Empire State Mines — 2025 Technical Report

Standfirst The Empire State Mines concentrator, commissioned in 1970 and refurbished in late 2017, currently operates intermittently at 200 t/h to 220 t/h, processing 6,500 to 7,000 tons per week on a one-shift, four-day schedule.

The Empire State Mines (ESM) concentrator, located in the Balmat mining district, processes mineralized material mined from the ESM deposits. Originally commissioned in 1970 and last shut down in 2008, the concentrator was refurbished in late 2017 and began processing mineralization in 2018. The flowsheet for the current operation includes crushing, grinding, sequential lead and zinc flotation circuits, concentrate dewatering circuits, and loadout facilities. The operating philosophy, as documented in the 2021 technical report, is fundamentally different from traditional continuous processing; the facility is designed to operate at its rated throughput but only for as many hours as necessary to match mine production.

Critical Data

Overview

From a metallurgical perspective, the optimal way to operate a concentrator is on a continuous basis to minimize the usual occurrences of sub-standard metallurgy on start-up and product losses on shutdown. However, in practice, the ESM concentrator operates intermittently, which from a metallurgical standpoint introduces some amount of instability during start-up and shutdown. The facility processes material from both underground and open pit sources, with the underground mine plan indicating mill feed with lead values in the order of 0.02%. At this low level, it is not necessary or economic to run the lead circuit, and the current operation uses the lead flotation circuit to pre-float talc and magnesium, as excessive talc in final concentrates results in high magnesium content and incurs penalties.

Key Process Stages

The crushing circuit receives coarse material from either the surface crusher or the shaft hoist via a 36-inch conveyor equipped with an electromagnet for tramp removal. The material discharges into the feed chute of a 6-foot by 14-foot Tyler Tyrock Screen, Model F-900, with a 1.5-inch deck opening. Oversize material reports to a secondary crusher, an Allis Chalmers Hydrocone, Model 1084 EHD (84-inch diameter, extra heavy duty) equipped with a 300 hp motor. The crushing circuit design criteria are documented in Table 17.1 of the technical report, with the actual crusher throughput capacity higher than indicated by production records, which show the hoisting rate rather than crusher capacity.

Fine crushed mill feed is conveyed to the grinding circuit via a 36-inch conveyor equipped with a four-idler Merrick weightometer, which discharges to the rod mill. The rod mill is an 11.5-foot by 16-foot Allis Chalmers mill with a 1,000 hp Allis Chalmers synchronous motor. The ball mill is a 12.5-foot by 14-foot Allis Chalmers mill with a 1,000 hp motor, charged with 2-inch diameter balls and operated in closed circuit with two Warman 26-inch cyclones. The final grind size was normally 80% to 85% passing 65 mesh at typical mill feed rates of 200 t/h to 220 t/h. Media charges were left in the mills on shutdown, and minimal difficulties were found during mill start-up. The rod mill was relined in January 2018 by Metso in advance of recommissioning.

Cyclone overflow reports by gravity to the head end of the lead flotation circuit. The lead rougher circuit consists of a single bank of seven Wemco 300 ft³ cells, with all air inlet ports wide open as slide gates are not in use. The current lead flotation cleaning circuit is single-pass, 1st stage cleaning only, with no additional cleaning stages. In its current state, the lead circuit functions as a talc pre-float, removing excessive talc from the feed.

The zinc rougher circuit consists of two parallel banks of Wemco 300 ft³ cells. The grinding circuit design criteria are documented in Table 17.2, with the existing circuit deemed adequate for future requirements.

Additional Interesting Data and Summary

Laboratory test work on the proposed mill feed indicates that there is no benefit in grinding any finer than was done in the past. If future plant test work demonstrates that finer grinding improves metallurgical performance, this could be accomplished simply by reducing throughputs and increasing operating time. While the mill has a design capacity of 5,000 t/d, underground mine production is typically no more than 1,400 t/d.

The open pit mine plan indicates that mill feed from open pit sources will have lead and silver grades high enough to produce a saleable lead/silver concentrate. Various options for utilizing the existing lead circuit are put forward for consideration, including maintaining the circuit in serviceable condition for short-term lead spikes, bringing the lead circuit back to its original design by adding 2nd and 3rd stage cleaners, or installing a single vertical cell as final cleaning stage. The second and third options are intended to produce a marketable lead concentrate, which may require handling higher-grade mineralization from open pit sources separately to avoid diluting lead values. At the low lead feed grade of approximately 0.02%, it will not be necessary or economic to run the lead circuit, and the concentrate could simply be pumped to the final tails pumpbox.

Key Processes

• Crushing circuit with primary jaw crushers and secondary cone crusher (Allis Chalmers Hydrocone, Model 1084 EHD, 84-inch diameter, 300 hp motor)
• Grinding circuit with rod mill (11.5 ft × 16 ft Allis Chalmers, 1,000 hp) and ball mill (12.5 ft × 14 ft Allis Chalmers, 1,000 hp) in closed circuit with two Warman 26-inch cyclones
• Sequential lead and zinc flotation circuits, with lead rougher consisting of seven Wemco 300 ft³ cells
• Concentrate dewatering circuits and loadout facilities
• Talc pre-float function using lead rougher and 1st stage cleaner

Source: Empire State Mines 2021 NI 43-101 Technical Report, 2021.

Project website: Empire State Mines 2021 NI 43-101 Technical Report