Dual Filter Media for Solvent Extraction Electrowinning
One of the leading methods of extracting metals such as copper from their ores is solvent extraction and electrowinning (SXEW). This process can be enhanced by the use of garnet-anthracite dual filter media prior to the electrowinning step, affording significantly improved results from the final electrochemical process.
Overview of SXEW
SXEW is the electrodepositon of metals (electrowinning) from their mined ores that have been placed into aqueous solution by a leaching process, which has been nominally purified using a solvent extraction process. In a simple electrolysis set-up, the metal-containing leach solution is the electrolyte.
What is Solvent Extraction?
Solvent extraction is the chemical process of applying sequential aqueous and/or organic solutions to another solution to remove undesired impurities, and ensure that the desired product is located in a clearly-defined solution, in the case of solvent extraction ahead of electrowinning, this will be the aqueous phase.
The process of leaching results in a solution of metal ions and other impurities from the ore in a weakly acidic aqueous solution, producing a ‘pregnant leach’. This extractant solution is emulsified with an organic phase (‘diluent’) - typically oil based in metallurgical applications, often with high aromatic content, though aliphatic diluents have been used - and the desired metal ions move into the now ‘loaded’ organic phase. The old aqueous phase (‘raffinate’) is removed, taking some undesirables with it, and can be reused for leaching.
As the future electrowinning process requires metal ions in aqueous solution, the ‘loaded’ organic phase is next emulsified with a new strongly acidic aqueous phase (a ‘lean electrolyte’, typically sulphuric acid) into which the desired metal ions pass. The now stripped organic phase can then be discarded but is typically reused. The aqueous phase can move to the next phase, and is referred to as the ‘pregnant electrolyte’.
What is Electrowinning?
First used by Davy to extract sodium metal from molten sodium hydroxide, and then by Maximilian to extract copper, electrowinning is an electrochemical process that employs electricity to extract metals from solution(1). A current is passed from an inert anode through the ‘pregnant electrolyte’ (that has been acquired from the treatment by solvent extraction of the leach, as above) from the ore, and the desired metal is deposited on the cathode, which can be collected manually. Electrowinning is often associated with a much purer extraction compared to that of smelting. After an electrowinning process, ‘spent’ electrolyte can be reused to extract further metal ions from the ‘loaded’ organic phase from earlier.
Filtration can Enhance the SXEW Process
With any chemical process, small quantities of residual solids can prevail through a solvent extraction process - especially at scale. Other contaminants can include residual amounts of the organic phase, and thus the compounds dissolved within. A filtration process removes any of these undesired materials, providing a cleaner and purer electrolyte for the electrowinning process, thereby increasing the efficiency of that process and reducing the likelihood of any byproducts. A common feature in ores of copper and cobalt from East Africa, and the rocks in which they are found, is silicate-type compounds. Without adequate treatment, silicate/silica compounds (colloidal, gel-type, or dissolved) can cause severe problems in the SXEW process such as ‘crud’ formation, difficulty and/or slowing during phase separation and losses to the raffinate. Untreated, silica compounds can reach the electrowinning system and, due to the highly acidic nature of the electrolyte, polymerise and potentially deposit on the electrodes - significantly reducing the efficiency of the system(2).
How and why?
In the industrial SXEW process, filtration occurs between solvent extraction and electrowinning and is a key step in ensuring optimal electrowinning. Across the industry, many different filtration pathways are used - mostly based on size exclusion. A sequential approach to filtration is common, whereby the aqueous phase is stepped down to only containing contaminants on the low-end micron scale. This has inherent advantages in and of itself - a step-down process is more effective than a single filtration step.
Depending on process location, scale and the desired isolated metal, different filtration media are used. Furthermore, as solvent extraction occurs at an aqueous-organic interface, the presence of any foreign material could impact and interfere with the chemical and physical performance of the phase transfer. As the SXEW process is commonly run using a process that reuses ‘spent’ or ‘stripped’ aqueous and organic solutions, removal of contaminants is crucial prior to solution re-use. Increasingly popular - especially for extraction of copper - is a dual anthracite and garnet filtration medium, this is a solvent extraction filter media. A typical filter design is referred to as a ‘column’, where a dense layer of filtration media is packed into a large tube and the filtrate is pumped through it from the top. In the case of dual filtration media, layers of media are separated by a thin band of sand, with the coarser media towards the top of the column.
Why Anthracite and Garnet as a Dual Filter Media for Solvent Extraction?
Anthracite for solvent extraction and garnet for solvent extraction are often used together as dual filter media for solvent extraction. Both are common naturally occurring products.
Garnet filter media is found in nature with almost no cases of impurity; as a filtration medium, it is used in crushed form. Resistant to chemical treatments, abrasion and expansion due to heat, it is a reliable filtration medium for long term, high throughput applications. Garnet filtration has been demonstrated as effective as low as 10 microns. This making garnet for solvent extraction to be a very appropriate media to use.
Anthracite filter media is a type of coal with the highest levels of carbon content and the fewest impurities amongst its peers. As over 479 kilotonnes were produced in 2016, anthracite is a cost effective filtration media, with high grade and ultra high grade anthracite being used extensively in metallurgical applications. To gain greater surface area, anthracite for solvent extraction is finely ground before being used in a filter.
In a series configuration, several deionised beds containing anthracite and garnet are used SX filter media. Anthracite is used as a coarser, top-layer filter media, removing the vast majority of organic residues. It is followed by finely ground garnet, which acts to remove solid contaminants down to the micron scale. This dual filter media for solvent extraction is highly effective, inexpensive and scalable.
Certain production facilities in Zambia have employed six anthracite/garnet dual media filter columns in parallel, ahead of the electrowinning step, producing up to 800 m3 per hour of clean, advance electrolyte(4). According to some studies, modern facilities employing an SXEW process for the realisation of copper from its ore achieve in excess of 90% current efficiency when also using dual media filtration(5,6), with annual production rates in 2014 of almost 200 kT and 15 kT of high purity copper and cobalt respectively.
In filtration set-ups using ground anthracite and garnet, particle sizes of between 3 and 100 mesh are used in a loading range of 0.5 to 2.4 m3 per m2 of filter media, and flow rates of 3.5 to 189 litres per minute can be achieved(7).
In addition to copper and cobalt, SXEW has been useful for the extraction of zinc, iron(8) and nickel(9) from their ores in high purity, or when they have been co-located with copper or cobalt.
- Dual filtration media for solvent exchange enhances the electrowinning process by preventing organic residues and undesirable solids from reaching the electrowinning stage
- Solvent exchange is the method of using sequential organic and aqueous solvents to ensure metal ions are highly concentrated in a solution suitable to electrolysis, and other compounds are discarded
- Electrowinning is the electrochemical process where solid metal is deposited on an electrode upon application of current in an electrolyte
- SX filter media is used in dual filtration using anthracite filter media and garnet filter media as a robust means to prevent contamination of the electrowinning process, and the overall SXEW cycle
- SXEW using such dual filter media can be used to isolate copper and cobalt, amongst others, from their ores in high purity
- A. Philip (ed.) The Electro-plating and Electro-refining of Metals, Lockwood, London, 1902
- K. C. Sole, F. K. Crundwell, N. Dlamini and G. Kruger, 9th Southern African Base Metals Conference, Livingstone, 2018
- K. C. Sole and O. S. Tinkler, J. S. Afr. Inst. Min. Metall., 2016, 116, 553-560
- K. C. Sole et al. Hydrometallurgy, 2005, 78, 52-78
- G. A. Kordosky, J. S. Afr. Inst. Min. Metall., 2002, 102, 445-450
- US Patent US3842001A, 1973 (expired)
- S. Sandoval et al. J. S. Afr. Inst. Min. Metall., 2016, 116, 497-507
- G. Cote, Solvent Extr. and Ion Exch., 2000, 18, 703-727
- G. Bacon and I. Mihaylov, J. S. Afr. Inst. Min. Metall., 2002, 102, 435-443