This 2013 Preliminary Economic Assessment (PEA) update by ERCOSPLAN describes the hot leaching/crystallization processing route proposed for the Holbrook Basin project, designed to produce up to 2.5 million tonnes per annum (MTPA) of muriate of potash (MOP) from sylvinite/carnallitite ore.
Article Body
The Holbrook Basin project in Arizona is the subject of a 2013 Preliminary Economic Assessment (PEA) update prepared by ERCOSPLAN, which details the recovery methods and processing facilities required to produce a final MOP product. The process design is based on the characteristics of the mineralized material, which contains approximately 8% K₂O and nearly 5% insolubles. Due to these ore characteristics, a hot leaching/crystallization route was selected as the reliable processing method over the alternative flotation process. The document notes that flotation remains an unverified option, with testing required on appropriate material to determine its feasibility.
The processing plant design assumes a conventional underground mining operation, with solid ore hoisted to the surface. The process flow involves ore storage, crushing, cold decomposition, hot leaching, clarification, vacuum cooling crystallization, and product drying. The plant design accommodates multiple production scenarios, with process calculations completed for final product capacities of 1.0, 1.5, 2.0, and 2.5 MTPA MOP. The preliminary process calculations described in the report are based on the 2.5 MTPA scenario.
The processing route is designed to handle the specific mineralogy of the Holbrook deposit, which includes both sylvinite and carnallitite ore types. The presence of carnallite necessitates a cold decomposition step ahead of hot leaching, which allows for the removal of MgCl₂ from the process stream. The design achieves a final product specification of 95.5% KCl, with the report detailing the complete mass balance for the operation.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Design production capacity | 2.5 | MTPA MOP | Process calculations based on this scenario |
| Alternative capacity scenarios | 1.0, 1.5, 2.0 | MTPA MOP | Process amounts provided for all scenarios |
| Ore K₂O content | Approx. 8 | % | Basis for process selection |
| Ore insolubles content | Nearly 5 | % | Contributes to process selection |
| Hot leaching temperature | Over 110 | °C | Brine preheating temperature |
| MgCl₂ content in decomposition brine | Considerably below 320 | g/l | Brine delivered to mashing unit |
| MgCl₂ content in discharge brine | 430 | g/l | Brine discharged from process |
| Product moisture | 0.2 | % | After drying |
| Product KCl content | 95.5 | % | Representative product specification |
| Product NaCl content | 3.4 | % | Representative product specification |
| Product MgCl₂ content | 0.1 | % | Representative product specification |
| Product CaSO₄ content | 0.2 | % | Representative product specification |
| Product insolubles | 0.6 | % | Representative product specification |
| Product H₂O content | 0.2 | % | Representative product specification |
| Product specific gravity | 2.0 | g/cm³ | Representative product specification |
| Product melting point | 770 | °C | Representative product specification |
| Water solubility (25°C) | 357 | g/1,000 g | Representative product specification |
| Wet solids residue | 15.4 | MTPA | Annual processing residue |
| Brine residue | 997,000 | m³ | Annual processing residue |
| Water demand | Approx. 285 | TDH | Total water demand; Not stated for total volume |
Overview
The Holbrook Basin project exploits a potash resource that requires processing to produce a saleable MOP product. The mineralized material comprises sylvinite and carnallitite, with a mineralogical composition that drives the selection of the processing route. The high MgCl₂ content associated with carnallite and the elevated insolubles content make the hot leaching/crystallization process the preferred option, as opposed to flotation, which would require additional testing to verify its applicability.
The processing facility is designed to operate year-round with a conventional mining operation feeding ore from underground operations. The ore handling and processing system includes storage facilities, a crushing plant, and a processing building where the dissolution and crystallization steps occur. The plant design allows for equipment sizing and configuration to vary according to the operational needs of the different production capacity scenarios.
Infrastructure requirements include water supply, power, and natural gas connections. The water supply assessment, based on a study by Montgomery & Associates, indicates that the Coconino Sandstone aquifer could supply the required amount of water for processing operations, including process water and make-up water for cooling cycles.
Key Process Stages
The processing route begins with potash ore storage, where ore from the skip discharge is transported by belt conveyor. The ore, which must have an appropriate K₂O content, is then crushed in the mill building to achieve a grain size suitable for the subsequent dissolution step. Crushed material is separated by grain size, with oversized material returned to the crusher. The undersized material is fed by belt conveyor to the cold decomposition step in the process building.
Cold decomposition is the initial processing stage, where the ore is contacted with recycled brine. The brine delivered to the mashing unit contains MgCl₂ at concentrations considerably below 320 g/l. The percentage of clarifier overflow sent to evaporation controls the MgCl₂ concentration in the mashing unit. During cold decomposition, carnallite is decomposed, and the KCl/NaCl solids are separated from the MgCl₂-rich brine. These solids are then fed to re-pulping and subsequently to the hot leaching unit, while a portion of the brine is sent to evaporation to control the MgCl₂ balance.
Hot leaching is the primary separation stage, where material from cold decomposition is contacted with preheated mother liquor. The brines are preheated to over 110°C before being used for dissolving, recovering energy from the KCl cooling crystallization circuit. The hot liquor dissolves KCl preferentially due to its strong temperature-dependent solubility, while NaCl remains largely undissolved because of its low temperature dependency of solubility. This separation mechanism is independent of MgCl₂ content in the brine and represents the primary means of separating KCl from NaCl.
Following hot leaching, the hot suspension is separated into a hot solution with less undissolved material and a mass flow containing the main parts of the remaining undissolved material, primarily NaCl, insolubles, and CaSO₄. The hot solution is clarified to remove nearly all undissolved material, and the clarified solution proceeds to vacuum cooling crystallization. The undissolved solids from the separation steps are washed and filtered, with filtrates returned to the dissolving unit, and the wet NaCl solids become the final process tailings.
Vacuum cooling crystallization exploits the significant change in KCl solubility with temperature, while NaCl solubility does not change significantly during cool-down, so only KCl crystallizes. MgCl₂, being highly soluble, does not crystallize in this step. The crystallized solids are separated from the mother liquor, and the depleted brine is reused as mother liquor in hot leaching. The surplus mother liquor not required for hot leaching is used in cold decomposition.
A carnalite separation step removes the main portion of MgCl₂ fed into the process with the potash ore. The crystallized solids from this step consist of over 90% carnallite and are directed to cold decomposition. The discharge brine contains approximately 430 g/l MgCl₂ and exits the process, carrying with it the majority of the MgCl₂ that entered with the ore.
Product drying reduces the moisture content of the de-brined solids to 0.2% water. The dried product leaves the process building by belt conveyor and is delivered to product storage facilities. Each storage unit has a capacity of 30,000 tonnes, and product is reclaimed from storage to a transport belt conveyor for rail or truck loading.
The final product specification requires a minimum of 95% KCl with maximum 0.2% moisture. The representative chemical composition includes 95.5% KCl, 3.4% NaCl, 0.1% MgCl₂, 0.2% CaSO₄, 0.6% insolubles, and 0.2% H₂O.
Additional Interesting Data and Summary
The preliminary process calculations for the hot leaching/crystallization procedure, including cold pre-decomposition, yield specific process amounts for each production scenario. For the 2.5 MTPA scenario, processing leaves 15.4 MTPA of wet solids and 997,000 cubic meters of brine per year as residue. The water demand for processing, including process water and make-up water for cooling cycles, is approximately 285 TDH total dynamic head, and the total water demand is not stated as a volumetric figure. According to the Montgomery & Associates study, the required water could be supplied by the Coconino Sandstone aquifer.
The process design accounts for the presence of both sylvinite and carnallitite in the mineralized feed. The inclusion of cold decomposition addresses the carnallite content, which would otherwise introduce excessive MgCl₂ to the hot leaching circuit. The process achieves effective separation of KCl from NaCl and removes the bulk of soluble impurities from the product stream.
The assessment evaluated four production scenarios ranging from 1.0 to 2.5 MTPA MOP, allowing for staged development or design optimization. The plant layout and equipment configurations would vary according to production capacity, but the fundamental processing steps remain consistent across all scenarios.
Key Processes
- Hot leaching/crystallization is the selected processing route, chosen over flotation due to high carnallite content and nearly 5% insolubles in the ore
- Cold decomposition ahead of hot leaching removes MgCl₂ and decomposes carnallite, with brine concentrations maintained considerably below 320 g/l MgCl₂
- Hot leaching operates with brines preheated to over 110°C, exploiting KCl's strong temperature-dependent solubility while NaCl remains largely undissolved
- Vacuum cooling crystallization recovers KCl from the clarified hot solution, with the brine reused as mother liquor in hot leaching
- Carnallite separation crystallizes over 90% carnallite for recycle to cold decomposition, discharging approximately 430 g/l MgCl₂ brine from the process
- Product drying achieves 0.2% moisture in the final MOP product containing 95.5% KCl
Source: Sylvinite/Carnallitite Mining in the Holbrook Basin, Arizona , 2013 PEA Update, 2013.
Project website: Sylvinite/Carnallitite Mining in the Holbrook Basin, Arizona, 2013 PEA Update
