Sonora Lithium Project — PFS Technical Report

The PFS presents a proposed two-stage plant to process hectorite clays through beneficiation, gypsum roasting, and hydrometallurgical recovery to produce battery-grade lithium carbonate and potassium sulfate.

Report context

The Pre-Feasibility Study Technical Report for the Sonora Lithium Project, dated with the document number 101304-FS-0005-Tech Report rev0.docx, describes a proposed processing facility for the recovery of lithium from hectorite clays located in Sonora, Mexico. The study evaluates a flowsheet based on metallurgical testwork and preliminary economic evaluations, with the plant designed for staged construction and continuous 24-hour operation, 365 days per year.

Processing route

Flowsheet development

During the development of the PFS, different flowsheet options were investigated for the recovery of lithium from the Sonora hectorite clays, including acid pre-leaching of the ore, acid bake, atmospheric leaching, and potassium sulfate roasting. Gypsum roasting was selected based on testwork and preliminary economic evaluations.

The Sonora Lithium Plant is proposed to be built in two stages. Stage 1 design involves processing approximately 1.37 Mt/y of Run of Mine feed, at 0.39% Li and 1.68% K (first two years), to produce battery-grade Li₂CO₃ and potassium sulfate (K₂SO₄) for sale. The K₂SO₄ produced is expected to be sold as a high-quality Sulfate of Potash fertiliser. About 77,000 t/y of sodium sulfate is produced in Stage 1; however this is not expected to be saleable and is therefore stored in a lined tailings storage facility. Stage 2, planned for start-up in Year 3, involves adding a duplicate 1.37 Mt/y train to treat a total of 2.74 Mt/y of ROM feed, at 0.35% Li and 1.49% K.

The operating schedule for the plant is a continuous 24 hours per day operation, using two 12-hour shifts per day, 365 days per year. Design plant availabilities are typical at 90% (7,882 h/y) for the beneficiation plant and 83% (7,270 h/y) for the extraction and precipitation plants.

Beneficiation circuit

The purpose of beneficiation is to reject as much of the non-lithium bearing minerals (gangue) while maximising lithium recovery. Initial testwork showed that it is possible to reject about 70% of the feed mass (calcite and silica) while recovering 60% of the lithium into the -20 µm fraction using wet screening and classification.

The larger particles (mostly quartz) in the ore are initially rejected via wet scrubbing and screening. Additional gangue (mostly calcite) is then rejected using hydrocyclones with finer calcite subsequently being removed using reverse flotation, where the carbonate gangue floats while the lithium bearing clays sink.

Key considerations in the development of the beneficiation flowsheet included testwork showing improved beneficiation performance when the ore is treated wet rather than dry, with operational performance also expected to be better for wet beneficiation when the ore is received wet and possibly sticky. However, dry beneficiation represents a potential opportunity which could reduce capital and operating costs as the beneficiated clay would not need to be dried prior to roasting. Sufficient liberation of valuable minerals is expected using a scrubber while ensuring that power input is minimized. Screening was not used due to the high number of screens required; hydrocyclones were selected for the relatively fine cut size and to reduce capital cost. The combined concentrate, consisting of fines and flotation sinks, is proposed to be dewatered using conventional high-rate thickening, pressure filtration, and atmospheric drying prior to roasting.

Ore Preparation and Classification: ROM ore is delivered by Front End Loader to the Mineral Sizer which reduces the particle size to -150 mm. The Mineral Sizer product is discharged onto the Scrubber Feed Conveyor. This material, together with water, is fed to the Drum Scrubber in which the agglomerates and clay lumps are broken up and form slurry. The Drum Scrubber has a 15 mm aperture inner screen and 6 mm outer screen. The +6 mm oversize is mostly calcite and quartz and is discarded. The scrubber product (-6 mm) is pumped to the Classification Circuit which consists of primary, secondary, and scavenger hydrocyclones. The clay slurry is separated into three fractions: +150 µm fed to the tailings belt filter; -150 + 20 µm fed to the reverse flotation circuit; and -20 µm fines fraction fed to the Concentrate Thickener.

Flotation and Dewatering: The -150 + 20 µm slurry undergoes reverse flotation where the floats contain the waste stream with a high percentage of quartz and calcite, and the sinks contain the concentrate stream with a high amount of lithium. The flotation circuit consists of Rougher Flotation, 1st Cleaner Flotation, and 2nd Cleaner Flotation. Each flotation circuit includes conditioning tanks as well as flotation tank cells. The rougher circuit comprises a dilution tank to add water to achieve 50% solids and four 20 m³ tank cells. Each cleaner circuit includes two 20 m³ tank cells. Flotation reagents include collector, soda ash for pH adjustment, and sodium silicate as a dispersant. The sinks from each of the flotation cells report to the concentrate thickener. The classified overflow product along with the flotation concentrate is thickened in the Concentrate Thickener and filtered by plate and frame Concentrate Filters. Flocculant is added to aid settling in the thickener. The concentrate filter cake is stacked and stockpiled, which allows for drying and a decoupling between the upstream beneficiation plant and downstream extraction plant. The primary cyclone underflow and flotation tailings (floats), at 60% solids, are pumped to a 66 m² belt filter. The filter cake, at 20% moisture, is conveyed and then stacked prior to transport to the Tailings Storage Facility using Front End Loader and trucks.

Extraction and precipitation circuit

Testwork showed that lithium extractions of 87% could be achieved with an optimised ratio of reagents to maximize the uptake of sulphur dioxide and thereby reduce the consumption of gypsum. Testwork indicates that it is feasible to use the recovered K₂SO₄ in the roasting circuit to reduce gypsum consumption; the base case flowsheet produces K₂SO₄ for sale.

The most important impurities that need to be managed for the production of battery-grade Li₂CO₃ are sodium and calcium sulfate. Magnesium, manganese, silica, aluminium, and iron are removed by precipitation and ion exchange in impurity removal. Calcium is minimised by a combination of adding soda ash and ion exchange. Sodium, potassium, sulfate, and chloride are reduced by washing the lithium carbonate crystals to remove the contaminants on the surface.

Roasting: The beneficiation concentrate, along with gypsum, are reclaimed from stockpiles using variable speed belt feeders and transferred to the Paddle Mixer which blends the concentrate and gypsum. The combined feed is introduced to the roasting kiln where the reaction of lithium and calcium sulfate (gypsum) occurs to form lithium sulfate (Li₂SO₄). Initially chemically bound water is released as the feed is preheated on its way to the higher temperature zone. Calcite (CaCO₃) and gypsum are calcined to lime. Once the optimum roasting temperature of 1,000°C is achieved, a one hour residence time is required in the hot zone due to the slow kinetics of the reactions involved. Heat input and off-gas volumes are carefully optimised in order to reduce gas velocity and thus decrease the dust load carried to off-gas cleaning systems. The product calcine exiting the kiln is cooled with preheated fresh air. The cooled calcine is transferred to Calcine Leaching for the recovery of the water soluble lithium sulfate.

Leaching, Thickening and Filtration: The calcine is mixed with recycled PLS filtrate and regenerated IX solutions to achieve 50% w/w solids in a 200 m³ Calcine Leach Tank. The Li₂SO₄, along with any metal sulfate impurities of iron, magnesium, calcium, aluminium, sodium, and potassium, are water soluble and leach into solution. The leach tank temperature is controlled to a target of 70°C with a water cooling coil. The leached slurry is separated into a lithium rich PLS and a clay residue, which contains little lithium, using a high-rate thickener and belt filters. The Leach Thickener is a 20 m diameter high-rate thickener which thickens the slurry to 65% w/w prior to pumping it to the Leach Filter Feed Tank. The thickener overflow (PLS) flows by gravity to Purification for impurity removal. Two parallel 150 m² vacuum belt filters produce a washed filter cake with less than 20% w/w moisture. The filter cake is conveyed to a stockpile for transport to the Tailings Storage Facility using a Front End Loader and trucks.

Purification and Evaporation: The Purification area consists of two tanks in which sodium carbonate solution is added to the PLS to precipitate calcium as CaCO₃. The precipitated CaCO₃ is removed via a 30 m² horizontal plate and frame filter and discharged into a bunker for transport to the lined Sodium Sulfate Pond. The filtered solids are washed with fresh water to maximise the recovery of residual soluble lithium. The filtrate is collected and pumped to the Polishing Filter to remove any residual solids prior to evaporation. The Evaporator uses forced circulation by mechanical vapour recompression to increase the PLS lithium concentration from 5.5 g/L to 14.3 g/L to maximise the amount of lithium carbonate precipitated as battery-grade lithium carbonate.

Ion Exchange: The purpose of the ion exchange circuit is to remove any multi-valent cations in solution (calcium, magnesium, manganese, aluminium). The IX package consists of three IX columns in a lead–lag–regeneration configuration to enable continuous operation. The regenerated solution is recycled to the Leach Tank to be used as dilution water. The purified PLS is then pumped to Precipitation.

First Lithium Carbonate Precipitation, Filtration, Washing and Drying: The First Battery-Grade Lithium Carbonate Precipitation circuit consists of four agitated tanks operated in batch mode, with three tanks in operation at any time. Sodium carbonate (Na₂CO₃) solution is pumped into the precipitation tanks in a 1.1:1 stoichiometric mole ratio for conversion of the Li₂SO₄ into Li₂CO₃. The temperature in the precipitation tanks is maintained at 95°C by indirect steam heating via immersed heating coils. Any vapour produced in a precipitation tank is cooled in the condenser and the condensate returned to the precipitation tank. At the end of the batch, the precipitate is allowed to settle and the slurry is then pumped to the 1st Precipitation Centrifuge. The 1st Precipitation Centrifuge dewaters the slurry and washes it with hot demineralised water. The 1st Precipitation centrate is collected and pumped to glaserite evaporation. The Li₂CO₃ centrifuge cake, at 8% w/w moisture, is discharged to the 1st BG Product Dryer Feed Bin. The BG Dryer uses indirect heating provided by LNG to reduce the moisture to less than 0.1%. The dryer off-gas is filtered in the BG Dryer Baghouse. The dryer discharge along with the captured dust is transported to the BG Product Silo via bucket elevator.

First Battery-Grade Product Handling: The First BG Li₂CO₃ is transported in one tonne bulk bags. If the product is to be micronised, it is transferred to the BG Microniser by rotary valve and screw conveyor. The BG Microniser reduces the particle size from P₉₀ 100 µm to 5 µm. A cyclone and baghouse capture any dust. A screwfeeder then transfers the microniser discharge and captured dust to the First BG Bagging and Palletising Package. One tonne bulk bags are semi-automatically filled and placed onto pallets. A forklift then transfers the loaded bags on pallets into a 20 ft shipping container.

Glaserite Evaporation: The solution from the First Lithium Carbonate Precipitation, the recycled sodium sulfate filtrate, and the Decomposition Filter centrate are sent to the Glaserite Evaporator Feed pond. Sulphuric acid is added to convert any Li₂CO₃ to Li₂SO₄ to prevent it crystallising out of solution. The solution is evaporated at 100°C to form glaserite crystals (Na·3K(SO₄)₂). The glaserite is then separated from the solution via a semi-batch operation of the Glaserite Filter. The glaserite filter cake is sent to the Decomposition Tank. The filtrate is pumped to Second Lithium Carbonate Precipitation.

Second Lithium Carbonate Precipitation, Filtration and Washing: Second lithium carbonate precipitation consists of three agitated tanks operated in batch mode with each tank at a different stage of operation to achieve pseudo-continuous operation. This process is otherwise identical to the First Lithium Carbonate Precipitation circuit and includes precipitation, filtration by centrifuge, and washing. The Li₂CO₃ cake with 8% w/w moisture is further processed in the bi-carbonation circuit to produce Second BG lithium carbonate.

Bi-carbonation Circuit, including Filtration, Drying and Packaging: Lithium carbonate cake from Second Precipitation is fed to the bi-carbonation circuit for further purification to produce BG lithium carbonate. The Li₂CO₃ is batch fed into the Bicarb Dissolution Tank and is re-slurried with recycled centrate. The dissolution process is maintained at 25°C to maximise the concentration of lithium in solution. The bi-carbonation centrate is pre-cooled via heat exchanger and immersed cooling coil utilising chilled water as necessary. Carbon dioxide gas is bubbled into the bi-carbonation dissolution tank to convert lithium carbonate to more soluble lithium bicarbonate (LiHCO₃). Off-gas produced in the Bicarb Dissolution Tank is cooled in a condenser and the condensate returned to the tank. The excess CO₂ is collected and recycled to the Bicarbonate Process through a CO₂ Blower. The Bicarb Dissolution Tank solution is pumped to the Bicarb Crystallisation Tank and filtered to remove any residual insoluble impurities left over from the dissolution process. In the Bicarb Crystallisation Tank the solution is heated to 95°C to re-crystallise the Li₂CO₃. The Li₂CO₃ solution then is filtered and washed with mineralized water similar to the First Lithium Carbonate Precipitation circuit. The Li₂CO₃ cake is discharged to the 2nd BG Product Dryer Feed Bin. The 2nd BG Li₂CO₃ is then dried and packaged similarly to the First BG product.

Services

Reagents used in the process include sulphuric acid received at 98% w/w concentration by bulk road tanker; sodium carbonate received by bulk road tanker; caustic soda delivered to site as solution in bulk; gypsum dumped from 20 t trucks onto a pad; carbon dioxide supplied as liquid by bulk road tanker; flotation reagents including NBC-4 collector supplied as a liquid in ISO containers; Superfloc MF-345 flocculant supplied as a powder in 800 kg bulk bags; and cooling water reagents such as biocide, algaecide, and hypochlorite.

Water used in the Process Plant includes raw water and fire water pumped from bore fields; demineralised water produced in an on-site reverse osmosis plant for centrifuge washing, clean re-pulping, chiller make-up water, and boiler water; cooling water; gland seal water; potable water; chilled water for the Bi-carbonation Dissolution Tank; and condensate and waste water from cooling tower and boiler blow down.

Plant air is dried using refrigerated driers and stored in a receiver for use throughout the process plant including instrument air requirements.

Liquefied Natural Gas is supplied to the plant by tanker and stored in bullets to be provided by the supplier.

Steam is produced in a natural gas fired Steam Boiler Package to meet the various steam demands throughout the plant.

Key reported parameters

*Note: Most criteria have been derived from the metallurgical testwork program undertaken by SGS and Bacanora, which were used in developing the mass balance that forms the basis for the sizing of process plant equipment.*

Technical qualifications

The PFS is a preliminary study. Different flowsheet options were investigated during development including acid pre-leaching, acid bake, atmospheric leaching, and potassium sulfate roasting, with gypsum roasting selected based on testwork and preliminary economic evaluations. The process design criteria in Table 17.1 have been derived from metallurgical testwork. The report notes that dry beneficiation represents a potential opportunity which could reduce capital and operating costs but has not been adopted in the base case. Testwork indicates that it is feasible to use recovered K₂SO₄ in the roasting circuit to reduce gypsum consumption, however the base case flowsheet produces K₂SO₄ for sale.

The sodium sulfate produced in Stage 1 (about 77,000 t/y) is not expected to be saleable.

The report includes a summary flowsheet figure (Figure 17.1) which forms part of the technical description but has not been reproduced in this text.

*Source: Sonora Lithium Project PFS Technical Report, 101304-FS-0005-Tech Report rev0.docx, Section 17 Recovery Methods.*