Pressure Acid Leaching Panel Discussion - ALTA 2019

The panel discussion was held Wednesday 22 May 2019, immediately following the Pressure Acid Leaching Forum, held as part of the Nickel-Cobalt-Copper sessions at ALTA 2019 in Perth, Australia.

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Panel Chair: Alan Taylor (AT), ALTA Metallurgical Services (Australia)

Panel Participants (left to right): Warren Salt (WT), NobelClad (USA); Boyd Willis (BW), Boyd Willis Hydromet Consulting (Australia); Bryn Harris (BH), NMR360 (Canada); Cory Kosinski (CK), Sherritt Technologies (Canada); Lourdes Valle (LV), BHP (Australia); Paul Tucker (PT), Bantrel Co (Canada); Hermann Scriba (HS), LiNiCo (Australia); Fiona Sofra (FS), Rheological Consulting Services (Australia); Tim Harrison (TH), Clean TeQ (Australia); Mike Miller (MM), Mike Miller Consultancy (Australia); Mark Benz (MB), MRB Consulting (Canada)

Editor’s Note: The contributions of the panel members and delegates are not presented verbatim, but rather have been paraphrased and condensed for clarity and brevity. The topics are presented in the order in which they were discussed. Also, it is not feasible to include all contributions made during the discussions, and they are limited to some that are representative of the key points raised and debated. 

Improvements to the HPAL Process for Laterites

AT (chair) opened the discussion by asking the panel whether there is any room for improvement in the current concept for pressure acid leaching of laterites?

Thickening and Rheology

PT (panel) identified feed thickening and rheology as a key area for improvement. He pointed out that the ore throughput is dependent on the feed thickener underflow pulp density, so that improving thickening is a key issue as it increases the ore feed rate to the autoclaves. In Cuba a lot of research has resulted in achieving well over 50% solids.  On the other hand, only 23% could be achieved even with doubling the flocculant addition at the Bulong operation. 

FS (panel) added that studies for improving thickening need to start with the mine, especially when the ore contains material with a high clay content. A lot can be achieved by controlling the dispersion of the clays, but that can't be done once the material has been wetted. We need to look at adding certain salts during the initial phase of slurrying the material.  This can lead to huge improvements in thickening and dewatering though it will cost money and requires investigation on its effectiveness.

Brent Moldovan (BM), International Atomic Energy Agency (Austria), floor asked whether any consideration has been given to pump technology and pipe size diameter. During his experience in uranium ore processing, a lot of effort was put into increasing the tailings density by reducing the water input and 45-48% solids was achieved in the thickener underflow.  The initially installed centrifugal pumps were replaced by larger capacity positive displacement pumps to eliminate the ingress of gland water and obtain laminar flow to avoid shearing the floccs going out to the tailings.  He asked whether any consideration have been given to applying these strategies to HPAL autoclave feed and tailings thickening.

FS (panel) advised that some of her previous projects have considered these issues. She agreed that centrifugal pumps do involve the detrimental addition of gland water, but they can also enhance dewatering as shearing the material lowers the rheology which may actually increase water recovery, especially if sheared material is recycled to the bottom cone of the thickener which results in a higher solids concentration that is still pumpable. Normally the trade-off is between the cost of PD and centrifugal pumping, but additionally once you are in laminar flow using PD pumps you have to make sure there is sufficient pressure to maintain a sliding bed.  If you are in laminar flow and laminar settling occurs along the pipe, which is likely to happen as you are reducing the yield stress and pumping the material, there is no mechanism to re-suspend the material, so blockages can easily occur.  Enough pressure has to be maintained to push the stationary bed along the pipe if it should occur.  Modified centrifugal pumps are also available that are able to pump material with much higher yield stresses, around or over 500 pascals, which have a modified suction side which physically draws the slurry or paste into the pump.  

MM (panel) added that it would be beneficial to train the geologists in minerology. They tend to be more involved in metallurgical accounting, trying to value a resource. There is less emphasis on what the minerals consist of and in a process such as HPAL the geological chemistry is not as important as the mineral itself. A case in point is if we have some particular clays which we know are really bad actors, and we see the mining units going over a particular area of the mine on a recurring basis, we know it’s going to adversely affect the autoclaves and the CCD thickeners.  The geologists tend to think that anything that is fine is a clay even if it is goethite. It would be helpful to equip geologists with instruments such as infrared for determining the particular mineral and the same instrument can be used to check the ore feed thus providing accountability between geologists and the plant.

James McQuie (JM), BHP Nickel West (Australia), floor asked MM (panel) to comment on how much work has been done to understand the key role of particle size in filtration and thickening. He pointed out that the aluminium industry is far more advanced than the base metals hydrometallurgy industry on understanding aspects such as seeding. 

MM (panel) responded that settling of tailings involves dealing with precipitate from the autoclave and the degree of foliage of the hematite in particular. Optimization of the autoclave leaching conditions can change the morphology of the hematite. If the acidity is low, the hematite precipitate tends to be platy which will result in slower thickening and inferior filtration characteristics.  If the acid level is high enough, spherical type hematite is formed when treating pure limonite. If saprolite is present, thickening characteristics deteriorate with increasing magnesium concentrations. if limestone or lime is added as a neutralising agent, seeding can be used within that circuit to maximize the particle size and shape to improve the filtration and settling characteristics.  

Indirect Heaters

AT (chair) asked for comments on the potential operational and cost benefits of indirect heaters in the HPAL circuit.

PT (panel) responded that the reduced dilution with direct heaters enables a higher flow of ore feed to pass through the autoclaves which gives a better return on the capital investment. Murrin Murrin ran a pilot plant on one of their trains which did increase the autoclave feed density.

BW (panel) advised that indirect heating is included in the proposed Platina scandium project. It is a small-scale project so that the cost of having to retrofit conventional direct heaters in the event of a failure of the indirect heaters would be low compared with a large plant.

Utilizing Acid in HPAL Residue

AT (chair) raised the issue of possibly utilizing the significant amount of acid present in the HPAL discharge. The most common current practice is to neutralize it with limestone or calcrete which significantly increases the amount of waste sent to tailings and adds to the opex. Going right back to the early days, AMAX, the first to seriously work on HPAL after Moa Bay, developed and piloted their Omnivorous Process which used the residual acid from limonite HPAL to pre-leach high magnesium saprolite which was then recycled to the limonite HPAL to complete the leaching. This was investigated for the Bulong project, but the saprolite had to be activated in a kiln and the economics were unfavourable. Later, the EHPAL Process was developed for Ravensthorpe which utilizes residual acid from limonite HPAL to complete the leaching of saprolite after partial atmospheric tank pre-leaching with acid.

HS (panel) added that the presence of both limonite and saprolite offers a good opportunity to utilize the residual acid as the saprolite can act like limestone to soak up the acid while recovering nickel, and referred to two example processes:  BGRIMM have developed an Inverse Leaching Process which uses an atmospheric tank limonite leach followed by secondary autoclave leach (at 150oC) for the saprolite, and a two-stage atmospheric tank leaching process for limonite followed by saprolite has also been developed.

Stephen Grocott (SG), Clean TeQ (Australia), floor said that apart from adding saprolite to neutralize it in-situ, the value of the residual acid in HPAL discharge is too low to justify the capex and opex of recovery and re-use. Also, filtration and recycle of solution for water management is problematical due to the recycle of impurities. For example, recycle of magnesium will result in the need for increased acid addition and will increase the free acid in the HPAL residue. 

MM (panel) pointed out that at a given residual solution acid concentration, the acid in the residue is proportional to the %solids in the autoclave feed, so increasing the %solids will reduce the residual acid to be neutralized with limestone. Another important consideration is to optimize the acid dosage to HPAL which will also be reflected in limestone consumption. In the EHPAL approach, saprolite addition is used to generate jarosite downstream of the autoclave discharge as a means of neutralizing the acid and obtaining a secondary leach from whatever acid is present.

Mike Dry (MD), Arithmetic (Canada), floor invited comments on the feasibility and implications of injecting SO3 directly into the HPAL autoclave thus bypassing the conversion to acid step.  The huge generation of heat should significantly reduce the amount of steam injection resulting in a higher acid concentration in the autoclave, which may enhance leaching.

MB (panel) responded that this would amount to a new unit operation and would require careful mathematics and energy balancing engineering. He would take a lot of convincing to move away from acid addition. In terms of improvements, he considers that a key issue is how many acid plants should be included.

Individual Pots Versus Multi-Compartment Autoclaves

AT (chair) raised the issue of using a series of individual pots for HPAL which has been used in the uranium industry and at a pilot plant scale for laterites. Potential advantages include by-passing a stage for descaling or agitator maintenance instead of shutting down the entire autoclave, and more efficient mixing which could improve recovery and reduce scaling.  

Jim Kyle (JK), Consultant (Australia), floor responded that in the mid-1990s the then Lakefield Oretest (now SGS) built their first pilot scale laterite autoclave as a series of four individual vessels.  The design was chosen for a number of reasons, mainly for the number of holes you could put into the titanium.  Although some scaling problems tended to occur within the connections between one autoclave to another, it worked very well, and the residence time distribution was the same as would expect be expected, and in fact occurred, in a single “submarine” type autoclave.  While the chemistry supported the concept, the industry didn’t like it.

MB (panel) added that Sherritt ran a number of studies which showed that the capex of the series of vessel concept is higher. Another consideration, from an operating cost and reliability perspective, is whether the projected benefits would be achieved in commercial practice.  The reality is that in a commercial operation the descaling schedule for the first vessel would be similar to the general maintenance schedule for entire autoclave system, which is about 6-12 months based on industry experience. So there is no true advantage apart from when there is an agitation or seal failure, which is typically due to human error.

Jim Groutsch (JG), MetEng (Australia), floor reported that the former Cawse HPAL operation included an initial sacrificial vessel to collect most of the scale ahead of the autoclave and then be isolated for descaling. However, when in operation, rapid scaling made it impossible to isolate the sacrificial vessel.

PT (panel) reported that the initial design of the Alacer Gold POX project in Turkey was based on seven pots, the first three in parallel and four in series to facilitate transport up to the mountainous plant site. In a peer review he recommended against this concept for two main reasons. The first was the 7m agitator shaft length which would be very hard on the mechanical seals when scale accumulates on the end. The second was the safety risk of entering an off-line pot for descaling adjacent to an operating pot. Instead, he recommended two long and skinny Getchell style autoclaves transported in sub-assemblies and welded at site, which was adopted for the project.

CK (panel) added that from a safety perspective, using individual pots increases the amount of potential leak points compared to one horizontal autoclave, and doesn’t present any obvious benefits.

Operation and Control Improvements

MB (panel) pointed out that there is huge room for improvement on how HPAL projects are operated.  While the high capex is an issue, high operating cost is having a greater effect on the viability of some of the operations. Achieving nameplate capacity and design on-stream time is very important. For new projects it would be advantageous  to invest in dynamic simulators for training and to support detailed engineering by projecting the required dynamic surge capacity between the various unit operations. The tools to do this are now available as demonstrated by the Sherritt/NTWIST presentation in the earlier HPAL Forum on the application of artificial intelligence for predictive process modelling using data from Ambatovy. 

LV (panel) raised the issue of why is it that 20 years on from the Cawse, Bulong and Murrin Murrin projects, despite a quantum leap in technical advances, we are still seeing a high rate of failure due to technical or economic issues or a combination of both? Based on her experience in complex metallurgical plants around the world as well HPAL plants, the most important attributes of the best operations lie in the area of soft skills such as the culture, the mindset and adaptive leadership of both external and internal stakeholders. It is difficult to put a dollar value on soft skills during project evaluation, yet they have the most profound effect on the value of the project throughout its lifecycle and this is something that has often been overlooked.

TH (panel) added that we as metallurgists also need to be a part of that process.

AT (chair) further stressed the need for co-operation between the various on-site operational groups such as locating the geology, mining and process managers in adjoining offices, and mandating a regular morning co-ordination meeting with the operations manager.

Safety, Hazardous Materials, and Tailings Disposal

Adrian Dickison (AD), Beca (New Zealand), floor asked where the industry is heading on the issues of safety, minimization of hazardous materials, and tailings disposal. He said that during a recent visit to a major HPAL operation to undertake a hazard identification exercise, his first exposure to this type of HPAL flowsheet, he was quite shocked about the large inventory and production of hazardous materials in the various plant and ancillary facilities. Also, compared with pyrometallurgy which produces a relatively inert slag, HPAL processing results in a huge tailings legacy issue. 

AT (chair) pointed out that the safety and hazardous materials issue is affected by the selection of the downstream  process, for example MSP production involves more hazardous chemicals than MHP.

AD (floor) added that once HPAL is selected for the front-end of the plant, everything else follows from it including the utilities and waste disposal facilities. The front-end HPAL components are also quite hazardous.  

AT (chair) pointed out that tailings disposal is a particularly big issue especially in the light of the disaster in Brazil, and at the last two ALTA’s there was a focus on dry stacking using large capacity filters. He asked whether this is a game-changer for the industry, especially for high rainfall areas where there is a greater risk of tailings instability.

Andrew Hawkey (AH), Diemme Filtration (Australia), floor reported that pressure filtration and dry stacking of HPAL tailings are certainly doable. Diemme have carried out a lot of testwork at Vale in New Caledonia and are now building 10 large machines that will be delivered at the end of this year with commissioning projected close to the end of next year.  

BW (panel) added that a Diemme pilot filter was integrated into the Platina pilot plan at SGS, and the tailings. filtered exceptionally well producing a crumbly cake with 27% moisture that could be snapped and abraded by hand. The outcome was that for the very small-scale Platina plant, a tailings dam would be much more expensive than filtration. However, the cost comparison will likely flip to favour a tailings dam for large scale plants with multiple filters.

MB (panel) added that some of these questions were in fact driving forces for configuring Clean TeQ’s Sunrise Project flowsheet to eliminate the use of H2S, and he believes that dry stacking is coming into the industry, at least for new projects, as a consequence of some of the things that have happened. Another issue is that with HPAL more solids are being impounded than in the plant feed, so there is a need to assess the potential of alternative processes where the waste is reduced by recovery of by-products, such as by using chloride chemistry. These issues need to be addressed at the scoping study level when alternative process routes can be evaluated.

AT (chair) observed that political action we are now seeing is likely to spread as has happened with cyanide restrictions. If we ignore the trend, we might find that we are eventually not allowed to build plants at all. Tailings dam failure has been a lingering problem for years and it is tragic that it has taken such loss of life to produce real action.

Benefits of Industry Information Sharing

JG (floor) raised the issue of the potential benefits of information sharing on important issues such as increasing plant availability and establishing realistic maintenance costs. He agreed that the potential failure of mechanical seals is one of the important issues though this can be alleviated with good pressure and equalisation systems even at 260 – 265 oC. One of the biggest issues is block valves which have limited duty cycles and are very expensive. 

Sharing information with regard to opex, capex and ramp-up time from existing operations would enable the industry to establish a benchmark for HPAL projects. The Rio Tuba (Coral Bay) operation, for example, has worked very well. He supported LV ‘s (panel) opinion that there is a lot to gained by soft skills and training in simulators where mistakes can be safely made. You only have to have one mistake by an operator or programmed into a control system at these pressures to result in expensive and long downtimes.

LV (panel) responded that in relation to the sharing of information, there have been numerous papers that have been shared about the Coral Bay plant from conception through to commissioning and operation which present the valuable lessons learned.  Predictability is one of the key factors that have made the plant successful.  The plant feed is known approximately three months in advance which provides a clear picture of the chemistry as a basis for predicting behaviour in the entire plant circuit and the life of critical HPAL items such as the cone and the let-down valves, and for addressing any challenges which may arise. During her five years at Coral Bay she never encountered a passing of the isolation block valve after the autoclave. Another point is that the culture of Coral Bay is to avoid chasing easy gains, and to be prudent in every action taken by staying within the design envelope of the equipment and process which results in predictability.

HPAL Versus NPI for Producing Nickel and Cobalt Sulphates

AT (chair) asked for comments on selecting a HPAL versus an NPI project for producing feed for making nickel and cobalt sulphates for the battery industry. He also asked whether the industry is locked into producing sulphates or should we consider options with lower transport costs.

SG (floor) said that while he could give a more technical answer, the fact that the biggest NPI producer in the world has selected HPAL for their project targeted at the battery industry answers the question.

In relation to the production of nickel and cobalt sulphates, this is driven by the battery manufacturers.  Some of them will take cobalt chloride for instance, and some will take hydroxides, however the dominant form is sulphates.  Clean TeQ studies have shown that it isn’t too difficult to change the sulphate to a carbonate or chloride, however it is an immature market and so they follow the specifications of the battery manufacturers. He agreed that there will be some diversification in the future, however the market is currently dominated by sulphates.

MB (panel) added that nickel sulphate was already a well-understood, fungible, and freely traded material from the long-established chemicals industry, so from a business perspective it was a natural starting point.  However, once we start getting truly integrated and dedicated supply chains, including major car manufacturers, we may see some real innovation.

HS (panel) said that the impression he got on the nickel sulphate issue from the conference is that things are happening very quickly, with companies moving into nickel sulphate in a big way in places such as Indonesia and the Philippines.  The last time the nickel price increased, which typically happens when there is a shortage, it was the nickel pig-iron (NPI) industry that took off, especially in China, capitalising on the increased selling price to overcome their production cost hurdle. A similar thing could happen this time with atmospheric tank and heap leaching. Their relative simplicity, lower capex, and suitability for small operations, may provide enough incentive to counteract the higher acid consumption compared with HPAL.

Production of MHP Versus MSP for Supplying the Battery Industry

AT (chair) said that there appears to be a trend towards producing MHP for HPAL projects targeting the battery industry and asked whether MHP was now preferred over MSP.

SG (floor) responded that he doesn’t see a strong preference either way. There are existing operations that have refineries based on sulphide feed be it MSP or matte, and there are those treating MHP feed such as the battery plant based on Ramu MHP which will double in size if the projected expansion of Ramu goes ahead.

Future Prospects for Nitric and Chloride Atmospheric Tank Leaching

AT (chair) added asked for comments as to whether there will be a revival of interest in nitric and chloride atmospheric tank leaching processes, and whether they can be developed to a commercial level in time to take advantage of the current and projected increase in nickel demand.

BH (panel) responded that if the industry had devoted just a fraction of the effort that they put into HPAL on developing chloride front-end processing, there would likely be a commercial operation now.  People are always afraid of chlorides, but, unlike nitric acid which doesn’t have any commercial operating plants, chloride plant operations already exist.  Four of the world’s major nickel refineries are in fact chloride-based, i.e. Kristiansand in Norway, Sandouville in France, Jinchuan in China (mixed chloride-sulphate) and Niihama in Japan (where Sumitomo send their mixed intermediates from HPAL). Chloride leaching of laterites gives about 200 Kg/tonne of clean, crystalline, benign residue that doesn’t take up too much volume and can be sent to an iron and steel plant because it would make a very good flux.  The residue fall is relatively small and easy to compact compared with 1.5 tonnes/tonne for HPAL. Finally, nickel-cobalt separation in chloride is far easier than it is in sulphate. He believes that there is time to develop the front-end chloride leaching of laterites and predicts there will be a commercial chloride laterite plant in the next five years, likely to be in Western Australia.

Effect of Nickel Price on Processing Development

Tony Eltringham (TE), Retired (USA), floor asked the panel how a doubling of the current nickel price would affect their views as expressed in the discussion. His impression is that the discussion is constrained by what we do now and what we can afford to do. The nickel laterite industry appears to be trying to develop a “one size fits all” approach whereas the historically cyclical nature of the mining industry indicates that it is necessary to have separate masterplans for today’s nickel price and for possible future market conditions to be able to successfully navigate the highs and lows of the price cycle.    

MB (panel) commented that if the nickel price was double what it is today, some of the topics discussed would still be relevant because it is always a shame when you have operating facilities running at 50-60% of name plate.  That is a tremendous loss and an even greater loss at higher nickel prices. We may be talking about fourth generation plants, but by no means is this technology anywhere near as mature as for example alumina refinery technology, and one of the reasons is because people generally do not cooperate and exchange information as discussed earlier.

Summary of Key Points

• Thickening of the autoclave feed is a key target for improvement in a HPAL circuit as it determines the ore throughput. Potential improvement strategies include dispersion of clays by blending and addition of selected salts during the initial ore slurrying phase based on rheological studies.  
• Thickening in the CCD circuit is also an important target as it reduces water loss to tailings. One approach implemented in the uranium industry is to replace the initially installed centrifugal pumps with positive displacement pumps to eliminate the ingress of gland water and achieve laminar flow to avoid shearing the floccs going out to the tailings.  
• Indirect autoclave feed heaters would reduce dilution and enable higher ore throughput. They have been tested at a pilot scale but not yet applied commercially. However, they have been selected for a small-scale scandium project under development.  
• Productive utilization of the relatively high acid content in HPAL discharge can potentially reduce acid and limestone consumptions and increase metal extraction. Progress to date is limited to the partial atmospheric tank leaching of saprolite ore which has been tested for a number of hybrid HPAL/AL processes and commercially applied in the EPAL Process.  
• The use of a series of individual pots instead of a single multistage horizontal autoclave potentially offers the advantages of by-passing a stage for descaling or agitator maintenance instead of shutting down the entire autoclave, plus more efficient mixing which could improve recovery and reduce scaling. It has been successfully applied on a commercial scale in the uranium industry and at a pilot scale for laterites. However commercial HPAL project studies have indicated higher capex and no significant advantages.
• Tailings disposal is a major issue, especially in the light of the disaster in Brazil, and there is a trend towards filtration and dry stacking. Testwork has shown that this strategy is readily applicable to HPAL tailings and has been selected for an existing HPAL major operation.
• Industry experience to date indicates that there is significant room for improvement by optimizing HPAL plant design, increasing on-stream time, and improving operational control. Available strategies include dynamic simulators for training and artificial intelligence for predictive process modelling.
• The successful Coral Bay operation developed and maintained a culture of avoiding chasing easy gains and being prudent in every action taken by staying within the design envelope of the equipment and process which results in predictability.
• Sharing information with regard to opex, capex and ramp-up time from existing HPAL operations would enable the industry to establish a benchmark for HPAL projects. The Coral Bay operation has shared valuable lessons learned on all aspects of the project and plant operation in numerous papers. 
• The fact that the biggest NPI producer in the world has selected HPAL for a major new project indicates that it is preferred over NPI production for supplying nickel and cobalt to the battery industry.
• The preference for nickel and cobalt sulphates is driven by battery manufacturers. Some will accept cobalt chloride or hydroxides, however the market is currently dominated by sulphates. More diversification will likely occur as the industry matures and integrated supply and manufacturing  facilities are developed.
• There does not appear to be a battery industry preference for either MSP or MHP.
• Compared with sulphuric leaching of laterites, chloride leaching results in a relatively small, clean, crystalline, benign residue that could be sent as flux to the iron and steel industry. The extensive experience gained with chloride-based major nickel refineries should make it feasible to develop a successful process for laterites in time to capitalize on the projected increase in demand for nickel.
• The nickel laterite industry appears to be trying to develop a “one size fits all” approach whereas the historically cyclical nature of the mining industry indicates that it is necessary to have separate masterplans for today’s nickel price and for possible future market conditions to be able to successfully navigate the highs and lows of the price cycle.    

The Editor acknowledges the work of the student volunteer from Murdoch University, Andro Thomas, for providing notes on the discussion.

Hydromet Processing of Ni-Co-Cu Sulphides is the featured topic for the ALTA 2020 Nickel-Cobalt-Copper Forum and Panel, to be held 25-27 May in Perth, Australia. 

The program includes a short course on The ART of HPAL - The Way to Success to be held 24 May, presented by Dr Naoyuki Tsuchida and Mr Fumio Iwamoto, both formerly of Sumitomo Metal Mining.  Course Outline

Editor: Alan Taylor, Metallurgical Consultant and Managing Director, ALTA Metallurgical Services, Australia

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