Mineral Ridge Project — 2020 Technical Report

The proposed processing plant will re-process gold heap leach residual material at 4,000 t/d using conventional comminution and carbon-in-leach cyanidation.

Report context

The Mineral Ridge Project is located in Esmeralda County, Nevada, USA. This NI 43-101 technical report, dated December 2020, presents a proposed processing plant design for re-processing gold heap leach residual material and, in a modified configuration, remnant open pit (ROM) material. The report documents process design criteria, flowsheet development, and equipment specifications based on testwork conducted primarily from 2014 to 2017.

Processing route

Proposed process overview

The proposed processing plant will be a conventional design that will re-process gold heap leach residual material at a rate of 4,000 t/d with an equipment availability of 92% (365 d/a). The process flowsheet developed for the heap leach material combines conventional comminution using ball mills with carbon-in-leach (CIL) cyanidation to recover gold and silver. The process plant will produce a gold and silver loaded-activated carbon product from the CIL circuit. The loaded carbon will be shipped off-site to a refinery to recover the gold and silver.

Reclaim and grinding area

The grinding plant will receive residual material from the heap leaching operation. This material will be reclaimed with a front loader at a feed rate of approximately 181 t/h. The reclaimed material will be fed to a reclaim mixing tank through a vibrating grizzly that will remove previous heap leach operation trash. The mixing tank will be agitated, and process water will be added to form slurry with approximately 55% solids. The slurry will then be pumped to a holding tank with a retention capacity of 60 minutes, and then pumped to the ball mill pump box. This material will have a top size of approximately 0.25 in and a passing 80% under 0.14 in.

The grinding circuit design will include two ball mills (12.5 ft by 25.5 ft) with a motor of 2,400 hp each, operating in parallel with a shared pump box and dedicated cyclone clusters for each ball mill. Pumps with variable frequency drivers will pump slurry to the hydrocyclone packs. Each hydrocyclone pack will have nine cyclones of 10 in diameter with an arrangement of seven operating and two in standby. The cyclone overflow will have a design for 80% passing size of 200 mesh (74 µm). The cyclone underflow will return to the two ball mills at a circulating load of 300% and a solids content of 77% w/w. The mills are designed to process 2,000 t/d each net, at 92% availability.

Pre-leach thickening

The hydrocyclone overflow material will flow by gravity to a trash linear screen to prevent oversized trash from reporting to the CIL circuit. The linear trash screen undersize will report by gravity to the pre-leach high-rate thickener (80 ft diameter). Flocculant will be added to aid the settling process. The underflow density of the thickener will be approximately 60% solids. The thickener overflow will be recycled to the process water pond for reuse.

Leaching circuit

The leaching circuit will consist of four tanks (50 ft by 52 ft) operating in series with a total capacity of 353,160 ft³ that will provide a total leaching residence time of 36 hours. The slurry density will be adjusted by recycling water from the process water pond for the CIL circuit to operate at 45% solids. Air will be injected into the tanks through spargers to improve oxygen dissolution during leaching. It is expected that the slurry will arrive at the CIL circuit at a pH of 10.5; adjustments will be made to maintain the required pH level as necessary. Sodium cyanide (NaCN) will be added to the first and third leaching tanks and its addition rate will be controlled based on cyanide analyses. Cyanide concentration in the slurry will be maintained at 0.03 to 0.06 lb/ft³ (0.5 to 1.0 g/L) sodium cyanide.

The slurry will flow by gravity between the tanks. Each tank will be equipped with twin impeller agitators, one interstage screen to prevent activated carbon from flowing co-current with the slurry, and one carbon advance pump for carbon movement counter-current with respect to the slurry flow. Each CIL tank will contribute nine hours of residence time.

Fresh or reactivated carbon will be added to the final tank after rinsing with fresh water. The designed activated carbon concentration in the leaching tanks is 0.94 lb/ft³ (15 g/L). Dissolved gold and silver in the slurry will be adsorbed onto activated carbon and finally removed from the circuit at the first tank. Loaded carbon will be bagged and transported to an external facility for final gold and silver recovery, and carbon regeneration and recycling. Every 14 days bagged loaded carbon will be transported off-site. The existing activated carbon handling infrastructure, which includes carbon receiving, attritioning, sizing, sampling, and loading, is suitable for the new process.

Tailings thickening and filtration

Barren slurry from the final leaching tank will flow to a carbon safety screen to prevent loss of carbon. The safety screen undersize slurry will report to the CIL tailings thickener feed tank. At the tailings thickener (80 ft diameter high rate thickener), the leaching barren solution will be thickened to produce an underflow product of 55 to 60% w/w solids. The thickener underflow will be pumped to plate-and-frame filters. The tailings thickener overflow will be recycled to the process water pond.

The tailings thickener underflow will be pumped to two operating pressure filters with a filtration volume of 580 ft³ each. Filtrate solution will report to the process water pond. Tailings filter cake at a designed moisture of 15% will be transported by existing grasshopper stacking conveyors to the heap leach pad. Since the lined pad will be used for depositing the filtered tailings, cyanide neutralization is not applied to the tailings prior to deposition.

Remnant material processing (ROM)

The proposed process plant can be modified to treat ROM open pit material at a nominal rate of 4,000 t/d. A crushing facility already existing on the property will be modified to produce a final crushed transfer material to the grinding circuit of 0.39 in (10 mm) T80. The crushing circuit will consist of a 32 x 48 jaw crusher (existing) with a close side setting (CSS) of 4 in that will be fed by a feeder with a maximum rock size of 24 in, controlled through the existing grizzly. The jaw crusher product will feed a 400 hp secondary cone crusher with a CSS of 5/8 in (16 mm). The cone crusher will work on a closed loop with a double deck screen (7/8 in and 5/8 in apertures) to produce a final product P80 of 3/8 in (10 mm).

The crushing circuit will discharge onto a stockpile with a storage capacity of up to 72 hours. Remote controlled feeders will be located under the stockpile to convey crushed ore into the ball mill circuit. An extra total power of 540 hp (270 hp each ball mill) will be necessary in the grinding circuit to accommodate the increase in the feed size from 0.14 to 0.39 in. The crushing plant will supply crushed material at a rate of 168 tph during one 12-h shift; the heap leach material will be processed during the other shift.

Testwork basis

The process flowsheet was developed based on parameters established from testwork conducted primarily from 2014 to 2017. The size selection of the grinding mills was based on the amenability of the reclaimed ore to grinding determined through test programs performed by laboratories. The CIL tank sizing was based on leaching times determined by testwork and using scale-up factors and experience.

Test programs evaluated several options for treating the reclaimed Mineral Ridge heap leach material. Samples showed regular to poor responses to conventional flotation. Although the mineralization responded reasonably well to a gravity concentration process followed by atmospheric leaching of gravity tailings in tanks, direct ore leaching was chosen as the best available alternative to re-process the heap leach residues. Future addition of gravity concentration has been considered in the design.

Certain assumptions were made during the development of the process flowsheet which were based on engineering experience from other cyanidation projects with similar ore characteristics.

Key reported parameters

Technical qualifications

The process flowsheet presented in this report is a proposed design only. The estimated gold and silver recoveries in the CIL circuit from the heap material are 91% and 24%, respectively. Refinery recovery is estimated at 99.4%. The report states that certain assumptions were made during the development of the process flowsheet based on engineering experience from other cyanidation projects with similar ore characteristics. The CIL tank sizing was based on leaching times determined through testwork and using scale-up factors. The major design criteria table (Table 1-6 / Table 17-1) presents design values for both heap leach and ROM scenarios, with recoveries differing between the two feed types.

Source: Titan Mining Corporation, Mineral Ridge Project, Esmeralda County, Nevada, USA, NI 43-101 Technical Report, Project number 20145, December 2020, Sections 1.15 and 17.0.