksperimentele data bewys.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/86538 |
| Date | 04 1900 |
| Creators | Mulwanda, James |
| Contributors | Dorfling, C., Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | xviii, 130 p. : ill. |
| Rights | Stellenbosch University |
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