La Negra Mine: Differential Flotation for Silver and Base Metals

Source: Unnamed_Company_57 (2026)
Website: https://miningdataonline.com/property/1981/La-Negra-Mine.aspx

Critical Data

Overview

Minera La Negra operates a 3,000 tonne-per-day concentrator in Querétaro, Mexico, employing a conventional crushing, grinding, and differential flotation process to recover silver, lead, copper, and zinc. The facility, historically running 336 days per year, is planned to restart with 337 operating days, treating 2,500 tonnes per day (842,500 tonnes per annum). The process is designed to produce three distinct concentrates: lead-silver, copper-silver, and zinc, in that sequence, maximizing silver recovery across all streams. The plant uses a mobile jaw crusher for specialized tasks, followed by primary, secondary, and tertiary crushing stages to achieve a feed size suitable for grinding. Grinding is performed by two parallel ball mill circuits in closed circuit with hydrocyclones, targeting a P80 of 75 µm. Differential flotation employs a suite of reagents to selectively recover lead, copper, and zinc while suppressing pyrite, arsenic, and iron. Thickening and filtration reduce concentrate moisture to nominal levels between 10% and 11% for shipment. The concentrator is integral to the La Negra mine’s restart plan, supported by an on-site laboratory for quality control. This recovery method is significant for its ability to produce high-grade silver-bearing concentrates from a complex polymetallic orebody, contributing to the economic viability of the operation.

Key Process Stages

• Stage 1: Primary Crushing – Mineralized material is tipped into a 60-tonne hopper with a 25” x 25” grate; oversize is broken by a BT200 hydraulic breaker. A Gator 30” x 42” jaw crusher with a 150 hp motor reduces feed to a P80 of 4 inches at a rate of 220 tph with 88% availability.
• Stage 2: Secondary and Tertiary Crushing – The secondary crusher (5½ ft Symons standard head cone, 200 hp) operates at 300–340 tph to produce a P80 of 1½ inches. The tertiary crusher (5½ ft Symons short head, 300 hp) further reduces to a P80 of 3/8 inches at 150–190 tph, with 86% availability.
• Stage 3: Grinding – Two parallel ball milling lines in closed circuit with hydrocyclones achieve a product P80 of 75 µm. Line 1: Marcy 10’ x 10’ mill (800 hp, 46 tph) with two D20 cyclones. Line 2: Allis Chalmers 9’ x 11’ mill (500 hp, 36 tph) and Taylor 7½’ x 11’ regrind mill (450 hp, 22 tph) with six D10 cyclones. Zinc sulfate and sodium cyanide are added at a 2:1 ratio to activate lead and depress zinc, copper, pyrite, arsenic, and iron.
• Stage 4: Differential Flotation – The lead circuit uses a 12’ x 12’ conditioning tank (12 min retention), four 350 ft³ Outotec cells, and Denver cleaners. Copper flotation includes two Wemco 300 ft³ cells, ten Denver scavenger cells, and three cleaning stages; ammonium bisulfite and S-7583 are added. Zinc flotation uses four Denver 160 ft³ cells with three cleaning stages; lime raises pH to 9.0–10.0 and copper sulfate activates zinc.
• Stage 5: Thickening and Filtration – Lead concentrate is thickened in an 18’ x 6’ thickener to 30–40% solids and filtered by PIPSA disc filters to 10.8% moisture. Copper concentrate is thickened in a 30’ x 10’ unit to 65–70% solids and filtered by a Clever plate filter to 10.9% moisture. Zinc concentrate is thickened in a 12’ x 12’ unit and filtered by PIPSA disc filters to 10% moisture. All concentrates are stored in a compartmentalized shed for truck shipment to Manzanillo.

Additional Interesting Data and Summary

The La Negra processing plant relies on a well-established reagent regime to achieve selective flotation. Key reagents include sodium cyanide (300 kg/t) and zinc sulfate (400 kg/t) for depression in grinding, AERO 404 (20 kg/t) and CC1064 (13 kg/t) in lead flotation, S-7583 (25 kg/t) and ammonium bisulfite (2,764 kg/t) in copper flotation, and lime (520 kg/t), copper sulfate (410 kg/t), and Aero5160 (10 kg/t) in zinc flotation. The on-site laboratory supports the operation with atomic absorption (Agilent AA240FS, Varian AA240), ICP-AES (Agilent 4210 MP-AES), and fire assay capabilities, ensuring rigorous quality control. Tailings management is addressed through existing impoundment facilities and a closure plan (TSF5 and TSF5A) as part of the broader environmental and social management system, which includes dust, surface water, soil, biodiversity, and cultural protection plans. Economically, the mine’s restart is projected to produce concentrates shipped to the port of Manzanillo, leveraging local contractors for trucking. The facility’s design recovery rates—82.1% for silver, 73.9% for lead, 84.4% for zinc, and 68.3% for copper—demonstrate efficient extraction from a polymetallic ore. Sustainability initiatives are embedded in the operating permits and closure planning, though specific energy and water consumption metrics are not detailed in the recovery methods section. The facility’s ability to process 2,500 tpd with 90–99% mill availability underscores its operational resilience. Looking ahead, the plant’s restart will support the 7.5-year mine life, with potential for optimization in reagent consumption and grinding efficiency. The integration of a mobile crusher for tailings buttress construction further adds operational flexibility. Overall, the La Negra concentrator exemplifies a conventional yet robust differential flotation circuit tailored to a silver-rich polymetallic deposit, positioned to deliver high-value concentrates to international markets.

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

Target Commodities: Silver, Copper, Zinc, Lead