Pressure Hydrometallurgy at Moa
The acid pressure leach process for the treatment of low magnesium content lateritic ore has been in operation at the Pedro Sotto Alba plant in Moa, Holguin, Cuba since 1959. The plant was originally constructed by Chemico for Moa Bay Mining Company, a subsidiary of Freeport Sulphur, but was taken over by the Cuban government in 1960. The plant recommenced operations in 1961, under Cuban management.
Under Cuban management the production at Moa gradually increased and improvements were made to the recovery of nickel and cobalt. In December 1994, Sherritt Inc. and General Nickel Co. S.A. announced the formation of a combined enterprise that included the Moa plant, now known as Moa Nickel S.A. The nickel and cobalt sulphides produced by Moa Nickel S.A. (13) are transported to the nickel and cobalt refinery at Fort Saskatchewan, Alberta, Canada now known as “Corefco” (The Cobalt Refinery Company Inc.), a second combined enterprise company, for processing to pure metal products.
At Moa, Nickel limonite ore is processed in a high-pressure acid leach to selectively dissolve nickel and cobalt from the ore. Concentrated sulphuric acid is the lixiviant.
The ore consists entirely of mixed oxides; sulphides, sulphates and carbonates are not found in the ore. The ore typically contains 1.3% Ni, 0.12% Co, 0.55% Mg, 4.8% Al, 4.3% Si and 2% Cr.
Extensive laboratory investigations demonstrated that high extractions of nickel and cobalt could be obtained with a minimum dissolution of iron and aluminum by operating the leach at temperatures in the range 230 to 260C. At these temperatures, acid consumption is minimized since the majority of the dissolved iron and aluminum hydrolyze and precipitate with the liberation of the acid initially used to dissolve these species.
The leach circuit consists of five independent trains, each having four Pachuca type vertical reactors connected in series. Ore thickener underflow pumps deliver slurry to a direct contact slurry preheater where the slurry is heated by low pressure (100 kPa) steam to between 70 and 80 degrees celcius The preheated slurry is delivered to a direct contact heater where the slurry is heated to 246 degrees celcius with high-pressure (4 500 kPa) steam. This steam is a combination of reactor vent gas and fresh steam supplied from the powerhouse and the acid plants. The use of direct steam for heating the feed slurry results in a reduction in the solids content of the slurry.
Each Pachuca reactor contains a draft tube and is agitated by the injection of high-pressure steam. Concentrated sulphuric acid is fed to the first reactor. Slurry passes through the reactors in the train by overflow pipes.
Slurry overflows from the fourth reactor through two parallel slurry coolers where the temperature is reduced to 135C. The low-pressure (100 kPa) steam used in the slurry preheaters is generated in these coolers. The slurry discharges to flash tanks and then to the counter current decantation (CCD) wash circuit. Low-pressure steam from the flash tanks is used to preheat the solution entering the sulphide precipitation autoclaves.
The parameter that has the biggest impact on metal extractions is the acid addition, which in turn sets the free acid concentration in the leach discharge solution. Metal extraction increases with increasing acid addition. Acid addition also affects the sedimentation characteristics of the leach discharge slurry, with the sedimentation of the slurry improving at higher acid concentrations in the leach discharge solution. Aluminum is the largest consumer of acid.
Since the plant commenced operation, metal extractions have averaged 94% and values in excess of 95% are now regularly obtained with good control of acid addition. The metal concentrations in the leach discharge solution are largely influenced by the solids content of the ore slurry feeding the leach reactors. The nickel concentration is generally in the range 6.5 to 7.0 g/L and the free acid concentration is in the range 30 to 35 g/L.
A major feature of acid leaching of limonite ores is the formation of scale in the reactor. The scale builds up to a thickness of 75 to 100 mm over a period of three to four months. Periodically each leach train is shut down and the scale is removed by pneumatic hammers. The majority of the brickwork in the leach reactors is the original lining installed in 1958. Small repairs to the brickwork are carried out as required when the leach train is shut down for scale removal.
Leach residue is separated from the metal-rich leach solution, raw liquor, in a seven-stage CCD circuit. Raw liquor is contacted with hydrogen sulphide gas to precipitate copper as copper sulphide and reduce ferric iron to the ferrous state, and hexavalent chromium to the trivalent state. Limestone mud is then added to the raw liquor to neutralize excess free acidity and raise the solution pH to 2.5.
Nickel and cobalt are selectively precipitated from the neutral solution by hydrogen sulphide gas in the sulphide precipitation plant. Zinc also precipitates, but magnesium, manganese, iron and aluminum remain in solution. The precipitation reaction is carried out in four, three-compartment, mechanically agitated, horizontal autoclaves.
The neutral solution is preheated to 82C with flash tank exhaust steam from the leach plant, and is then heated by 100 kPa steam, generated in the leach plant coolers, to between 116 and 121C. The autoclaves operate at 121C and 1 034 kPa. A continuous vent is maintained in each autoclave to remove inerts and control the hydrogen sulphide concentration at 60 to 70%. The vent gas is cooled, enriched with fresh hydrogen sulphide and used in reduction. Slurry discharges to the flash tank with the vent gas from the flash tank cooled by direct contact with water. The gas is compressed and recycled to the autoclaves with makeup of fresh gas. The water from the vent gas cooler is discharged with the precipitation thickener overflow solution as waste liquor.
The slurry from the flash tank, which has a solids content of about 3%, is fed to one of two parallel thickeners. Flocculant, Magnafloc 455, is added to reduce the suspended solids content of the overflow solution. The solids settle rapidly and an underflow solids content of up to 60% solids is obtained. This slurry is stored in tanks before transportation to the washing and drying section at the port, where the sulphides are bagged for shipment to the refinery in Fort Saskatchewan.
Three of the original autoclaves and linings are still in use. The autoclave linings are brick and rubber. All autoclave internals are Hastelloy C or titanium. After about 1 200 h of operation, an autoclave is shut down to remove scale buildup from the walls. The scale, which is chemically the same mixed sulphide, is recovered, ground and shipped together with the precipitated mixed sulphide product.
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