Wet milling; Treating asbestos tailings

Minimizing a worker’s exposure to asbestos fibre is a top pri for operators of mills that treat asbestos ore. Current Canadian regulations permit two fibres of asbestos per millilitre of air, while other countries have lowered the limit to 0.5 fibres per ml. Baie Verte Mines, a 100%-held subsidiary of Cliff Resources Corp., which operates an asbestos mine in north-central Newfoundland, thinks it has come up with a solution to air-borne fibre — a solution that could keep the operation going for another 18 years. To recover more fibre from its ore, the company has spent $18 million to construct a wet mill that in March began recovering more asbestos from the tailings stream of the existing dry mill. When that dry mill is decommissioned three years from now, the new mill will continue to treat asbestos tailings that have been stockpiled for the past 36 years. At a rate of 50,000 tonnes per year, that would suffice for about 15 years of operation.

“Experience in the prototype mill (constructed at the Woods Reef mine in New South Wales, Australia in 1981) showed that occupational exposure to asbestos fibre could be maintained at less than 0.08 fibres per ml,” says Dr. Philip Stewart, manager of new process development for Minerals Commodities (which holds a 42% interest in Cliff Resources). He and Dr. John Andrews (now of the Dept. of Chemical Engineering at Monash University in Australia) developed the process in a research project which began in 1977.

The Baie Verte mine was reopened eight years ago after an 18-year shutdown. (The open pit was operated by Advocate Mines from June, 1963 to 1981.) Ore is milled in a dry milling process which recovers 75% of the available fibre in the ore. Every year, about 75,000 tonnes of this material are pressure-packed and exported to Europe, India, Southeast Asia, Japan and South America.

About 100 tonnes of new asbestos product will be produced every day in the wet milling process now under construction. To start, it will treat the tailings stream from the existing dry mill. This material contains about 2.2% recoverable fibre. Recoveries are expected to be about 95%.

The new mill, which measures about 30 x 36 x 22 metres high, was constructed on a fast-track basis. This means the detailed design, by ShawMont Newfoundland Design Associates, was completed after equipment had been ordered. “This was necessary,” Stewart says, “to ensure the project schedule would not be adversely influenced by equipment deliveries or inadequate vendor information. So far, the project team has been able to work around vendor and equipment shortcomings without incurring excessive additional costs or affecting the product schedule.” Kilborn Engineering did the original feasibility study in 1986 and updated that study in 1988.

The wet milling process involves five steps: washing, classifying, degritting, fiberising/classifying and blending.

(The remainder of this article is reprinted from a paper presented by Dr. Stewart at the second annual meeting of the Newfoundland branch of the Canadian Institute of Mining and Metallurgy, held in St. John’s in November, 1989.)

Process water is drawn from nearby Steam Bath Pond. Prior to being introduced into the process, it is filtered and softened to ensure that there is no contamination of the final product or scaling on milling equipment.

A conveyor belt feeds tailings, sized to minus 16 mm, from the dry mill to the wet mill plant. This material is washed, to remove fibre from the waste rock, producing a slurry for further treatment. The waste rock is conveyed to the tailings area.

Fibrous material is then classified to separate crude from open fibre. Crude fibre is material that is still in tight bundles. Open fibre consists of material where the bundles have been fluffed up and individual fibres have been separated out. The open fibre is wet-screened to remove non-fibrous dust. Cleaned, open fibre is then sent to product storage tanks.

Crude firbre is screened to remove grit. This material is then opened, using a rod mill and high shear agitation. Following classification, open fibre reports to the product storage tanks.

Dewatered fibre is pelletized and dried

Cleaned, open fibbe is dewatered in a filter press supplied by Eberhard Hoesch & Sohne of Duren, West Germany. This material is pelletized and dried in a propane-fired dryer, manufactured by Aeroglide Inc. of Raleigh, U.S. The final product is free-flowing and dust-free. It is held in 1-tonne bulk bags, to permit quality evaluation prior to blending and packaging.

Dust from the open fibre circuit and grit from the crude fibre circuit are collected in a thickener and pumped to tailings. About 70% of the process water is returned from the thickener to the plant for immediate re-use.

The building and general arrangement of equipment have been designed to take advantage of gravity flow, wherever possible. As a result, there are four levels. Pumps and most vessels are on the firrt floor. Screens, cyclones, conveyors, dryer, pelletizer, etc., are on the upper floors. The gravity flow water treatment plant is on the third level. The advantages associated with a gravity flow design outweigh the cost of installing heavy tanks at this elevation.

In addition to performance, cost and maintenance requirements, a primary consideration in selection of process pumps was the type of seal employed. Early in the design process, a decision was made to eliminate (or at least minimize) seal water requirements. Consequently, pumps with expeller-type seals were selected wherever practical. This required careful selection of pump speed and impeller diameter to match the suction head each pump would experience in service and ensure good seal performanne. Wilfley rubber-lined (series K) and hard metal (series HD) pumps were selected for abrasive services. Worthington all iron (series FRBH) pumps with expeller seals were used elsewhere.

For service with slurry containing significant concentrations of open fibre, Worthington FRBH pumps with flushed mechanical seals were used because of concerns over fouling of expeller seals. Consequently, despite our best efforts, about one-third of the 30 process pumps in the plant require seal flushing water.

Floor sumps will be pumped using Flygt submersibles. These pumps were attractively priced (compared with vertical cantilevered models) with the additional benefit of having the entire pump and motor below the floor. This installation is more attractive and less likely to be damaged.

Pipe materials include stainless steel for instrument air, black steel for water and polyvinyl chloride (PVC) or high-density polyethylene (HDPE) for slurry services. Most of the black steel pipe is joined using Victaulic zero flex couplings to minimize installation cost. The PVC is solvent-cemented, while the HDPE is joined using the Victaulic “Hugger” system with rubber-lined steel fittings.

The “Hugger” system was selected for the HDPE pipe in an attempt to make the piping and arrangement of equipment as compact as possible. Fabricated HDPE fittings and, to a lesser extent, PVC fittings are extremely bulky and would have required significant rearrangement of some process equipment. Caution is required, however, when using the Hugger system because some common types of couplings and fittings are unavailable and because the system is incompatible with standard Victaulic fittings.

Valveless Piping and High-flow Thickener

With few exceptions, all slurry piping is valveless. This minimizes potential water hammer and eliminates flow restrictions, which could cause blockages in piping that transports fibre.

The thickener is a high-flow design from Supaflo Technologies of Australia. This 15.25-metre-diameter vessel will provide the same service as a conventional 30-metre model. A unique feature is its scalloped bottom. This design allows a significant weight saving, as opposed to conventional straight-bottom thickeners. The thickener has been installed inside a building, annexed to the mill. This ensures reliable opera
tion and start-up in the event of discontinuous operation during winter. The thickener has been installed six feet above grade at the lowest point to provide usable space for installation of pumps, maintenance areas, etc., below the vessel.

The rod mill, classifiers and cyclones used in the mill are all second- hand equipment obtained through Nelson Machinery of Vancouver and Minpro Ltd. of Mississauga, Ont.

The wet process facility is equipped with its own substation, motor control station (MCC), programmable “logic controllers” (PLCS) and control room. The substation and MCC equipment are manufactured by Westinghouse and the PLC are manufactured by Allen Bradley. Total plant connected load is 1,700 kw.

The PLC system uses single-ended switching of electrical loads and is connected with the MCC via a remote input/ output rack. In this arrangement, all control relays are implemented in the PLC software, eliminating the need for mechanical relays in the mcc. The plant control room features a “process mimic board” and a personal computer, with enhanced color graphics capability as well as supervisory and data acquisition functions. The pc serves as the main operator link with the PLC.