Project Description

If permitted, the Crandon Mine would include facilities to support mining, mineral extraction and separation activities, wastewater treatment and discharge, surface water mitigation, and mine waste disposal. This page summarizes the design and layout of the following four major project facilities and provides diagrams (maps) showing their locations:

• Underground Mine;
• Plant/Mill Site;
• Tailings Management Area; and
• Soil Absorption Site;

For each facility, the mining permit applicant has submitted detailed engineering design plans. In response, the Department has reviewed and commented on the layout, design, and operational procedures that have been proposed by the applicant for each facility. As a result of feedback from the Department and other interested parties, the design of many project facilities has changed since the inception of the project.

Underground Mine

In order to extract and process the Crandon Ore Body, Nicolet Minerals Company (NMC) has decided to develop an underground mine -- which would have approximate dimensions similar to the ore body, roughly 5,500 feet long east-west, 2,200 feet deep, and slightly greater than 100 feet wide -- and would contain:

• one vertical production shaft for access from the surface and two vertical ventilation shafts on the east- and west-end of the ore body;
• a series of 11 horizontal mining levels with drifts (tunnels) placed at approximately 200-foot vertical intervals located in the rock formations adjacent to the ore body, called mine workings;
• vertical raises would be constructed to enable access between horizontal mining levels;
• ramps would also be used to connect between horizontal mining levels;
• a workshop for routine vehicle maintenance;
• a primary crusher for breaking-up ore;
• crosscut workings which are used as advancement points to blast and mine out ore, approximate dimensions would be 13 feet by 16 feet; and
• roughly 318 rectangular blocks of the ore body, called stopes, would be excavated with controlled blasting techniques (approximate stope dimensions are 75 feet wide, 100 feet deep, and 200 feet height).

The geologic processes that formed the ore body produced two portions of potentially economic ore. The larger part of the ore body is a zinc-rich massive sulfide formation, which means that pyrite and the zinc-copper-lead sulfide minerals make up the bulk of the rock. A smaller part of the ore body is the copper-rich stringer zone, where pyrite and the copper sulfide minerals are confined to veins in a larger matrix of more common bedrock minerals.

The underground mine would be a highly-mechanized operation where the zinc-rich ore would be removed first over 16 years and the copper-rich ore would be removed second over 12 years of operations.

This thumbnail image links to a larger version of a cross section map in PDF format of the underground mine for the proposed Crandon Project.

Initially, the ore would be excavated and transported by load-haul-dump vehicles to an underground crusher, and then hoisted to the surface for further crushing prior to processing in the mill (Plant/Mill site). The metallic minerals in each ore type would be concentrated in the mill and transferred by rail out of Wisconsin for smelting and refining. During each phase of mining, concentrates of zinc, copper, and lead would be produced at the plant/mill. Minor gold and silver present in the concentrates would be recovered at the refinery. Pyrite, or iron sulfide, the main sulfide mineral in the ore body, has no commercial value. It would be separated from the tailings to produce another concentrate. The pyrite concentrate would be mixed with cement to produce a paste at the backfill preparation facility, returned to the underground mine, and pumped into the mined-out stopes for use as backfill.

Refilling the mined-out stopes is necessary to mine safely and efficiently. The backfill could be waste rock, natural sand and gravel aggregates, crushed rock, or tailings. Paste backfill is a recent mining backfill technology where tailings are thickened on the surface prior to pumping back underground. It results in a higher density of backfill than can be achieved by slurried tailings.

To enhance mine stability and minimize the potential for surface subsidence the company has agreed to leave a minimum of a 100 feet thick layer of weathered bedrock, called the crown pillar, in place between the mine and the overlying glacial sediments. In addition, the placement of cemented paste backfill into the mined-out stopes will also reduce the risk of surface subsidence by decreasing the amount of permanent void space left underground.

During mine operation, roughly 2,000,000 tons of ore per year would be processed, the equivalent of 5,500 tons of ore per day. A typical stope would contain roughly 170,000 tons of ore, and it is estimated that during mine production six to eight stopes will be active at any one time.

Of the 55 million tons of ore to be mined, approximately 58% would be returned to the mine as pyrite concentrate paste backfill, 27% would be pumped to the Tailings Management Area as depyritized tailings slurry, and 15% would be shipped out as metal concentrates.

During initial project development and as needed throughout the rest of the project, the company has proposed an extensive mine grouting program to minimize infiltration of groundwater into the underground mine workings. Accordingly, holes would be drilled into the bedrock at the top of the ore body, and the grout material (cement and additional additives) would be pumped under pressure into the bedrock's interconnected cracks, voids, and fractures in order to reduce mine inflow. Grouting has been used extensively to control water flows in mines, tunnels, and dams in other areas.

An extensive grout "ceiling" would be installed over approximately the eastern one-third of the ore body where the bedrock is highly weathered and is more conducive to the transport of groundwater. If mine inflow remained higher than expected, the company would complete the grout ceiling over the remainder of the ore body and, if necessary, extend a grout "curtain" vertically to a maximum of about 400 feet down the sides of the ore body into less weathered bedrock. The company has stated that access ramps, vertical shafts, and drifts would be grouted as necessary to minimize infiltration.

In their permit application, the company has proposed to limit pumpage of groundwater from the underground mine to an average of 600 gallons per minute (the equivalent to 864,000 gallons per day) over any 30-day period. The local agreement with the Town of Lincoln also specifies such a limit. If mine pumpage reached this threshold, the company proposed to suspend mining operations (such as blasting and moving/hoisting ore) and implement measures to reduce mine inflow until mine pumpage was reduced below the maximum permitted rate. The Department intends to closely monitor the amount of water to be withdrawn from the mine to ensure this limit is met if the project is approved.

After mining activities are complete, the mine would be allowed to reflood in order to remove air from the sulfide minerals and limit the release of additional contaminants. The movement of contaminants from the mine, and the level of contaminants in groundwater, is largely dependent on the flow of groundwater in the vicinity of the mine and the quality of the water present in the mine area. During closure, NMC would implement the closure plan as described in the Reflooded Mine Management Plan and would include: plugging the shafts at the bedrock surface, removing spilled ore, washing oxidation products from mine workings, removing and processing oxidized material from the stope access points, and accelerated re-flooding. Approximately 10% of the underground workings would remain open, including the access ramp, horizontal drifts, mine service areas, and vertical shafts and raises. Backfilling the underground workings completely is not practical and would be hazardous to mine ventilation and safety prior to final evacuation.

Plant/Mill Site

This thumbnail image links to a larger version of a map in PDF format of the Crandon Project Plant/Mill.

The proposed Plant/Mill site would contain a stockpile for ore extracted before the concentrator is built (pre-production ore), a stockpile for construction materials, the topsoil stockpile, the mineral concentrator facility, the low-sulfur waste rock storage area, the wastewater treatment plant, wastewater storage basins, rainfall runoff basins, treated wastewater storage lagoons, the access road from State Highway 55, a 2.7 mile railroad spur from the Wisconsin Central Limited trunk line, a parking lot, and additional support features. The approximate disturbance footprint would encompass 129 acres.

The mineral concentrator facility, located north of the main production shaft, would contain five major components: the grinding building, the reagent preparation and storage facility, the flotation building, the concentrate dewatering and loadout building, and the paste backfill preparation facility. These connected buildings would contain the facilities and equipment to recover the minerals and produce concentrates of zinc, copper, lead, and the pyritic paste backfill. The recovery processes would involve grinding, flotation, thickening, filtering, and concentrate storage/loadout, in addition to the facilities necessary for preparing the paste backfill.

The mineral recovery process would involve the use of numerous industrial chemicals, some in large quantities. Sulfuric acid, sulfur dioxide, and sodium cyanide are three examples of chemicals that would require special transportation, storage, handling, and use precautions. A separate spill prevention control and counter measure plan would be prepared for diesel fuel and petroleum-related products at the mine and mill. The applicant has proposed a Contingency Plan detailing responses to all potential spills or accidents in the Mine Permit Application.

Tailings Management Area and Reclaim Pond

This thumbnail image links to a larger version of a map in PDF format of the Crandon Project Tailings Management Area.

The Tailings Management Area (TMA) would be designed as a lined solid waste disposal facility where tailings plus waste rock and minor amounts of other waste will be permanently disposed. The TMA would include an access road from the Plant/Mill site, the tailings distribution pipeline, six rainfall runoff (stormwater) basins, soil borrow area, and the temporary high-sulfur waste rock storage area. The reclaim pond, a lined basin for recycling mill process water used to transfer tailings to the TMA, would be located immediately adjacent to the TMA. The total area of disturbance would be approximately 282 acres.

After clearing and topsoil stripping/stockpiling, the three TMA disposal cells would be built, operated, and reclaimed in sequence, and would provide surface disposal for a total of about 15 million tons of depyritized tailings and 1.6 million tons of waste rock. The design of each cell includes a composite liner (soil, geosynthetic clay liner, and a geomembrane), a leachate collection system consisting of piping and gravel and geocomposite drain layers, a final cover capping system (soil, geosynthetic clay liner, and a geomembrane), and surface and subsurface drainage systems in the cover soils. The cover and liner are designed to minimize water infiltration and oxygen penetration into the tailings mass, and exfiltration of contaminants from the facility into the groundwater. These systems are intended to work together to limit impacts to groundwater quality around the TMA. In fact, the facility design must provide for the protection of groundwater quality (based upon numeric standards) at the edge of the design management zone located 1,200 feet from the outside edge of the facility.

The tailings cells are proposed to be operated to maintain non-acidic, or neutral conditions as a means to minimize the potential for development of acid drainage. Neutral conditions are enhanced by partial flooding of the tailings surface during operations, removal of pyrite from the tailings to the degree practical, and burial of pyrite-rich waste rock within the saturated tailings.

Each TMA cell, the pre-production ore storage area, and the waste rock storage areas would be underlain with a barrier liner system to minimize leakage to groundwater. For each lined facility, the barrier liner starting from the surface down would consist of:

• 18 inches of on-site glacial till;
• 24 inches of granular drainage materials and collection piping on the bottom slopes, and geocomposite drain on sideslopes;
• geotextile protective pad (and filtering material);
• geomembrane;
• a geosynthetic clay liner (GCL); and
• 12 inches of low permeability soil (sieved on-site glacial till).

This thumbnail image links to a larger version of a cross section map in PDF format of the Crandon Project Tailings Management Area with design information on the line/cover system.

The reclaim pond would be constructed with a double composite liner (geosynthetic clay liner and geomembrane) with a geocomposite drain leachate collection system between the liners. Water depths over the liner of the reclaim pond are designed to be much higher than water depths over the TMA liner. The double liner system is intended to reduce exfiltration rates from the reclaim pond to levels comparable to those expected of the TMA.

After filling with tailings and pumping down the interior pond, each tailings cell would be covered with a grading layer of glacial material and/or low-sulfur waste rock. The grading layer accelerates consolidation of the tailings, defines final cover slopes, and reduces dusting and contact with air. The rest of the barrier cover system would include the same basic components present in the barrier liner system but would also include 6 inches of topsoil, 24 inches of on-site till (rooting layer) and 12 inches of cobbles (biotic barrier layer). The TMA surface would be restored to a native prairie-grassland environment.

NMC proposes to use geophysical leak detection surveying techniques as a means to detect leaks in the geomembrane installed in both the TMA liner and final cover systems, the reclaim pond, the wastewater lagoons, and the pre-production ore and waste rock storage areas. The surveys would be conducted before use of each facility, to detect rips and tears in the geomembrane component of the liner and cover systems. Leaks detected during the survey would be repaired at that time.

A temporary high-sulfur waste rock storage facility for waste rock with the potential to produce acid drainage would be built within the area that would later become TMA Cell 3. During the first 22 years of project operation this facility would store high-sulfur waste rock from underground mining activities. All of this material would eventually be disposed in the TMA in three foot thick layers and flooded with tailings slurry to minimize the potential for localized acid generation. The temporary waste rock storage facility would be eliminated during construction of TMA Cell 3.

Wastewater Treatment and Soil Absorption Site

Potentially contaminated water pumped from the mine and collected from lined facilities on the surface would be treated at the wastewater treatment facility located at the Plant/Mill site. In order to produce treated wastewater that would meet water quality standards for discharge to groundwater, the company has proposed to include a base treatment system and an advanced treatment system. In general, the water treatment system would include:

• addition of lime for neutralization and precipitation of dissolved metals;
• removal of metal precipitates and suspended solids by settling;
• addition of sulfuric acid for metal and sulfide precipitation;
• filtration;
• two-step reverse osmosis, a step necessary to meet numeric groundwater quality standards;
• evaporation for concentration and volume reduction of rejected water from the osmosis step; and
• processing evaporation water through an air stripper system for removal of ammonia.

This thumbnail image links to a larger version of a map in PDF format of the Crandon Project Soil Absorption Site.

Treated wastewater would then be pumped to two temporary storage lagoons located at the Plant/Mill site where effluent will be sampled and tested for compliance with effluent limits. If water in the ponds does not meet the prescribed standards it would be rerouted through the treatment process. Only water meeting the standards will be pumped 3.2 miles to the Soil Absorption Site (SAS) for discharge. The SAS distribution pipeline would be located adjacent to the railroad spur for the majority of its route, to minimize surface disturbance impacts. The pipeline is designed to handle about 700 gallons per minute to accommodate the proposed 600 gallon per minute mine pumping rate and some excess. Treated wastewater that meets the water quality requirements would be discharged or used as mitigation water for upland lakes impacted from mine inflow and groundwater drawdown.

The proposal to discharge treated wastewater into the Wisconsin River via a 38 mile pipeline has been replaced by the SAS as the proposed method for discharging treated wastewater.

The SAS would consist of six constructed seepage cells. From a central distribution station, treated wastewater would flow by gravity into one of six wet wells located at each seepage or soil absorption cell. Adjustable flow control devices would be provided to control flow going into each of the six wet wells. Within the absorption cell, similar to a household septic drain field, the effluent would be distributed by perforated-drain piping that would discharge at regularly spaced intervals into a gravel bed. The water would then infiltrate into the underlying, highly permeable glacially deposited sand units. The total area of disturbance would be approximately 70 acres.