Centrifuge technology in SX process provides solvent loss reduction

The solvent extraction (SX) process is a mining operation where metals such as copper, platinum, gold, cobalt, nickel, zinc and uranium are recovered using chemical agents. After the SX process is the electrowinning (EW) process, also called electroextraction, which is the electrodeposition of metals that have been put in solution through a process commonly referred to as leaching. Electrorefining uses a similar process to remove impurities from metals. Most commercial SX plants suffer from the formation of crud — a solid-stabilized emulsion that commonly accumulates at the aqueous/organic interface in the settlers of solvent-extraction stages. It is caused by a variety of substances entering the SX circuit, ranging from wind-blown dust, entrained solids from leaching, impurities in plant solutions, and even insects attracted by the lights in the SX plant. While a thin layer of crud at the aqueous/organic interface can aid the coalescence of fine droplets, excess crud can interfere with phase separation and severely reduce the efficiency of the settlers. If allowed to build up in the settlers, crud can start moving to the next mixing stage. This can rapidly cause a “crud run,” resulting in severely increased entrainment. This causes contamination of electrolytes with the leach solution and organic phase. The transfer of organic to the electrowinning can cause “cathode burn.” The entrainment of electrolyte into the extraction section of the solvent extraction can result in loss of copper to the raffinate (in this case, when refining copper). Entrainment of organic in the raffinate will result in increased organic losses. While removal of crud from SX plants does not usually pose a major problem, crud typically has an organic content of more than 50 percent. Losses of organic with the crud can cost plants hundreds of thousands of dollars per year. Solving crud in the SX process with technology Centrifuges help to avoid losses of the organic solution while maintaining the efficiency of the solvent loading (SL) and the stripping process. The SX process helps to maintain the control of the pH value and extend cathode life in electrowinning to avoid plant downtime due to settler cleaning. A variable impeller allows for precise on-the-fly adjustment. This optimizes the purity of recovered organic solution even with fluctuating process conditions, and provides a maximum solvent recovery that reduces solvent losses by up to 60 percent. This process helps refineries that produce 20 t/a (22 stpy) of copper or more. Centrifuges are also used for this purpose in small, medium and large copper mines worldwide and are specifically used in mines that use the SX process. Plant manufacturers, design offices and engineering, procurement and construction (EPC) contractors, who are active in mining, can also benefit from the use of centrifuges for this critical process. Case study 1 — Bwana Mkubwa, Zambia For Bwana Mkubwa, a copper producer in Ndola, on the Zambian Copperbelt, the inability to process crud efficiently resulted in several SX plants struggling with a buildup of crud in the SX settlers. This caused severe problems in the SX process and reduced the availability of the process by as much as 10 percent. Zambia and the Democratic Republic of the Congo account for 1.2 Mt/a of the SX/EW of copper production, of which at least 80 percent is produced in plants using three-phase centrifuges for crud treatment. At a savings of $12/t of copper, that amounts to an annual savings of $12 million on just this one application in these two countries. The challenge. Twenty years ago, most copper refineries worldwide were struggling with significant financial losses caused by the formation of crud in their SX plants. Removal of the crud from the process, while essential, resulted in organic extractant and diluent losses that amounted to hundreds of thousands of dollars annually. Standard recovery techniques using filter presses were laborious and ineffective. Even after filtration, the crud still contained significant quantities of organic solution. The buildup of crud in the SX settlers often caused severe problems in the SX process and reduced the availability of the process by as much as 10 percent. The residual solids had to be treated as hazardous waste because the organic solution is extremely hazardous for the environment. The solution. Using a three-phase centrifuge, the organic could be recovered as a clean solution, which could be fed directly back to the SX plant. The savings on organic solution alone amounted to more than $12/t of copper produced. A plant producing 50 kt/a of copper would therefore save approximately $600,000 annually. There were additional savings in labor, filter cloths and maintenance. Finally, the efficient crud treatment process made it possible to keep crud levels in the SX plants under control. This led to additional savings, as the stability of the SX plant operation improved and its efficiency increased. Disposal of the residual solids also became easier as their organic content was very low. The process. Crud needs to be seen as a batch operation. The crud is sucked out of the settlers by operators using diaphragm or peristaltic pumps. The composition can change significantly from day to day. The crud is collected in a feed tank and agitated with a low shear agitator. The solids concentration in the crud normally ranges from two to 10 percent. The crud is fed to the three-phase centrifuge, and the operators make adjustments to the variable impeller to ensure that the organic solution comes out clean. It must be noted that most crud around the world do not separate into three phases under centrifugation, but rather four phases. A stable fourth phase builds up at the organic aqueous interface. By adjusting the variable impeller of the three-phase centrifuge correctly, the operator can ensure that this portion of ultra-fine solids is discharged as part of the aqueous phase. This is the reason the aqueous phase is not clear, but rather, a muddy brown. This aqueous phase has to be sent to tailings. It cannot be returned to the SX plants because it would simply provide seed for more crud to grow on. Summary. Twenty years later, most copper SX plants around the world are using three-phase centrifuges for SX crud treatment. More than 80 machines are currently in operation worldwide. When one considers that at present there are only about 59 SX plants in operation with copper cathode production exceeding 20 kt/a, the success of the three-phase centrifuge in this application is clear. The three-phase centrifuge normally becomes a viable option for copper SX/EW refineries producing 20 t/a of copper or more. With the savings it generates in organic recovery, a three-phase centrifuge can pay for itself in about four months. The reliability of the machine is also a major advantage as most of these machines are installed in remote regions. Three-phase centrifuges that were installed in Zambia 16 years ago are still operating without problem. While copper SX crud remains the main application for three-phase centrifuges in the hydrometallurgical field, they have also been successfully employed in uranium, molybdenum, lithium and zinc SX processes. The three-phase centrifuge has become the standard for crud treatment equipment around the world. Case study 2 — Multiple copper production projects in China Chinese companies supporting copper crud plants in Africa and Myanmar still use chamber filters for the copper crud as their standard. The chamber filter press is well established. However, through trial runs at several plants, process engineers learned they could save more than 60 percent solvent after using a three-phase centrifuge compared with chamber filters. The challenge. Many three-phase centrifuges were being used successfully in South America, the United States, Australia and Africa for copper SX crud applications. But Chinese companies had not adopted centrifugation yet and were still relying on chamber filters for the copper crud process. The solution. In a technical meeting in 2013, the China Wanbao Mining Myanmar YangTse Copper Mining project team showed interest in centrifuge technology. However, Flottweg could not convince the customer as China Nerin Engineering had designed their process using chamber filter technology, and their budget was limited to 30,000 euros. In 2015, the chamber filter press could not meet expectations. In April 2016, the three-phase centrifuge was commissioned in the copper crud application in Myanmar. Because it was the first Chinese application, there was a steep learning curve, which was addressed and overcome. Challenges were found around few settled fine solids, and higher-than-expected wear caused by excessive bowl speed. An adjustment was made to the separation disc design, and updates to the scroll provided heavier wear protection using tile protection. The result was a savings of 60 percent solvent after switching to the three-phase centrifuge versus the chamber filter. Consequently, neighboring mine Myanmar Wanbao Mining, which is situated 10 km (6 miles) away from YangTse, also incorporated the centrifuge technology. Conclusion. In 2018, YangTse bought their second centrifuge for their current works. A third SX plant was built in 2022 using three-phase centrifuges. Case study 3 — Various projects by Fishtec Ltda., Chile In Chile, mining companies have used SX for many decades in the copper refining process. The Chilean mining industry was not familiar with the three-phase centrifuge technology and used either filter press technology or were forced to discard too much solvent. They were losing product and generating a lot of waste on this new residual phase (CRUD — a mix of organic, aqueous, air and suspended solids). By constantly removing and treating the crud through three-phase centrifuges, companies found they reduced the input of new extractant to SX plants by between 40 and 60 percent, with significant financial savings. The challenge. By using filter press technology, the Chilean mining industry was discarding too much solvent, losing product and generating a lot of waste. The solution. Well established in Chile in the fishing industry since the 1990s, this has always been Fishtec Ltda.’s core business. In 1995, Fishtec was asked to test a sample of crud from a small mining company, Pudahuel Mine. Interestingly, the test showed that this product was remarkably like a sardine production. The similarity was that there are four phases in a mix that must be divided into three. The three-phase centrifuge achieves great results thanks to the adjustable impeller. The fourth phase was separated via the heavy liquid, and the organic was recovered totally clean. The new application for the three-phase centrifuge became the best option in this process, improving product quality, reducing waste disposal cost and providing lower operational and maintenance costs. After this trial, and because SX was gaining acceptance globally as the most common process in copper mines, the Chilean market opened and three-phase centrifuges were installed at different mines: Escondida Coloso (BHP Coloso, 1995) and Quebrada Blanca (Teck, 1995). Fishtec’s strategy was to “wow” customers with the consistently satisfactory performances. Since then, Fishtec has opened the market and achieves almost 95 percent of the SX plants by using three-phase centrifuge technology. In 2020, a pilot unit was installed for Codelco Salvador. Codelco is a Chilean national company with six active mine sites, two of which have three-phase centrifuge units in their SX operations. The results were very good, and the client took the initiative to purchase a new three-phase centrifuge unit. The recovery of solvent achieved by the three-phase centrifuge on site is the best solution for a steady-state application with predictable performance. The process. Fishtec provides a complete solution for the client, adapting the three-phase centrifuge to the site where it is installed. This is true for both greenfield and existing plant sites, adding all the interfaces and aspects that the customer requires on site — from a pump to an operator for the centrifuge. Conclusion. The experience in this field has provided positive results to show the savings and simplicity of the operation. With constant treatment of the crud, the client will reduce the input of new extractant to the SX plant. The savings are simple to calculate, generally achieving a reduction in the range of 40 to 60 percent of the input of new solution. The payback depends on how the machine is operated. A rate of 24/7 will be the best-case scenario to achieve the recovery rate of six to nine months. On the other application treating the residual material “borra,” the volume is reduced by almost 60 percent and recovers 15 to 40 percent of the material as viable crud. This makes the system valuable equipment for the complete crud treatment plant.

Centrifuge technology in SX process provides solvent loss reduction

Authored by: Robert Klug

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