FALCONBRIDGE IN TIMMINS: KIDD CREEK: MILLING AND BEYOND

Clean, bright, airy and spacious — not uncommon for office space, but for a flotation mill? Or more unusually still, a smelter?

When it built its plant, Kidd Creek not only constructed a modern, functional mill and smelter; it had people in mind. It recognized that if its workers were satisfied with their surroundings and carried out their jobs in a healthy environment, there would be far fewer problems. As an added bonus, they would gain a reputation for dependability and for being the producers of a premium product.

Look after the people, as the saying goes, and the machines will look after themselves. But why did Texas Gulf build a mill at such an odd location, 25 km from the mine? Moreover, why did it build a smelter when there were other smelters in the region? These are questions that have cropped up many times over the years.

Why the location? Not exclusively because of the logistics, 15 miles (24 km) east of Timmins and on a principal highway, nor because it was close to a main railway and similarly situated with respect to a powerline. These would all have been seen as favorable factors but not compelling on their own account. The mine alone dictated the requirements. These included a heavy-duty, all-weather road, a major powerline and a railway spur to handle the daily tonnage of concentrates, or run-of-mine-ore. All the expensive logistical requirements had to be built from scratch.

Then the engineering studies at the mine site came up with two suprises that determined where the greater part of the plant would have to be. First, there was little bedrock within acceptable digging distance of surface, certainly not enough to support a 10,000-ton-per-day mill without piles and extensive concrete. Second, in a land seemingly overflowing with lakes and creeks, there was an insufficient volume of fresh water for dependable year-round use. And, indirectly related to the latter, there was the lack of space for disposal of 100 million tons of tailings or more.

Hence, the search for a new site and its present location — far enough from large centres of population to avoid down-wind atmospheric annoyance but still within convenient driving distance for most of the workforce.

There is the second question: Why go to the expense of building a smelter when there are other smelters in the area, especially considering the high cost of today’s modern smelter?

So far as the zinc smelter and refinery were concerned, the sheer volume of zinc concentrates that the mine would produce meant that they would have to be shipped, notwithstanding the technical hurdles involved and the discouraging expense of shipping costs as well as custom smelting charges. There is very little smelter capacity left in the U.S. Over the past 25 years, more than 60% of U.S. zinc smelter capacity has either succumbed to obsolescence or terminated operations to avoid the expense of adapting to environmental legislation. Undoubtedly, Canadian zinc smelters could have accomodated some of Kidd Creek’s concentrates but nowhere near the volume that was available.

Therefore the choice was determined by external cicumstances and in May, 1969, plans to start construction were announced. The zinc smelter was commissioned in 1972. (For reference, the mill started treating Kidd Creek’s open pit ore in late 1966.)

The rationale for building a copper smelter was wholly commercial and feasibility studies were under way by 1966. For various reasons, the copper smelter and refinery were not realized until mid-1981, and in the intervening 15 years copper concentrates were shipped to Noranda’s Horne smelter and refined by its Montreal-based subsidiary, Canadian Copper Refiners.

A mining company prefers to smelt and refine its own ores if that option is available. The company then has control of smelter charges and it has the opportunity to extract marginal values and unusual elements that would otherwise accrue very largely to the custom smelter’s account.

The Kidd Creek Mill

Ore at the 1990 production rate of 4.5 million tons per year (4.1 million tonnes) is crushed to minus six inches at the mine site. It is hauled by company-operated railway to the metallurgical site, just under 17 miles (27 km) away, and delivered to a 1,000-ton-per-hour fine crushing plant.

The mine produces two types of ore and until 1987-1988 it was the practice to crush, grind and concentrate the two types separately. The mill itself had four grinding-flotation lines, one of which was reserved for the lesser volume of ore that carried significant lead and silver values. Since 1988, the volume of this type of ore has diminished to the point that separate treatment is no longer warranted. The ores are now blended, no discrimination is made between them and the line that was once dedicated is now on standby or takes the not-infrequent surges of mine production. Present grade of the ore approximates 3.3% copper, 4.6% zinc, 0.15% lead and one to two ounces silver per ton.

The ore is ground to 50% minus 325 mesh in a rod mill/ball mill circuit (see accompanying illustration). The rod mills are 10.5 by 16 ft. (3.2 by 4.9 metres) lined with Noranda Ni-Hard wave liners and end liners of Domite chrome-moly. Pulp from the rod mill passes to a cyclone shared in common with its ball mill counterpart, and the oversize passes to a rubber-lined ball mill, 12.1 by 18 ft. (3.7 by 5.5 metres). One of the innovations in the milling circuit is the use of water recovered from the smelter at a temperature of 30degc. The warm water hastens the chemical reactions taking place in conditioning of the pulp prior to flotation, consequently reducing the conditioning time.

Both copper and zinc flotation circuits require the regrinding of middling products as a large proportion of the ore minerals are fine-grained and intergrown. The regrind ball mills are 7.9 by 12.1 ft. (2.4 by 3.7 metre) and reduce the middlings to 78% minus 325 mesh.

Returning to the generalized flow sheet, the copper and zinc circuits are similar. An initial rougher concentrate is given a 3-stage cleaning with the rougher tailings being reground and given a second rougher and a final scavenging float (the details of the circuit are understandably more comprehensive). Final copper concentrate grades are about 25% copper, 3.7% zinc, 0.60% lead and 12 ounces silver per ton. Final zinc concentrate grades are about 53% zinc, 0.8% copper, 0.05% lead and 1.5 oz. siver per ton.

Primary copper flotation is carried out using fifteen Wemco 66 machines in three banks of five cells, followed by cleaning in Denver No. 30 DR machines (100-cu.-ft. cells). Secondary roughing and scavenging is performed by thirty-two to thirty-six Wemco 66 machines at each stage. The Wemco 66 machine has a capacity of 60 cu. ft (1.7 cu. metres). Zinc roughing and scavenging involves thirty-four Wemco 66 machines followed by 29 of the same machines for cleaning.

Reverse Flotation

The former separate treatment of copper-zinc ores carrying important silver and lead values produced a poor grade of zinc concentrate which assayed 50% to 51% zinc. This low-grade material subsequently was upgraded to 53% to 54% zinc by the process of reverse flotation. In this sub-process, the gangue mineral was in fact pyrite and the object of the reverse flotation was to remove the gangue in order to leave an enriched tailing, which in this case was a high-grade zinc concentrate. Specifically, low-grade zinc concentrates from the standard 3-stage cleaning were first subjected to high pressure steam (125 pounds per square inch) at 85degc. This aggressive conditioning broke down and destroyed the molecular, thick film of attached collectors and presented a fresh mineral surface for reconditioning, thus permitting depression of sphalerite by sulphur dioxide and re-activation of the pyrite. Conditioning was followed by rougher and scavenger flotation with 3-stage cleaning of the pyritic concentrate. The eventual and valuable product was the “tailing.”

As noted, this sub-process is no longer used and a zinc concentrate assaying 53% to 54% zinc is regularly produced from the more tractable ores handled since 1987.

Mill tailings at 17% solids are pumped at the rate of 10,000 tons per day to a roughly circular area 2.75 miles (4.6 km) from the mill. The area of the tailings depository is 1,200 ha. The disposal arrangement is unique and entails the discharge of the tailings pulp, thickened to 62% solids, at the centre of the circular area. A very low-sloping cone of solids is generated from this central discharge point and the present slope of four degrees is not expected to exceed six degrees by the time the operation comes to an end. A partly clarified effluent migrates to the periphry where it is impounded by a concentric levee and it is then directed to a polishing pond where lime is added and final clarification carried out. More than 60% of the mill water is recycled from the tailings pond.

A focus of sustained interest and continuing innovation at the Kidd Creek mill is the optimization of metallurgy by computerization. Overall coverage of the control system is shown in the attached figure; it is a powerful tool and virtually all-encompassing. Alarms indicate fault conditions on lubricating oil pressures; bearing and motor temperatures; thickener and drive torques and gland water pressures. The Modcomp computer (acquired in 1983) runs programs calculating assays from digitized x-ray voltages. It also provides tonnage totals, process monitoring, set point control and copper-lead-zinc division alarm reports. In addition, there are daily metallurgical accounting and cumulative monthly and yearly summaries from assays of daily composite samples using Matbal 3, developed by the Canada Centre for Mineral and Energy Technology. This is a materials-balancing program for mineral processing circuits. All data are available by printer or individual terminal.

Flotation control is exercised through monitoring of pulp and froth levels and reagent addition. These in turn are dependant on xrf analysis of the process streams for assays, pulp densities and a number of other factors. The degree of control varies from wholly manual to a strategy of computer supervision.

Two x-ray fluorescence analyzers are each capable of handling 15 process streams and assay for copper, lead, zinc and iron. Cuts of the process streams are made every 20 minutes and the results, which include pulp densities and recoveries as well as assays, are available every 15 minutes.

Overall, the effects of centralized control have been a major saving in reagent costs, a reduction in operating manpower (through natural attrition) and, most important, a marked improvement in metallurgy.

Smelter and Refineries

With a nominal capacity of 132,000 tons per annum (120.000 tonnes), the Kidd Creek zinc unit ranks third in capacity out of a Canadian total of four (Cominco, Noranda, Falconbridge and Hudson Bay Mining & Smelting).

Eighty-five per cent of the output is produced by an orthodox leach-roast-electrowinning process, and since 1983 the balance has been made up from a pressure leach (autoclave) system. The leach system is based on Sherritt Gordon technology. This is the world’s second commercial zinc autoclave plant, the first having been established by Cominco at the Trail smelter in British Columbia.

In the orthodox plant, zinc concentrates are calcined in Lurgi, fluo-solids roasters and the sulphur dioxide cleaned and passed to a Monsanto absorption sulphuric acid plant. The calcine is leached with sulphuric acid to dissolve zinc oxide, the crude zinc sulphate rigorously purified and then electrolyzed. The pure zinc cathodes are melted and recast into saleable form.

The plant initially produced a nominal 113,000 tons (or 102,500 tonnes) per annum, but later studies showed that much of the capacity was underutilized. The sulphuric acid plant was throttling production and creating a bottleneck. It was well within the capabilities of the rest of the plant to increase output. The only practical way for realizing the full potential of the plant was to employ a autoclave system for generating more zinc sulphate for the electrolytic plant. The obvious first choice of a marginally larger sulphuric acid plant was out of the question. It would have to have been “between sizes” and custom-made.

The autoclave treats zinc concentrates pulped in sulphuric acid, at a pressure of 160 p.s.i. (1,100 kpa) and a temperature of 130deg to 145degc. Zinc extraction averages above 95% with iron and elemental sulphur passing into the tailings in solid form. The zinc sulphate produced by autoclaving passes into the calcines circuit and is processed in the common stream.

If the greater part of the zinc smelter is a well-known, long-established technology, then the Kidd Creek copper smelter is the opposite. It was claimed to be the world’s most technologically advanced copper smelter when it first appeared in 1981 and it is improbable that this standing will have changed over the past 10 years. The basis for the accolade is Mitsubishi’s continuous smelting process. Dry copper concentrates enter one end of the furnace and blister copper exits the other. The key is continuous processing. The traditional batch nature of copper smelting is avoided.

THE KEY TO THE MITSUBISHI SYSTEM IS CONTINUOUS PROCESSING. THE TRADITIONAL BATCH NATURE OF COPPER SMELTING IS AVOIDED.

Ever since the end of the Second World War, there has been a major effort to improve the technology of copper smelting. Until then, there had been only one major change in the extractive metallurgy of copper since the introduction of the reverberatory furnace at the end of the seventeenth century. The major improvement was the air-blown converter. The copper smelting industry had, of course, grown enormously since its early days, but the changes were in size rather than technique. Copper smelting was still a batch process and increases of output came about by building larger furnaces and building more of them. Since the Second World War, several processes have been developed, the Mitsubishi process being one of them.

As practiced at Kidd Creek, bone dry copper concentrates carrying about 0.5% moisture are blended with flux and recycled slag. The mixture is blown into the 33.8-ft.-diameter smelting furnace (10.3 metres in diameter) through 1.5-inch (38-mm) pipes. Each of the feed pipes is enclosed within a 3-inch-diameter (76-mm) lance through which oxygen enriched air is blown. In the furnace’s enriched atmosphere, the sulphide oxidizes rapidly and melts to a matte and slag that runs into the well of the furnace. From the well, the slag flows by gravity to an electric furnace where the two products separate. This second furnace is the slag-cleaning furnace. It discharges a continuous stream of molten slag assaying 0.7% copper. This is granulated with a stream of water and discarded as a final product. Simultaneously, a 68% copper matte is syphoned from the furnace well and dicharged continually into a cylindrical converting furnace 26.9 ft. in diameter. This furnace is equipped with lances similar to those in the smelting furnace and in an analogous manner the matte is reduced to blister copper. The slag produced here is averages 15% copper and is recycled.

KIDD CREEK RANKS THIRD IN CAPACITY OUT OF FOUR CANADIAN ZINC UNITS

That is the essence of the process — continuous, totally enclosed, no external solid or gaseous fuel and a rich sulphur dioxide gas for acid manufacture. From the converting furnace, the blister copper passes to a holding furnace, then to two anode funaces where it is partly fire-refined and then to a Hazelett continuous casting machine. At that machine, a continuous stream of molten copper is transformed into a uniform strip of 0.71 inches (18 mm) thick. It is then stamped, without leaving any waste, into precisely sized anodes for electrolytic refining.

The copper smelter and refinery started operation in 1981 at a design capacity of 65,000 tons (59,000 tonnes) and produced 31,600 tons (28,700 tonnes) that year. Progressive increases in smelter capacity were achieved by increased oxygen to the smelting and converting furnaces and corresponding modifications to the acid plant such that, in 1990, blister copper production from the smelter was 116,000 tonnes. In 1986, the refinery was expanded and is now capable of producing 104,000 tons (94,000 tonnes) per year.

KIDD CREEK V.P.

The vice-president and general manager of the Kidd Creek operation, W. Warren Holmes, was born and educated in South Porcupine, Ont., practically in the shadow of the Dome mine headframe. He graduated in 1964 from Queen’s University as a mining engineer. He joined Noranda Mines (now Noranda Inc.) as an engineer-in-training at its Quebec operations.

In late 1965, he was transferred to the Timmins area and appointed general superintendent of Pamour Porcupine Mines in 1974. After a 2-year absence, during which time he gained a Master of Business Administration from the University of Western Ontario, Holmes became mines manager and then vice-president and general manager at Pamour.

In 1986, he joined Falconbridge as manager of mines in Sudbury, rising to vice-president and general manager in 1989. Late in 1990, he was given his current position at Kidd Creek.

CUSTOM FEED AT SUDBURY

Total nickel production from Falconbridge’s Sudbury smelter amounted to 37,000 tonnes in 1990. About 3,600 tonnes of that total came from recycled metal through Falconbridge’s custom feed business. As well, custom feed contributed one million pounds, or 36%, of cobalt and about 5% of the copper processed through the smelter. Not bad for a sideline.

But in the years ahead, it could become even more significant. The nickel last year came from 39,000 tonnes of feed. “This year, that figue will probably rise to 45,000 tonnes,” says Michael Humphries, manager of smelter custom feed. He hopes to double custom feed capabilities within five years. Custom feed includes primary sources such as concentrates and ores, secondary sources such as recycled aerospace alloys (turbine blades, etc.) and other nickel-containing metals, and tertiary sources such as catalysts and plating residues. The custom feed business is a welcome source of income from Falconbridge. Says Larry Seeley, director, metallurgical operations: “Custom feed income provided by the Norway refinery and Sudbury allows us to provide capital for future mine development and exploration.”