Hemco Gold Recovery: Crushing, Leaching & Merrill-Crowe Optimization

Source: Mineros S.A (2026)
Website: https://www.mineros.com.co/operations/nicaragua

Critical Data

Overview

Mineros S.A. operates the Hemco Property in Nicaragua’s Región Autónoma de la Costa Caribe Norte, a gold mining complex with three processing plants: the Hemco Plant, La Curva Plant, and Vesmisa Plant. The flagship Hemco Plant processes underground and artisanal feeds and reprocesses tailings from La Curva, using a comprehensive gold recovery flowsheet that includes three-stage crushing, two-stage grinding, agitated cyanide leaching, counter current decantation (CCD), and Merrill-Crowe precipitation. Established historically and continuously upgraded—most notably with a US$1.7 million crusher upgrade in 2019 and a new primary ball mill in 2014—the plant achieved an overall gold recovery of 89.4% in 2025. The significance of the Hemco Plant lies in its ability to handle up to 2,200 tpd after planned grinding circuit modifications, stabilizing throughput that averaged 1,957 tpd in 2025. Key process steps integrate cyanide addition during grinding to initiate leaching early, followed by a nine-tank agitated leach circuit, an eight-thickener CCD system, and a Merrill-Crowe precipitation stage that produces doré bars. These gold recovery methods are optimized for variable feed grades and artisanal sources, making the Hemco Property a benchmark for mid-scale gold processing in Central America.

Key Process Stages

• Stage 1: Crushing – Ore from underground and artisanal miners passes a 14 in. grizzly and a double-deck scalping grizzly producing three size fractions. The +3 in. and +3/8 in. × 3 in. materials feed a primary jaw crusher, followed by secondary and tertiary cone crushers in closed circuit with vibrating screens. The upgraded crushing circuit (2019, US$1.7 million) increased capacity from 100 to 150 tons/hr and includes a new dust collection system.
• Stage 2: Grinding – The grinding circuit consists of one primary ball mill (Dominion 11 ft × 14 ft, 750 kW) and five secondary ball mills configured in parallel. Hydrocyclone classification achieves a cyclone overflow P80 of approximately 75 µm. Cyanide solution is added to the hydrocyclone underflow to initiate gold dissolution. Planned changes will designate Dominion 1 and 2 as primary mills, with Sepro and M5 as regrind mills, achieving a total grinding capacity of 2,200 tpd.
• Stage 3: Leaching – Slurry from grinding is thickened to 45%–50% solids in a primary thickener before entering a series of nine agitated leach tanks (tanks No. 0 through 8). Cyanide and lime maintain pH between 11.0 and 11.5. The first stage (tanks 0–4) is followed by an intermediate thickening step; the underflow enters a second stage (tanks 5–8). Total retention time ensures maximum gold dissolution.
• Stage 4: Counter Current Decantation (CCD) – The CCD circuit comprises eight 50 ft diameter Denver thickeners arranged in two parallel trains of four each. Solution flows counter current to the slurry, washing gold-bearing solution from the solids. Fresh barren solution (from Merrill-Crowe and tailings pond reclaim) is added to the last thickener; overflow moves upstream, while underflow (final tailings) is pumped to the San José tailings facility via two 100 m³/h positive displacement pumps.
• Stage 5: Merrill-Crowe and Refining – Clarified pregnant solution is deoxygenated in a vacuum tower, then mixed with zinc powder to precipitate gold and silver. The precipitate is collected in filter presses, mixed with flux (sodium carbonate, borax, silica sand, fluorspar), and smelted at ~1,350°C in tilting furnaces. The resulting doré bars contain 18%–30% gold and 43%–72% silver, with a silver-to-gold ratio of 3.11 for 2025.

Additional Interesting Data and Summary

Beyond the core recovery circuit, the Hemco Plant includes a dedicated sand washing screen, a laboratory constructed at a cost of US$2.7 million (2020–2021) that processes 800 samples per day using three atomic absorption spectrometers, and a modern tailings disposal system. The San José tailings facility, commissioned in 2017, is fully lined with HDPE, has a design capacity of 27.5 Mt, and a planned life of over 50 years. By 2025, the third expansion lift to 250 MASL was 90% complete, with completion expected by mid-2026. Water is reclaimed from the tailings pond and recycled to the mill or used in a cyanide destruction circuit during the rainy season. The older Aguas Claras facility now serves as a rainwater reservoir, while the Concha Urrutia area (≈2 Mt of tailings) is undergoing closure. Economic investments include US$1.2 million for a new primary ball mill in 2014 and continuous optimization projects. The two satellite plants—La Curva (producing gravity/flotation concentrates sent to Hemco) and Vesmisa (batch processing of high-grade artisanal ore >30 g/t Au at ~250 tonnes/week)—complement Hemco’s throughput. A proposed Porvenir Plant (2,000 tpd) will incorporate Cu-Pb flotation, cyanidation, and zinc flotation with cyanide destruction, targeting a grind size of P80 73 µm. Mineros S.A. has demonstrated a commitment to increasing throughput and recovery through equipment upgrades and process modifications while maintaining environmental compliance with lined tailings storage and water management systems. The Qualified Persons (QPs) from SLR Consulting consider the processing plants to be operating well given the age of the equipment, with no anticipated issues regarding energy, water, or process materials. Future outlook includes the Porvenir plant (subject to feasibility) and continued tailings lift construction, ensuring the Hemco Property remains a significant gold producer in Nicaragua.

Key Processes: Flotation, Cyanidation, Gravity Separation, SAG Mill, Ball Mill, Crushing

Target Commodities: Gold, Silver, Copper, Zinc, Lead