There is no known formal industrial mineral ore deposit model for alunite. The characteristics for a model and some exploration criteria are derived from three publications: Hall (1978), Hall and Bauer (1983), and Hofstra (1984).

The local alunite deposit has been described, in the above-mentioned publications, as hydrothermal alteration of calc-alkaline volcanic rocks.

Alunite mineralization is found on four ridges that occur within the Blawn Mountain Project. Acid sulfate alteration associated with a shallow, possibly laccolithic intrusion altered the silicic-alkalic rhyolite porphyries, flows and tuffs belonging to the Miocene Blawn Formation and the Oligocene Needles Range Group. Alteration tends to be in linear bodies reflecting the role of normal faults in controlling the mineralization. Alteration is zoned away from the point of hydrothermal fluid upwelling. The mineralized ridges are erosional remnants of a once larger altered area.

Krahulec (2007) described the appearance of rocks from the silica cap and quartz-alunite zone as follows, “The Silica Cap is a zone of intense silicification believed to be the near-surface manifestation of the hydrothermal channel-ways. The silica is typically buff, dense, and massive but may be quite porous and vuggy locally and resemble a siliceous sinter… On the surface the Quartz-Alunite alteration zones are composed of white to cream to buff to gray to pink, generally fine grained, punky to dense, intermixed alunite and silica with only minor amounts of other impurities, mainly iron… Alunite also occurs locally as coarse (>0.5in.), lathy, typically pink crystals in veins. Kaolinite becomes increasingly important, at the expense of alunite, in the Quartz-Alunite zone near the boundary with the Hematite-Clay zones and also where the QuartzAlunite zones are cut by faults (Walker, 1972). Dickite (a high-temperature member of the kaolinite group) is reported by Whelan (1965) and Thompson (1991) in the Quartz-Alunite zone”.

Krahulec gives the following description of the two geometries, “The cone-shaped (narrow end at the base) zones are interpreted as the primary area of strong hydrothermal upwelling . . . and the adjoining flat-bottomed zones are recognized as permeability-controlled areas above the paleoground-water table where steam-heated H2S is oxidized to H2SO4. Only the central portion of Area C (Area 1) at Blawn Mountain is clearly a funnel-shaped zone. The other flat bottomed alunite zones are strongly controlled by the higher porosity and permeability of the host volcanic rocks, while the hydrothermal cones are largely independent of these factors (Hofstra, 1984)”. Krahulec continues this discussion by quoting Hofstra, “…The control of permeability on the degree of alteration intensity is most important near the margins of Quartz-Alunite altered zones. Alteration is pervasive and unaffected by variations in the permeability of the host rocks”. The alteration zones tend to be thicker in cone-shaped areas than in flat-lying areas. It is possible that there were more cone-shaped feeder zones but they were eroded or are buried under valley fill.

Mining operations will use a conventional open-pit, truck and shovel mining approach. This is a typical and standard approach for many surface mining applications and takes advantage of the flexibility of the mining equipment. For Blawn Mountain, Area 1 and Area 2 will be developed in phases that will allow for optimizing the ore grades encountered in the deposit, while providing flexibility to the operation.

The mining plan for the PFS uses a nominal 3.75% K2O ore grade cut-off for Area 1 and a 3.50% K2O cut-off for Area 2. These cut-off grades were utilized in the development of the pit shells and mining areas for Area 1 and 2. In addition, two low-grade ore stockpiles will be developed, one for Area 1 and Area 2 respectively. Ore that is less than 2.75% K2O is considered waste material and will be placed in the waste dumps. The Area 1 low-grade stockpile will contain ore ranging in grade from 2.75% K2O to 3.75% K2O and the ore in the Area 2 low-grade stockpile will range from 2.75% K2O to 3.50% K2O. This low-grade ore will be fed to the processing plant after mine operations have concluded in year 28.

The mining production schedule is driven by the capacity of the processing plant. The ROM ore production schedule for direct plant feed is approximately 3.4Mtpy. Mining operations commence in Area 1 in Year 1. For purposes of this PFS, Year 1 is anticipated to be 2020. Year 1 is considered a construction period for stockpile area development, haul-road construction and initial mining area development. Year 1 is considered a “ramp-up” period and during this time the mining area within Area 1 will be developed in such a way that full production can be achieved beginning in Year 2 (2021). The initial phase of mining in Area 2 is developed in Year 3 in coordination with operations in Area 1.

The ROM ore will be processed as envisioned, by crushing, reduction roasting, extracting SOP by leaching the calcine with water, solid/liquid separation, evaporation of brine, crystallization as well as drying and packaging of SOP product for markets. Provisions have been made in the process plant to conserve energy and water through treatment and reuse of effluents and disposal of residues in an environmentally sound manner.

The ROM ore at minus 12in is delivered to the 400t dual loading hopper that feeds two parallel primary roll crushers, one operational and one stand by. The crushed ore will be conveyed to a collection bin which will feed two parallel screening/secondary crushing units.

The collection bin will feed two dry crushing screens. Screen oversize will report to two secondary cage mills. The crushed product from the cage mills will be recycled back to the dry screens via bucket elevators. Screen undersize (P80 – 1mm) will be conveyed to the calciner feed bin. A dust collection system will be installed to manage the dust generated from dry crushing.

Crushed ore containing alunite and inert solids at approximately 98% solids (2% moisture) is
dehydroxylated and roasted to decompose the alunite. The calcine produced contains a mixture
of SOP (K2SO4) and alumina (Al2O3). A start-up air heater is used during the system start-up to bring the flash roaster up to the auto-ignition temperature of the fuel.

The calcine produced from the roasting step is quenched with water and the slurry is pumped to the water leach circuit. Calcine particles entrained in the roaster off-gases are separated in a cyclone followed by an electrostatic separator (ESP). The dust collected in the ESPs is also recycled to the water leach circuit.

The gamma-alumina phase occurs in a porous cubic structure which can be leached with sodium hydroxide (NaOH). It reverts at high temperatures to the recalcitrant alpha form with hexagonal close-packed structures. The temperature limits on thermal processing are therefore, required to ensure that the gamma-alumina crystals are the end-product of alunite roasting operations. Alunite decomposition reactions are therefore carried out at 550°C. The maximum temperature in the roaster not to exceed 600°C.

Air-to-gas heat exchangers are used to remove heat in the roaster off-gases to ensure that the gas temperature entering the ESP is in the range of 275-300°C. Reducing conditions are created by injecting excess fuel in the roaster to convert most of the generated SO3 from the decomposition of aluminum sulfate to usable SO2 in the production of sulfuric acid. The conversion of SO3 to SO2 will be regulated based on the amount of excess fuel delivered to the roaster. An estimated 2190 tpd of sulfuric acid (H2SO4) is manufactured at the project site from sulfur dioxide (SO2) produced by the decomposition of alunite during the thermal processing of ROM ore.

Based on bench-scale test results at HRI, it is proposed that the calcine discharged from the roaster will be leached with water at a pulp density of 35% solids. After dissolving approximately 90% of the K2SO4 in the calcine, the leach residue slurry at approximately 33% solids is filtered in a battery of belt filters and the filtrate (brine), containing approximately 10.5 to 11.0% dissolved K2SO4, is pumped to the evaporator/crystallizer circuit. The filter cake at about 90% solids, is washed with water and the washed filter cake, consisting of inert solids Al2O3 and undissolved K2SO4, is conveyed at approximately 90% solids to the tailings facility for dry stacking.

The leached slurry at the end of the leach cycle is filtered by using vacuum belt filters and consists of inert solids originally present in the ROM ore, Al2O3 produced in the roaster, an aqueous solution containing approximately 90% of the K2SO4 extracted during leaching, plus approximately 10% of unleached K2SO4 solids. Solid/liquid separation is one of the most significant unit operations at the project site because of its importance in energy conservation both in the calciner/roaster and evaporator/crystallizer circuits, water conservation through reuse of effluents as well as its impact on the size and type of water and wastewater treatment facilities.

Once the dissolved K2SO4 content of the filtrate from the calcine leach circuit has reached an estimated 10.5 to 11.0% range through recirculation of the filtrate, approximately 35% of the filtrate will be pumped as feed to the crystallizer circuit. The remaining 65% of the filtrate and cake wash water, as required, will be recycled to quench the calcine solids discharged from the roaster and to maintain the solids content of the slurry in the leach tanks at 35%.

Recovery of SOP crystalline product from the filtrate containing K2SO4 in the solution consists of an integrated crystallizer system followed by separation of the SOP crystals from slurry, drying, sizing and packaging of the product for shipment.

The product from the crystallizer is transferred to a thickener. The thickened slurry discharged from the thickener is then fed to a centrifuge where the crystals are separated from the liquid in the slurry. The wet crystals are transported to the dryer/cooler. The centrate from the centrifuge is collected in a tank and returned to the crystallizers. The dried product conveyed from the crystallizer system will be treated and packaged to meet market specification.