This technical report describes the proposed process plant design for the Steenkampskraal rare earth project, based on bench-scale and mini-pilot plant metallurgical testwork.
Report context
The process flow design, as well as the Metallurgical and Hydrometallurgical Plant design and costing, was undertaken for the Steenkampskraal Project by independent consultants ULS Mineral Resource Projects (Pty) Limited. The design was independently reviewed in a document entitled "Steenkampskraal Monazite Mine: Preliminary Design Review" (April 2014). The fundamental premise upon which the process plant design is based was the requirement for an annual production of 5,000tpa TREO+Y2O3.
Processing route
Comminution circuit
A standard load, haul and dump operation will bring the drilled and blasted material out of the mine to various destinations: the waste dump, the low grade stockpile or the high grade stockpile. The mineralised material will be transferred from either the low or high grade stockpile (48 hour surge capacity) by front end loader and dumped into a feed hopper. The comminution plant consists of a two stage, open loop crushing system in which the RoM will be fed onto a static screen, and into a hopper, which feeds a two stage crushing circuit comprising a primary jaw crusher and secondary cone crusher. The product from the primary jaw crusher reports to a classification screen and is separated into three size fractions: +12mm, -12mm+1mm and -1mm material.
The +12mm crushed RoM will be conveyed to the secondary cone crusher for further crushing and returned to the sizing screen. The -12mm+1mm size fraction will be stored in two, 240t crushed RoM storage silos which each provide a 63hr buffer storage capacity ahead of the concentration plant DMS unit operations. The -1mm size fraction from the crusher plant screening operation will report to a fines handling system for further processing.
The -1mm crushed RoM material will be combined with the slurry produced in the mining operation and passed through a fines handling circuit, located in the milling circuit structure, where gangue material is removed via LIMS and wet high intensity magnetic separation (WHIMS), with a filter press for removal of non-magnetic material, two classification cyclones, and various screens, pumps and transfer vessels. The product slurry will be retained in a buffer storage vessel and thereafter fed to the mill sump and into a two stage grinding/milling circuit. The comminution circuit will operate 8hrs per day.
Concentrator circuit
The -12mm+1mm material will be transferred from the crushing circuit storage silos to the concentrator circuit which includes a DMS plant in which a dense media cyclone separates the low density gangue material from the denser REE bearing concentrate. The DMS product will be conveyed to a buffer storage bin and thereafter fed, at a controlled rate, to the grinding or milling circuit. The DMS rejects will be utilised to cap the residue containment ponds (RCPs).
The upgraded mineralised material in the buffer storage bin is then fed to the milling circuit, which will reduce the solid size from 100% -12mm to a target of 100% -45µm. The milled solids are discharged into the mill discharge sump where they are combined with the solids from the fines handling circuit. The resulting slurry is pumped to the classifying cyclone with the underflow returning to the milling circuit. The remainder of the solids from the classifying cyclone are sent through a magnetic separation circuit to remove magnetic fractions, resulting in a slurry containing the concentrated REEs. The concentrate will be sent into a thickening circuit and then to the Hydrometallurgical Plant area.
Hydrometallurgical Plant
The dewatered product from the concentrator plant will constitute the feed to the Hydrometallurgical Plant, an annual steady supply of approximately 18,454tpa REE concentrate at a grade of ±30% TREO+Y2O3 (or 25% mass by mass m/m TREE). The plant will operate 24hrs per day with surge storage capacity. The Hydrometallurgical Plant is located inside a covered facility with access control for radiation risk monitoring and management.
The process flow design includes the following circuits:
- Sulphuric acid crack/bake: REE minerals are cracked in hot sulphuric acid at 280ºC for three hours to produce a solid REE sulphate.
- Water leach: Solid REE sulphates are dissolved to produce a sulphate solution containing all the REEs.
- Double salt precipitation circuit: REE double salts (NaREE(SO4)2) are precipitated by addition of sodium sulphate. The LREE and HREE streams split after the double salt precipitation, with thorium deporting to both streams and impurities such as copper, aluminium, silica, phosphorus and iron deporting to the HREE stream.
- LREE recovery circuit: Includes conversion of the LREE double salts to hydroxides and drying so that cerium hydroxide can be selectively removed in a hydrochloric acid leach. Thorium and low value cerium remain in the leach residue and will be stored in the underground radioactive material storage vault.
- HREE recovery circuit: Includes a thorium removal stage consisting of thorium, iron and aluminium precipitation by addition of lime, plus copper and base metals precipitation.
- LREE stream: Includes a solvent extraction process to separate lanthanum and the closely associated radioactive element actinium from the other LREEs.
- Final precipitation circuit: Both LREE and HREE streams pass through to produce a LREE and HREE Recovered REE carbonate product that will be despatched to a toll-treatment facility for further separation.
Plant infrastructure and reagents
The Hydrometallurgical Plant includes tanks for re-pulping, neutralisation and precipitation; filters, pumps and sump pumps; a sodium sulphate recovery process unit; reservoirs and storage bins for reagents; and safety showers and eyewash stations. The majority of reagents (sulphuric acid, hydrochloric acid, hydrated lime and sodium hydroxide) will be stored in a designated storage area to the northwest of the Hydrometallurgical Plant site.
Water supply will be sourced from the identified supply aquifer by a three borehole wellfield. The existing reverse osmosis plant will produce potable water and top up water for utilities. A new reverse osmosis plant will provide the remainder of the high quality process water system requirements. Tailings from the Hydrometallurgical Plant will be pumped to RCPs, dewatered via filter presses, and disposed of into the RCP.
Radiation control measures
The design of the Steenkampskraal Process Plant was significantly directed by the radioactive plant feed. Measures include dust suppression in dry processing areas; plant section location and zoning into green (very low radiation), orange (medium radiation) and red (high radiation) zones; barrier protection including linings of vessels and piping; equipment drainage designed to eliminate material holdup; automation to minimise operator interaction with mineralised material; and CCTV monitoring for visual feedback.
Phased construction
The Steenkampskraal Process Plant will be constructed in two phases to reduce initial capital funding requirements. The surface historic TSF material is a simple and readily available plant feed which can be treated at low operational costs. The strategic decision was made to construct the Hydrometallurgical Plant and milling circuit first, which will treat the historic TSF material in the first year of production, while the new access portals to the underground mine are developed and the Metallurgical Plant is constructed.
Key reported parameters
| Parameter | Units | Value | Basis |
|---|---|---|---|
| Annual production target | tpa TREO+Y2O3 | 5,000 | Design requirement |
| Metallurgical Plant nominal design RoM production rate | tpa | 76,000 | Design |
| Crusher plant maximum capacity | tpa | ~146,000 | Design |
| Concentration Plant nominal design annual processing rate | tpa | 62,882 | Design |
| DMS maximum capacity | tpa | ~146,000 | Design |
| Mineral concentrate annual production | tpa | 18,454 | Design (mass balance) |
| Mineral concentrate mass pull | % | 29.3 | Design |
| Product grind size (100% passing) | µm | 45 | Design target |
| Hydrometallurgical Plant annual concentrate processing rate | tpa | 18,454 | Design |
| Concentrate grade to Hydrometallurgical Plant | % TREO+Y2O3 | ~30 | Design (mass balance) |
| Acid bake temperature | °C | 280 | From testwork |
| Acid bake duration | hours | 3 | From testwork |
| Diluted RoM head grade | % TREO+Y2O3 | 8.98 | Mass balance based on mini-pilot plant testwork |
| Overall plant recovery before toll-treatment | % | 85.0 | Design |
| Overall plant recovery after toll-treatment | % | 83.0 | Design |
| Metallurgical Plant stage recovery (fines handling) | % | 99.29 | Mass balance based on mini-pilot plant testwork |
| DMS stage recovery | % | 99.5 | Mass balance based on mini-pilot plant testwork |
| LIMS stage recovery | % | 98.9 | Mass balance based on mini-pilot plant testwork |
| Hydrometallurgical Plant recovery | % | 86.4 | Design |
| LoM average diluted RoM production | tpa | 65,574 | Mine design |
| LoM total feed to plant | t | 918,474 | Mine schedule |
| Mine plan recovery rate | % | 79.1 | Mine design |
| Design base case dilution | % | ~35 | Design |
| Total process plant employees | number | 178 | Operational plan |
Project website: https://www.steenkampskraal.com/
Technical qualifications
The process plant design and costing was developed through iterative processes including consolidation of bench-scale and mini-pilot plant metallurgical testwork results. Equipment selection and sizing was undertaken in sufficient detail to provide engineering information for capital cost and operating cost estimation at an accuracy of ±15%. The mass balance provided is based on mini-pilot plant testwork performed at Mintek, but the applicability of the mass balance to the actual Steenkampskraal Process Plant will depend on the head feed grade, the mining dilution and metallurgical characteristics of the process plant feed. The extraction of co-products (thorium, uranium, scandium, gold and silver, copper, helium, gallium, germanium and phosphate) has not been specifically designed and costed as part of the feasibility study in the absence of, or based on minimal testwork, and has been excluded from the feasibility study financial model.
Source: Steenkampskraal Project Technical Report (2013), Section 16 Recovery Methods.
