Browsing by Author "Snyders, Cornelius Albert"

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    The adsorption and elution of Pt-, Pd- and Au cyanide using activated carbon
    (Stellenbosch : Stellenbosch University, 2015-03) Snyders, Cornelius Albert; Bradshaw, S. M.; Akdogan, G.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    ENGLISH ABSTRACT: In order to exploit lower grade and complex platinum group metal resources, cheaper and more efficient alternatives to the conventional mill-float-smelt-refine route are being sought. Leaching of platinum and palladium with cyanide has been proposed a number of times as a promising precious group metals (PGM) process option, and although platinum extractions are problematic, progress into the understanding of cyanide leaching of PGM containing ore and concentrate has been made. The platinum and palladium leaching will typically take place at elevated temperatures, which can range from 55°C on heaps to 180°C in autoclaves, with a better degree of leaching occurring with higher temperatures. Although this process for Pt and Pd extraction is a promising process option, research regarding the feasibility of the subsequent upgrading and recovery of the pregnant PGM leach solution, however, has been lacking. Since the carrier-phase extraction of gold using activated carbon offers significant advantages over other processes in terms of simplicity, the high pre-concentration factor, rapid phase separation, and relatively low capital and operating costs, activated carbon was deemed the most suitable sorbent for a Pt and Pd adsorption and stripping process. Very little is published on the adsorption of PGM cyanides onto activated carbon and when the effect of impurities such as base metals and thiocyanate together with a suitable elution method, are considered, no information could be found in the open literature. This study was launched and in general it was found that the activated carbon process does seem to be a viable process consideration for the upgrading of PGMs in a cyanide leach stream. Adsorption rates for dilute PGM solutions (0.15mg/L Pt, 0.38 mg/L Pd, 0.1 mg/L Au) in a stirred vessel indicated a high rate of adsorption within the first 60 minutes (giving more than 98% recovery of precious metals). A comparison of the Pt isotherm (25°C) to Au isotherms from literature indicated a similar loading capacity, while that of Pd was found to be significantly lower. In common with most diffusion controlled processes, an increase in the adsorption rate of platinum, palladium and gold cyanide with an increase in temperature was observed, while experiments with consecutive contacts of the PGM cyanide solution onto the activated carbon revealed that with an increase in temperature, the amount of PGMs that were adsorbed, decreased with each loading. In the absence of free cyanide and base metals, it was found that after 4 consecutive contacts, 99% of the total amount of platinum and palladium adsorbed at 25°C, compared to 85% of the platinum and 83% of the palladium at 50°C. No difference could be seen between the adsorption of gold cyanide at 25 and 50°C after 4 contacts. It has also been established that the detrimental effect of free cyanide on the adsorption of PGMs will increase as the temperature increases. The detrimental effect of the presence of Cu and Ni was found to depend on the amount of these base metals adsorbed, which in turn will depend on the cyanide concentration and the solution temperature. Adsorption of Pt and Pd has been found to be significantly more affected by temperature, cyanide and base metals than the adsorption of gold and needs to be carefully taken into consideration with the design of a PGM adsorption circuit to ensure sufficient Pt and Pd recovery. It is therefore highly likely that an activated carbon recovery process for Pt and Pd cyanide will not be as robust as the gold CIS (carbon-in-solution) process, which is considered to be one of its main advantages. The feasibility of eluting platinum and palladium cyanide complexes from activated carbon was investigated. It was found that platinum and palladium elute from activated carbon almost to completion in 4 to 5 bed volumes (BV) at 80°C, while the elution of gold at this temperature is slow, with a significant amount of gold (≈ 55 %) still to be eluted after 16 bed volumes. An increase in Pt and Pd elution kinetics was demonstrated with an increase in temperature with 99% recovery achieved at 4 BVs with an elution temperature of 95°C. Cyanide pre-treatment has been found to have a large influence on PGM elution. The effect of the NaCN concentration shows an increase in the recovery of Pt, Pd and Au as the cyanide increases from 0 to 2 %, after which the recovery starts decreasing again as the NaCN concentration increases from 2 to 4%. The NaOH concentration was also found to affect the PGM recovery and at 0% NaCN, an increase in the recovery is seen, while at a higher cyanide concentration (2 and 3% NaCN) a decrease in the PGM recovery occurs when the NaOH concentration is increased from 0.22% to 1.65%. A general decrease in Pt, Pd and Au recovery was seen as the ionic strength of the elution water increased and is consistent with literature on Au elution. The effect of a hydrochloric acid pre-treatment, which forms part of the process to remove calcium build-up from the activated carbon, was investigated, and for all the cases the Pt and Pd recovery increased when an acid pre-treatment was performed, compared to no acid pre-treatment. In none of the cases did any of the Pt, Pd or Au elute with the acid or the following rinsing water. The acid pre-treatment performed at 70°C removed a significant 64% to 75% of the Ni present and an additional 9.1% to 10.5% in the following rinsing water step. In the presence of copper cyanide, the elution order has been found to be copper, palladium, platinum and gold, which is the opposite order of adsorption preference. The cyanide pre-treatment has also been found to have a major influence on the elution of Cu and can be explained by the difference in the absorbance strength between the different copper cyanide complexes. The presence of Cu did not have a negative effect on the elution of the PGMs at strong pre-treatment (2% NaCN and 0.55% NaOH) conditions, but at weak pre-treatment conditions (0% NaCN) the recovery of Pt and Pd was reduced by between 10 and 18% after 5 BVs when Cu was present. The presence of 100 mg/L KSCN salt added to the leach solution during the adsorption stage, reduces the elution recovery of the PGMs at 4 BVs from 90% for Pt and Pd, when not present, to approximately 70% when present. The addition of the additional K+ ions reduced the recovery by less that 4% at 4 BV, which indicates that the possible formation of a PGM bonding with thiocyanate ([Pt(SCN)4]2- and [Pd(SCN)4]2-), which adsorb onto carbon, but doesn’t adsorb with water, cannot be ruled out completely. A maximum amount of 0.15% for Pt, 0.28% Pd and 0.6% Au was found to report to the pre-treatment solution at 25°C. For higher pre-treatment temperatures, the amount of Pt and Pd reporting to the pre-treatment solution increased significantly to approximately 8% at 80°C, while the increase in gold was marginal to 0.8% at 80°C. For the higher loading on the activated carbon (7000 mg/kg Pt and Pd), which is expected to be a better representation of plant conditions, 0.07% Pt, 0.11% Pd and 0.12% Au reported to the pre-treatment solution. The PGMs reporting to the pre-treatment solution is attributed to the distribution of the PGMs on the carbon particle, and even though very small, a certain amount of these PGMs would readily desorb or wash off the carbon. It has thus been established here that the stripping of adsorbed platinum cyanide complexes from activated carbon consists of a two step batch process, which involves the pre-treatment of the metal-loaded activated carbon with a relatively strong sodium cyanide and sodium hydroxide solution, prior to the elution step with de-ionized water at 80°C. Through the development of a mathematical model to describe this process, it was found that the rate of release of the platinum ions is governed by the amount of platinum and sodium on the activated carbon and the concentrations of these ions in the bulk of the liquid. This is mathematically described by a modified Freundlich isotherm equation and the mass transfer diffusion equation. The dependency of the platinum elution rate on the sodium concentration on the activated carbon, as suggested by gold related literature (Van Der Merwe and Van Deventer, 1990, Stange, 1990), is confirmed. Furthermore it has been found that the rate of platinum elution interchangeably depends on the equilibrium of the Pt ions at the carbon-liquid interface and the mass transfer of these Pt ions from the interface to the bulk liquid. As both of these rate-limiting factors were found to depend on the sodium concentration, the dominant platinum elution rate limiting factor shifts as the sodium concentrations change as the elution progresses. Four main time periods are used to simplify and to describe this process. The benefits of fundamentally understanding this process can ultimately lead to improved elution, better process control, shorter elution times, smaller elution columns or assist in the development of a continuous elution process.
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    Modelling the thermal, electrical and flow profiles in a 6-in-line matte melting furnace
    (Stellenbosch : Stellenbosch University, 2008-12) Snyders, Cornelius Albert; Eksteen, J. J.; Bradshaw, S. M.; Stellenbosch University. Faculty of Engineering. Dept. of Process Engineering.
    The furnace at Polokwane is designed to treat high chromium containing concentrates which requires higher smelting temperatures to prevent or limit the undesirable precipitation of chromium spinels. The furnace has therefore been designed to allow for deep electrode immersion with copper coolers around the furnace to permit the operation with the resulting higher heat fluxes. Deep electrode immersion has been noted to result in dangerously high matte temperatures. Matte temperatures however can be influenced by a number of furnace factors which emphasize the need to understand the energy distribution inside the furnace. Computational fluid dynamics (CFD) has therefore been identified to analyze the flow and heat profiles inside the furnace. The commercial CFD software code Fluent is used for the simulations. Attention has been given only to a slice of the six-in-line submerged arc furnace containing two electrodes or one pair while focusing on the current density profiles, slag and matte flow profiles and temperature distribution throughout the bath to ensure the model reflects reality. Boundary conditions were chosen and calculated from actual plant data and material specifications were derived from previous studies on slag and matte. Three dimensional results for the current, voltage and energy distributions have been developed. These results compare very well with the profiles developed by Sheng, Irons and Tisdale in their CFD modelling of a six-in-line furnace. It was found the current flow mainly takes place through the matte, even with an electrode depth of only 20% immersion in the slag, but the voltage drop and energy distribution still only take place in the slag. Temperature profiles through-out the entire modelling domain were established. The vertical temperature profile similar to Sheng et al. 1998b was obtained which shows a specifically good comparison to the measured temperature data from the Falconbridge operated six-in-line furnace. The temperature in the matte and the slag was found to be uniform, especially in the vertical direction. It has been found that similar results with Sheng et al. (1998b) are obtained for the slag and matte velocity vectors. Different results are, however, obtained with different boundary conditions for the slag/matte interface and matte region; these results are still under investigation to obtain an explanation for this behaviour. The impact of the bubble formation on the slag flow was investigated and found to be a significant contributor to the flow. With the bubble formation, it is shown that possible ‘dead zones’ in the flow with a distinctive V-shape can develop at the sidewalls of the furnace with the V pointing towards the centre of the electrode. This behaviour can have a significant impact on the point of feed to the furnace and indirectly affect the feed rate as well as the settling of the slag and matte. These results are not validated though. Different electrode immersions were modelled with a constant electrical current input to the different models and it was found that the electrode immersion depth greatly affects the stirring of the slag in the immediate vicinity of the electrode, but temperature (which determines the natural buoyancy) has a bigger influence on the stirring of the slag towards the middle and sidewall of the slag bath. The sensitivity of the model to a different electrode tip shape with current flow concentrated at the tip of the electrode was also modelled and it was found that the electrode shape and electrical current boundary conditions are very important factors which greatly affect the voltage, current density and temperature profiles through the matte and the slag. A detailed investigation to determine the electrode tip shape at different immersions, as well as the boundary conditions of the current density on the tip of the electrode is necessary as it was proven that the model is quite sensitive to these conditions. Several recommendations arose from this modelling work carried out in this investigation. Time constraints, however, did not allow for the additional work to be carried out and although valuable results were obtained, it is deemed to be a necessity if a more in-depth understanding of furnace behaviour is to be obtained. Future work will include the validation of the results, understanding the liquid matte model, investigating the MHD effects and modelling different furnace operating conditions.