Recovery of Precious Metals Using Precipitation, Adsorption, Electrowinning, Supercritical Fluids and Bio-mediated Approaches
RECOVERY OF PRECIOUS METALS BY PRECIPITATION, ADSORPTION, ELECTROWINNING AND BIOSORPTION/ BIOACCUMULATION: AN OVERVIEW
Extensive literature exists on the recovery of precious metals by cementation/precipita- tion, adsorption, agglomeration, bio-mediated approaches, solvent extraction, ion exchange, and so on, but there are limited studies dealing with real-time leach liquors from secondary sources and most of the work has been conducted with simulated/synthetic solutions that hinder the efficient implementation of the technologies at commercial scale.
To fulfil the sustainability targets and move towards a circular economy, the recovery of precious metals from processed urban mine liquors to production is an absolute necessity. Recovery processes are essentially employed to obtain metals of high purity for further applications, depending upon the concentration of leach liquors, availability of cheap electric power, and other economic considerations. This chapter reviews the feasibility of precious metal recovery from cyanide, halide, thiosulphate, and thiourea-based leach liquors by precipitation, adsorption, nano-accu- mulation. and electrowinning. The fundamental principles and the effects of impurity metals on the recovery process are highlighted for each approach. Bio-metallurgical recovery of precious metals from various secondary liquors by biosorption are also discussed. The metal-binding ability of various biomaterials including algae, fungi, bacteria, and yeast, as well as certain biopolymers and bio-waste materials, is discussed with reference to precious metals.
RECOVERY OF PRECIOUS METALS BY PRECIPITATION/ CEMENTATION
Precipitation involves the conversion of soluble metal ions to insoluble form by chemical reaction with metal ions and precipitating agents. The choice of precipitant depends upon its selectivity and crystalline size (for easy filter and wash), and it should be possible to form quite stable and insoluble precipitates for quantitative recovery in later steps. Generally, precipitates are formed at a specific pH range; since most of them are re-dissolved outside this range, most are more insoluble in cold solutions and decompose at high temperature. The presence of a complexing ion can inhibit the precipitation process and oxidizing or reducing conditions are mostly necessary for efficient precipitation. Co-precipitation can occur if contaminants are soluble under the precipitation conditions. Metal ions in various leach liquors of secondary wastes/urban mine sources can be effectively precipitated as reduced metals, metal hydroxides, metal carbonates, metal sulphides, and metal oxalates, and cemented to attain selective recovery.
The direct precipitation of metals from an aqueous solution of their salt is of great interest. Thus gold is precipitated from chloride solution obtained by aqua regia leaching of gold bullions by ferrous ions or oxalic acid as follows:
Palladium and platinum are precipitated by formate ions:
Based on these reactions, gold and the platinum metals are selectively separated from spent electrolytes during the electrolytic refining of gold on a commercial scale. Similarly from chloride-based liquors, platinum can be precipitated as the ammonium hexachloroplatinate and the metal is recovered by thermal decomposition of the precipitate (Habashi et al„ 1987):
The precipitation of a metal from an aqueous solution of its salts by another metal is known as cementation, because the precipitated metal is cemented on the surface of the added metal. This phenomenon was first described by alchemists as transmutation. A piece of iron was dipped in copper sulphate solution and coated with a layer of metallic copper. The cementation process can be predicted in terms of electrode potentials. ТА metal with more positive potential, in the electromotive series, will displace a metal with a less positive potential.
The electrochemical order of metals in a KCN solution indicates the sequence (from positive to negative) as:
Each metal dissolves more readily than the metals on its right, and will precipitate those metals from the solution. According to this sequence, aluminium will displace gold and silver more readily than will zinc.
In the case of halide leach liquors, cementation is one of the simple ways to recover precious metals, especially from commonly used chloride leaching. Reductive precipitation of gold is performed by contacting the leach liquor with a metal having a potential above that of the precious metal in the electrochemical series. In the case of Au-,+- the metals (M generally stands for iron and aluminium) which can form trivalent ions in chloride solution are usually employed due to their half-cell reactions as follows:
The precipitation reaction of gold can be commonly written as:
In principle, the metal M should be dissolved to precipitate the gold from leach liquor, hence the pH is always maintained in the acidic range.
Based on the above reactions, gold and platinum metals can be separated on a commercial scale from spent electrolytes during the electrolytic refining of gold (Habashi et ah, 1987). Further studies were performed by Yousif (2019) on selective precipitation of Pt, Pd. and Rh from spent catalytic converters. Prior to precipitation, the distillation of HC1 was conducted by evaporation at 190°C, which also leads to hydrolysis of palladium and rhodium species after dilution. The individual precipitation of Pt in the presence of Pd and Rh was performed by adding NH4C1 (290 g/L) at 40C with continuous stirring. A yellowish Pt precipitate of (NH4)2[PtClJ was obtained, which was filtered, washed with ammonium chloride solution (140g/L), calcined at 800C, and dried to obtain a fine Pt powder of more than 99.5% purity. To selectively precipitate palladium in the presence of rhodium, the filtrate from the platinum stage was evaporated and sodium chlorate was added slowly (3g) with constant stirring until a bright red precipitate of (NH4)2[PdCl6] was formed, which was calcined at 900°C. Finally, rhodium from the filtrate was precipitated with KOH as lemon-yellow rhodium hydroxide (Rh(OH),). filtered, water washed, air-dried (decomposed to Rh,0,) and ignited at 1150‘C to produce a grey Rh metal powder of 95.4% purity.
Recovery of precious metals by cementation from cyanide leach liquors were first practised on an industrial scale by MacArthur with zinc shavings in the 1890s. In 1900, C. W. Merrill introduced zinc dust to achieve more efficient recovery of gold (Habashi et ah, 1987). The addition of zinc also evolved hydrogen gas that can contribute to gold precipitation; however, gold is not precipitated by hydrogen at atmospheric pressure. The following reactions take place:
Barin et al. (1980) proposed an overall chemical reaction by considering the hydrogen evolution in gold cementation by zinc, as below:
Cementation is a heterogeneous redox reaction controlled by the rate at which auro- cyanide and cyanide ions are transferred to the zinc surface (Nicol. 1979; Finkelstein, 1972; Fleming, 1992). The reductive precipitation of precious metals by zinc on an industrial scale was further improved by introducing oxygen instead of air to the leach liquor.
A minimum of 0.1 g/L to 1.7 g/L NaCN concentration is critical for cementation of precious metals like gold (Nicol, 1979; Barin et al., 1980). The concentration of metal itself has a direct influence on the cementation rate, which is essentially a first- order reaction controlled by the transfer rate of metal-cyanide ions. Although a change in solution pH (in the range of 9 to 12) has no appreciable effect on cementation, a higher pH may lead to formation of intermediate hydroxides, which can retard or sometimes stop the cementation process. Finkelstein (1972) reported that anions such as sulphate, sulphide, thiosulphate, and ferrocyanide might reduce gold precipitation yield by 1-2% from 10~3 M cyanide solutions. The free sulphates may precipitate as gypsum to reduce reactivity in the cementation process. Nicol (1979) found that sulphide ions can passivate the zinc surface even at lower concentrations of lxlCH M. For efficient cementation recovery of gold, the leach liquor should not contain > 5 ppm suspended particles and > 1 ppm dissolved oxygen, with a free cyanide concentration > 0.035 M at pH in the range of 9 to 11.
Reductive precipitation of precious metals from the impregnated (thiosulphate- based) leach liquor using inorganic zinc metal (Merrill-Crowe process) or organic acid (predominantly oxalic acid) is a common process in gold recovery; however it is not very effective in the thiosulphate-ammonia system. The metal precipitants often have a deleterious effect on thiosulphate ions, producing unwanted cations and thus complicating the lixiviant recycling process. The contamination of solid products is often a result of either undissolved (excess) precipitant or co-precipitation with other metal ions, necessitating further purification. Copper is a reasonable choice, as gold- depleted copper solution can be directly recycled to the leaching stage. Precipitation by the addition of sulphide salts or by chemical reduction with sodium borohydride, hydrogen or sulfphr dioxide has also been investigated (Awadalla and Ritcey, 1991; DeschSnes and Ritcey, 1990; Johnson and Bhappu, 1969). These techniques are not much favoured, as they are less selective and tend to precipitate most metals from solution as well as hindering the recycling of the leach liquor. The electro-reduction of aurothiosulphate ions to deposit on the cathode is especially problematic in the presence of a great excess of unwanted cations of copper, which get co-deposited on the cathode product. This results in a devalued product requiring further purification. Side reactions involving the oxidation or reduction of thiosulphate may also interfere (Aylmore, 2001). This lowers the efficiency of electrowinning by increasing the energy input required to recover the desired metals from solution, making it an unvi- able option for recovering the precious metal.
Reduction precipitation from thiourea leach liquor is one of the main ways of recovering precious metal. It is mostly aluminium, iron, and lead that are used for this purpose. However, reports reveal that using A1 as the precipitant metal does not yield complete recover)', leaving a 2% gold ion in the solution (van Lierde et al., 1982). The US Bureau of Mines has reported ~99% recovery of precious metals from thiourea leach liquor at the cost of6.4 kg consumption of Al for each kg of the precious metals (gold and silver). In principle, thiourea leach liquor contains iron, hence, iron is preferred for cementing gold by the following reaction (Groenewald, 1976; Zouboulis et al., 1993):
In comparison to Fe, using Pb powder for gold cementation yielded quite significant results from HCl-thiourea solution (Wen. 1982). The cementation reaction with lead powder can be written as (Tataru 1968):
A comparatively lower recovery using Fe is described in the study by Wang et al. (2011), finding that the presence of oxidants in leach liquor negatively affects the gold cementation reaction. The ferric ion significantly hinders the cementation process because of the increased redox potential of the solutions, which signifies the presence of a suitable electron acceptor. It may cause the Fe powder to be consumed by ferric ions, as follows:
The removal of iron powder lowers the availability of this reductant for the cementation of gold, resulting in a negative effect on gold cementation. The addition of tri-sodium citrate to the system can potentially control the redox potential of the solution by forming a ferric-citrate complex, useful for enhancing gold recovery by controlling the ratio of Fe-,+/Fe2+.