The January 2017 updated prefeasibility study for Potash Ridge's Blawn Mountain project in Utah describes dry crushing, flash calcination, a sulphuric acid plant, water leaching of the calcine, belt filtration and crystallization to 250,000 t/y of sulphate of potash.
Blawn Mountain is an alunite deposit in Beaver County, Utah. Potash Ridge Corporation commissioned the technical report, an updated prefeasibility study (PFS) with an effective date of January 10, 2017 and a report date of January 18, 2017, prepared by Millcreek Mining Group and Resource Development Inc. The project envisions processing open pit run-of-mine (ROM) ore by crushing, roasting, extracting sulphate of potash (SOP) by leaching the calcine with water, solid/liquid separation, brine evaporation, crystallization and drying. The process plant is also designed to conserve energy and water by treating and reusing effluents and to dispose of residues in an environmentally sound manner. The combination of unit operations rests on test work completed in 2012 and 2013 at HRI in Golden, Colorado, at Pocock Industrial in Salt Lake City and on thermal processing modeling by FLSmidth in Bethlehem, Pennsylvania.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| ROM ore rate | 3.4 | Mt/y | At 2% moisture |
| Operating schedule | 330 | days per year | 24 hours per day |
| Nominal plant capacity | 425 | tph | Design |
| ROM top size | 12 | in | Delivered to a 400 t hopper |
| Crushed product | Minus 3 | in | Roll crushers, one duty and one standby |
| Dry grinding product | 1,000 | µm P80 | To the calciner |
| Roasting temperature | 550 | °C | Not to exceed 600 °C |
| Roaster residence time | About 2 | seconds | Flash roaster |
| Sulfuric acid plant | 2,090 | t/d | 93% H2SO4 |
| Flue gas SO2 and SO3 | 9.44 | vol. % | Estimate for the acid plant |
| Leach slurry | 35 | % solids | 80 °C, water as lixiviant |
| Leach residence time | 75 | minutes | Two tanks in series |
| K2SO4 extraction | 90 | % | Bench-scale |
| Brine | 10.5 to 11.0 | % K2SO4 | To crystallizer |
| SOP product | 250,000 | t/y | 92.5% K2SO4, 50% K2O |
| Wellfield water | About 800 | gpm | Withdrawal |
| Process workforce | 148 | people | Average |
Overview
The Blawn Mountain flowsheet turns alunite into sulphate of potash by thermal decomposition and water leaching. Crushed ore at 98% solids is dehydroxylated and roasted to decompose the alunite. The calcine contains a mixture of leachable SOP (K2SO4) and alumina (Al2O3). Roaster off-gases go to a sulphuric acid plant, and the calcine is leached with hot water. The brine is evaporated and crystallized with mechanical vapor recompression, and the crystals are centrifuged, dried and sized. Alumina and silicate leach residues are dry stacked.
Key Process Stages
Crushing and dry grinding
Up to 425 stph of ROM ore minus 12 inches goes to a 400 t dual loading hopper that feeds two parallel primary roll crushers, one operating and one standby. Each has a 4 to 1 reduction ratio and makes a minus 3 inch product. A collection bin feeds two dry crushing screens, with oversize going to two secondary cage mills and mill product recycled to the screens by bucket elevators. Screen undersize at P80 1 mm goes to the calciner feed bin, and a dust collection system manages the dust. The ore has a crushing Bond work index of 7.06 kWh/ton and an abrasion index of 0.40 g.
Drying and calcination
A flash calciner removes the chemically bound water and decomposes the alunite to potassium and aluminum sulfates, and a flash roaster completes the decomposition to aluminum oxide and SO3, converting most of the SO3 to SO2. Alunite decomposition is carried out at 1022 °F (550 °C), with a maximum of 1112 °F (600 °C), because the porous gamma-alumina phase must be the end product and the alpha form is recalcitrant. Three cyclones in series transfer heat from the product to incoming air, and an electrostatic precipitator cleans the off-gas. The report states that FLSmidth's criteria draw on cement industry experience and desktop simulation, and that no pilot plant testing had been completed.
Sulfuric acid plant
An estimated 2,090 t/d of sulfuric acid is made on site from the SO2 in the roaster off-gas, which is cleaned and processed to recover 93% H2SO4. No steam is available from the acid plant because of the low SO2 concentration in the feed.
Leaching and solid/liquid separation
Calcine is leached with water at 35% solids and 80 °C for a total of 75 minutes in two baffled, covered, insulated and steam-jacketed tanks in series, extracting about 90% of the K2SO4. Leach slurry is thickened, thickener overflow goes to the evaporator/crystallizer, and underflow is dewatered on belt filters. The cake, at about 90% solids, is washed with three displacements and conveyed to the tailings impoundment, where a radial stacker dry stacks it.
Crystallization and product
About 35% of the filtrate is pumped to the crystallizer once its K2SO4 reaches 10.5% to 11.0%, and the other 65% with cake wash water is recycled to quench the calcine and hold the leach at 35% solids. Mechanical vapor recompression was selected to optimize energy use. Crystallizer product is thickened and centrifuged, the thickener overflow and centrate return to the quench tank, and the crystals go to drying, sizing and packaging. The design makes 250,000 t/y of SOP at 92.5% K2SO4, or 50% K2O.
Additional Interesting Data and Summary
Water and energy measures under evaluation include dry crushing in place of wet grinding, which removes a drying step ahead of the calciner and saves natural gas, and raising leach slurry density from 35% to 40% or 50% solids.
Two power options were studied: a new 138 kV transmission line of about 46 miles from a substation near Cedar City with gas-fired boilers for about 200,000 lb/h of steam, or an on-site co-generation facility. A cost comparison favoured turbine engines over piston engines for co-generation. The processing workforce averages 148 people over the project life.
Key Processes
- Roll crushing and dry screening with cage mills
- Flash calcination and flash roasting of alunite
- Sulfuric acid production from roaster off-gas
- Hot water leaching of calcine
- Thickening and belt filtration with dry stacking of residue
- Mechanical vapor recompression crystallization
- Centrifuging, drying and sizing of SOP
Source: Technical Report, The Blawn Mountain Project, Updated Prefeasibility Report, Beaver County, Utah, January 18, 2017.
