The 2024 technical report describes the three-stage crushing, agglomeration, and heap leach recovery process with a Merrill-Crowe plant for gold and silver doré production at the Soledad Mountain Project in Kern County, California.
The Soledad Mountain Project in Kern County, California, operates an open pit mine and a processing facility that uses heap leaching with dilute sodium cyanide solution to recover gold and silver. Run-of-mine ore is delivered to a crushing and screening plant located south of the Phase 1 heap leach pad. The crushing plant is a three-stage circuit designed for an average throughput of 375,000 dry tons of ore per month. The key comminution devices are primary and secondary cone crushers, a primary screen, and a high pressure grinding roll (HPGR). The HPGR discharge is conveyed to an agglomeration drum where cement and fresh water are added. The agglomerated ore is then transported by overland and grass-hopper conveyors to a stacker and placed on the heap leach pad. Cyanide bearing process solution is added to the ore at transfer points on the first three grass-hopper conveyors. The leachate, or pregnant solution, is collected in a network of perforated pipes and directed to a pump box, then pumped to the Merrill-Crowe plant. The plant has been in operation since late 2015.
Critical Data
| Parameter | Value | Unit | Notes |
|---|---|---|---|
| Average throughput | 375,000 | dry tons/month | Target crushing rate |
| Nominal crushing rate | 800 | tph | Equivalent to 64% availability |
| Crushing rate, 2016 | 703 | dry tph | 3,790 operating hours, 43% availability |
| Crushing rate, 2017 | 649 | dry tph | 5,477 operating hours, 63% availability |
| Crushing rate, 2018 | 669 | dry tph | 5,271 operating hours, 60% availability |
| Crushing rate, 2019 | 772 | dry tph | 5,337 operating hours, 61% availability |
| Crushing rate, 2020 | 793 | dry tph | 5,018 operating hours, 57% availability |
| Crushing rate, 2021 | 821 | dry tph | 5,488 operating hours, 63% availability |
| Crushing rate, 2022 | 803 | dry tph | 5,245 operating hours, 60% availability |
| Crushing rate, 2023 | 808 | dry tph | 5,344 operating hours, 61% availability |
| Coarse ore stockpile capacity | 40,000 | tons | Live capacity of 10,000 tons |
| Coarse ore stockpile live capacity | 10,000 | tons | Provides 12.5 hours of crusher operation |
| Nominal heap lift height | 30 | ft | Multi-lift single-use pad design |
| Primary leach cycle | 75 | days | Drip emitters used for irrigation |
| Average water use | 404 | gal/min | 2021 through 2023 |
| Laboratory throughput | 600 | samples/day | Fire assay samples |
| Laboratory throughput | 600 | samples/day | Solution samples |
| Plant staffing | 87 | personnel | Total including all process areas |
Overview
The Soledad Mountain Project uses a conventional heap leach operation for gold and silver recovery. Ore is crushed in a three-stage circuit, agglomerated with cement, and stacked on a multi-lift, single-use heap leach pad. Dilute sodium cyanide solution is applied to dissolve the precious metals, and the pregnant solution is processed in a Merrill-Crowe plant to produce doré. An Intermediate Leach System (ILS) allows low grade pregnant solution to be recycled as barren solution.
Key Process Stages
Ore is delivered to a dump hopper with a 250-ton capacity. Oversize ore is broken with a permanently mounted hydraulic rock breaker, and ore is withdrawn by a variable speed vibrating grizzly feeder. Grizzly undersize and jaw crusher product is conveyed to a coarse ore stockpile.
The coarse ore stockpile has a total capacity of 40,000 tons and a live capacity of 10,000 tons. Ore is fed to the primary screen by two Syntron-type vibratory feeders and one belt feeder located in the reclaim tunnel.
The secondary crushing stage includes the primary screen and a cone crusher operating in closed circuit, which feeds the HPGR. The HPGR operates in open circuit and is the tertiary crushing stage. The HPGR product is sampled to form a shift composite for ore grade information and HPGR performance. Monthly composites are used for onsite column testing to benchmark actual heap leach performance.
Agglomeration is achieved in a drum where Portland Type 2 cement is added as a binder ahead of the drum, along with fresh water. The agglomerated ore is conveyed and stacked on the heap leach pad.
The heap is irrigated with dilute cyanide solution by drip emitters for a primary leaching cycle of 75 days. Pregnant solution is collected in a network of perforated pipes and pumped to the Merrill-Crowe plant. The Merrill-Crowe process includes clarification, deaeration, precipitation with zinc dust, filtration, retorting, and smelting to produce doré.
Additional Interesting Data and Summary
The heap leach pad is a multi-lift, single-use pad with individual lifts designed at a 30-foot nominal height. Additional underlying lifts continue to leach to reach ultimate gold and silver recoveries. Replacing one of the vibrating pan feeders under the primary crushed ore stockpile with a belt feeder has resulted in a potential throughput increase up to 420,000 tons per month. The target crushing rate is 375,000 tons per month. Historical operating data from 2016 through 2023 show variations in operating hours and availability. The site assay laboratory processes 600 fire assay samples and 600 solution samples per day. Process staffing totals 87 personnel across the crushing plant, leach pad, Merrill-Crowe plant, refinery, and laboratory. Crusher, leach, Merrill-Crowe, and laboratory operators work rotating 12-hour shifts, while salaried personnel work four, ten-hour shifts.
Key Processes
- Three-stage crushing with primary jaw, secondary cone in closed circuit with a screen, and tertiary HPGR in open circuit.
- Agglomeration with cement and fresh water in a drum before heap stacking.
- Heap leach with dilute sodium cyanide solution applied by drip emitters over a 75-day primary cycle.
- Intermediate Leach System (ILS) recycling low grade pregnant solution as barren solution.
- Merrill-Crowe recovery including clarification, deaeration, zinc dust precipitation, filtration, retorting, and smelting to doré.
- Column testing of HPGR product composites to benchmark heap leach performance.
Source: Soledad Mountain Project , 2024 Technical Report, 2024. Project website: [Not stated]
Project website: Soledad Mountain Project, 2024 Technical Report
