Characterizing the role of polyacrylamide additives in copper electrowinning

Coetzee, Christiaan (2018-12)

Thesis (MEng)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Additives have been used for many years in the copper electrowinning process to improve cathode morphology. The industry has leaned towards polysaccharides in the form of Guar. Polyacrylamides however have been considered as possible alternatives. To get more value from the functionality of these additives it is necessary to understand the influence of structural chemistry on the resulting performance during electrowinning. This project investigated structurally different polyacrylamides (PAMs) and compared them to Guar and characterized their effect on copper electrodeposition. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques were used to characterize the effect of molecular weight (MW) and ionic charge of polyacrylamides on electrodeposition. The findings were compared against results from a Guar product, utilized in the industry. During EIS and CV experiments copper was electrodeposited onto a platinum surface from a synthetic electrolyte containing 35 g/L copper (Cu2+), 160 g/L sulfuric acid (H2SO4), and 25 mg/L chloride (Cl-) at 45ᵒC. The EIS and CV experimental work were conducted at additive concentrations of 2 mg/L and 10 mg/L. EIS equivalent circuit modelling results indicate that an increase in polyacrylamide concentration and a decrease in additive molecular weight cause an increase in the overall system resistance. The charge transfer resistance (RCT) values at 2 mg/L additive concentration gradually increased from 3.36 Ω to approximately 7 Ω as the PAM additive MW decreased from very high to low. CV results of additives at 2 mg/L show that there was no significant effect on nucleation overpotential. However, significant plating polarization was observed for lower molecular weight additives, decreasing in significance as the molecular weight of the additive increased. The measured average current density at 0.22 V vs. SHE on the return sweep during CV experimental work systematically increased from 277.35 A/m2 for the Low MW PAM additive to 326.39 A/m2 for the Very High MW additive. At the same experimental conditions, an average current density of 337.00 A/m2 was measured in the absence of any additives. CV results at 10 mg/L additive show that there was significant nucleation polarization and plating polarization, correlating well with the EIS results. The industry standard organic additive Guar had no significant impact on the charge transfer resistance or the polarization behaviour at both 2 mg/L and 10 mg/L concentration. No correlation between the ionic content of the additives and system polarization were observed. Scanning electron microscopy (SEM) micrographs of copper deposits after 10 s and 30 s of electroplating on stainless-steel cathode tips at 2 mg/L additive concentration provided strong qualitative validation for the polarization trends observed on a fundamental level. The effect of polarization or increased charge transfer resistance was identified in the structure of the deposits. A spiky, dendrite like crystal structure was observed due to fast unrestricted deposition when no additives were added to the system, while smoother spherical 3-D crystal structures were obtained when using Guar. The polyacrylamide additives systematically produced a flatter 2-D compact crystal growth structure with a decrease in additive molecular weight. Electrowinning experiments on stainless steel plates in a laboratory (capacity) scale setup at 300 A/m2 and 40 ᵒC for 24 hours of electrodeposition were conducted to determine if microscale deposit structures will be sustained after multi-layers of deposition have occurred. Bench scale electrowinning results showed that PAM additives acted to radically reduce localized growth compared to copper cathode plates produced in the absence of any additive and in the presence of Guar. The copper plate in the absence of any additives developed abundant localized growth and dendrite structures. The mean surface deviation (Sa) value, which is a measure of surface roughness, was calculated for all copper plates that was produced in the presence of each organic additive. The calculated Sa value for the copper plate in the absence of any additive and in the presence of Guar was 82.22 μm and 32.87 μm respectively. The Low MW PAM additive produced the smoothest, brightest copper deposit of all the deposits and had the lowest calculated Sa value of 18.18 μm. The Very High MW PAM additive was the only additive to show inconsistency between the polarization behaviour trends, and the predicted deposit structure obtained in the bench scale experimental work. Current efficiency (CE) values between each electrowinning experiment varied with less than 1%. Such insignificant variation indicates that the organic additives had minor impact on the CE of the electrowinning experiments. It was concluded that both qualitative and quantitative analyses are necessary to evaluate the performance of organic additives in copper electrowinning and that they should be used in conjunction with one another. In future work, it is recommended that a molecular weight range should be proposed for optimal additive performance by determining the numerical molecular weights of the additives. Varying the most significant operating conditions like current density, temperature, and impurity type and concentration in the presence of the selected organic additives will provide insight into ideal operational ranges in the presence of these organic additives. SEM micrographs of cross sections of the resulting cathode plates in bench scale electrowinning work will aid in further characterization of the crystal structure that developed in the presence of each organic additive respectively.

AFRIKAANSE OPSOMMING: Bymiddels word al vir baie jare in die koper elektroherwinning proses gebruik om katode morfologie te verbeter. Die industrie het na polisakkariedes in die vorm van guar geneig. Poliakrielamiede word egter as moontlike alternatiewe oorweeg. Om meer waarde uit die funksionaliteit van hierdie bymiddels te kry, is dit noodsaaklik om die invloed van die strukturele chemie op die resulterende werksverrigting tydens elektroherwinning te verstaan. Hierdie projek het struktureel verskillende poliakrielamiede ondersoek en dit vergelyk met guar en hul effek op koper elektroneerslag gekarakteriseer. Sikliese voltammetrie (SV) en elektrochemiese impedansie spektroskopie (EIS) tegnieke is gebruik om die effek van molekulêre gewig en ioniese lading van poliakrielamiede op elektroneerslag te karakteriseer. Die bevindinge is vergelyk met die resultate van ’n guarproduk wat in die industrie gebruik word. Tydens EIS en SV eksperimente is koper neergeslaan op ’n platinum oppervlak vanuit ’n sintetiese elektroliet by 45 °C wat 35 g/L koper (Cu2+), 160 g/L swaelsuur (H2SO4), en 20 mg/L chloried (Cl-) bevat het. Die EIS en SV eksperimentele werk is uitgevoer by bymiddelkonsentrasies van 2 mg/L en 10 mg/L. EIS ekwivalent stroombaan modellering resultate dui aan dat ’n verhoging in poliakrielamied-konsentrasie en ’n afname in bymiddel molekulêre gewig ’n verhoging in die algehele stelsel weerstand veroorsaak. Die ladingoordrag-weerstand (RCT) waardes by 2 mg/L bymiddelkonsentrasie het geleidelik van 3.36 Ω na ongeveer 7 Ω verhoog soos wat die poliakrielamied bymiddel molekulêre gewig verlaag het van baie hoog na laag. SV resultate van bymiddels by 2 mg/L het gewys dat daar geen beduidende effek op kernvorming oorspanning is nie. Beduidende platering polarisering is egter waargeneem vir laer molekulêre gewig bymiddels, wat verminder in beduidendheid soos die molekulêre gewig van die bymiddel afgeneem het. Die gemete gemiddelde stroomdigtheid by 0.22 V (teen standaard waterstof elektrode) op die terug veeg gedurende SV eksperimentele werk het sistematies verhoog van 277.35 A/m2 vir die lae molekulêre gewig poliakrielamied bymiddel na 326.39 A/m2 vir die baie hoë molekulêre gewig bymiddel. By dieselfde eksperimentele kondisies is ’n gemiddelde stroomdigtheid van 337.00 A/m2 gemeet in die afwesigheid van enige bymiddels. SV resultate by 10 mg/L bymiddel wys dat daar ’n beduidende kernvorming polarisering en platering polarisering was, wat goed korreleer met die EIS resultate. Die industrie standaard organiese bymiddel, guar, het geen beduidende impak op die ladingoordrag-weerstand of die polarisering gedrag by beide 2 mg/L en 10 mg/L konsentrasies gehad nie. Geen korrelasie tussen die ioniese inhoud van die bymiddel en stelsel polarisering is waargeneem nie. Skandeerelektronmikroskopie (SEM) mikrograwe van koperneerslae na 10 s en 30 s van elektroplatering op vlekvrye staal katodepunte by 2 mg/L bymiddelkonsentrasie het die polarisering tendens wat op ’n fundamentele vlak waargeneem is kwalitatief gestaaf. Die effek van polarisering of verhoogde ladingoordrag-weerstand is geïdentifiseer in die struktuur van die neerslag. ’n Spitsige, dendrietagtige kristalstruktuur is waargeneem as gevolg van die vinnige onbeperkte neerslag toe geen bymiddels tot die stelsel bygevoeg is nie, terwyl gladder sferiese 3-D kristalstrukture verkry is toe guar gebruik is. Die poliakrielamied bymiddels het sistematies ’n platter 2-D kompakte kristal-groeistruktuur geproduseer met ’n afname in bymiddel molekulêre gewig. Elektroherwinning eksperimente op vlekvrye staalplate in ’n laboratorium (kapasiteit) skaal opset, by 300 A/m2 en 40 °C vir 24 uur van elektroneerslag, is gedoen om te bepaal of mikroskaal neerslagstrukture gehandhaaf sal word na veelvuldige lae van neerslag plaasgevind het. Banktoetsskaal elektroherwinning resultate het gewys dat poliakrielamied bymiddels gelokaliseerde groei radikaal verlaag het in vergelyking met koper katode plate geproduseer in die afwesigheid van enige bymiddel en in die teenwoordigheid van guar. Die koperplate in die afwesigheid van enige bymiddels het oorvloedige gelokaliseerde groei- en dendrietstrukture ontwikkel. Die gemiddelde oppervlak afwyking (Sa) waarde, wat ’n mate van oppervlak grofheid is, is bereken vir alle koperplate wat geproduseer is in die teenwoordigheid van elke organiese bymiddel. Die berekende Sa-waarde vir die koperplaat in die afwesigheid van enige bymiddel en in die teenwoordigheid van guar is 82.22 μm en 32.87 μm onderskeidelik. Die lae molekulêre gewig poliakrielamied bymiddel het die gladste, helderste koperneerslag van al die neerslae geproduseer en het die laagste berekende Sa-waarde van 18.18 μm. Die baie hoë molekulêre gewig poliakrielamied bymiddel was die enigste bymiddel om inkonsekwentheid te wys tussen polariseringgedrag tendense en die voorspelde neerslagstruktuur verkry in die banktoetsskaal eksperimentele werk. Stroomrendement (SR) waardes tussen elke elektroherwinning eksperiment het gevarieer met minder as 1%. Sulke onbeduidende variasies toon aan dat die organiese bymiddels ’n geringe impak op die SR van die elektroherwinning eksperimente gehad het. Die gevolgtrekking was dat beide kwalitatiewe en kwantitatiewe analises nodig is om die werkverrigting van organiese bymiddels in koper elektroherwinning te evalueer, en dat hulle in medewerking met mekaar gebruik moet word. In toekomstige werk word dit voorgestel dat ’n molekulêre gewig bestek voorgestel moet word vir optimale bymiddel werkverrigting deur die numeriese molekulêre gewigte van die bymiddels vas te stel. Om die mees beduidende operasionele kondisies te wissel, soos stroomdigtheid, temperatuur, onsuiwerheid tipe, en konsentrasie in die teenwoordigheid van die gekose organiese bymiddels, sal insig verskaf in die ideale operasionele bestek in die teenwoordigheid van hierdie organiese bymiddels. SEM mikrograwe van deursnitte van die resulterende katode plate in banktoetsskaal elektroherwinning werk sal verdere hulp verleen in die karakterisering van die kristalstruktuur wat ontwikkel in die teenwoordigheid van elke organiese bymiddel, onderskeidelik.

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