Timothy G. Robinson

Electrowinning of Nickel from Purified Nickel Solutions

Electrowinning produces high-purity nickel metal by electrodepositing nickel from purified leaching solutions. Chloride and sulfate electrolytes are used. It is mostly used to recover nickel from the solutions produced by leaching sulfide mattes and sulfide intermediate precipitates. Chloride and sulfate electrowinning are both well-established. It seems, however, that future projects will be carefully assessed in terms of the occupational exposure to dissolved nickel salts.

Converting – Final Oxidation of Iron From Molten Matte

In the nickel industry, converting is the production of a low-iron matte, containing 0.5%-4% Fe, from a furnace matte that contains between 20% and 40% Fe. The iron in the furnace matte is most often removed from the matte into the slag by oxidation with air or oxygen-enriched air in Peirce-Smith converters. Nickel and other metals are not oxidized during this oxidation. These metals remain in the matte phase and are recovered during downstream processing. Peirce-Smith converting is a batch process that is chemically efficient but a poor collector of sulfur dioxide from inadvertent sulfur oxidation. An alternative to Peirce-Smith converting is flash furnace direct-to-low-iron matte production. It avoids the sulfur dioxide collection problem but it requires specialized technology for the recovery of nickel from the slag. The product of converting, the low-iron matte, is an excellent starting point for making nickel (and other metals) by leaching and other metal-production processes.

Chapter 1 – Overview

Worldwide, nickel, cobalt, and platinum-group metals are used extensively. Nickel is mostly used in alloys, for example in stainless steel, aerospace alloys and specialty steel. Cobalt is the key ingredient in rechargeable batteries. Platinum group of metals is largely used in the automobile industry. The purpose of this book is to describe how these metals are produced from the ores that are mined. Nickel is mined from two types of ores, laterites and sulfides which are given in a detailed flow chart. Laterites are found mostly in tropical regions and sulfides are mostly found in Canada and North Siberia. Laterites are complex ores and occur near the surface and mined using surface-mining methods. Laterite ores have three nickel-bearing layers, limonite, smectite and saprolite. Cobalt is also produced from Central African copper—cobalt ores. Platinum group metals are extracted from sulfide ores. Certain amount of nickel, cobalt and platinum group metals are produced from recycled end of used scrap. These methods help in conservation of energy and earths resources. This book can be broadly divided into four parts, which is extractive metallurgy of—nickel, cobalt, platinum group of metals, and recycling of these metals.

Smelting of Laterite Ores to Ferronickel

This chapter discusses industrial electric furnace laterite-to-ferronickel smelting and how smelting is controlled and optimized. The four main reactions that occur during ferronickel smelting are explained. Overall, these reactions are highly endothermic, that is, they require energy. Several ferronickel smelters are converting their power stations from fuel oil to coal. The recovery of nickel to ferronickel is between 90% and 98%. The loss of nickel is minimized by minimizing slag mass. Laterite smelting is done in electrically heated furnaces with suspended carbon electrodes. Two types of furnaces are used, either a rectangular furnace with six suspended electrodes or a circular furnace with three suspended electrodes. Great care is taken to avoid possible explosions by ensuring that water does not come into contact with molten ferronickel or slag. Great care is taken with all electrode maneuvers to avoid worker electrocution. Long-term shutdown of a furnace for major repairs and restart is accomplished by various procedures adopted. The typical life of a furnace before it needs to be re-bricked or rebuilt is10–15 years. Smelting of laterites to ferronickel in electric furnaces recovers nickel efficiently with little adverse impact on the local environment. The only disadvantage of smelting is the large amount of energy required by the electric furnaces. The last step in ferronickel production is reduction smelting of bone-dry, partially reduced nickel-rich calcine at 900 degree Celsius from the calcination/reduction kilns.

Upgrading of Laterite Ores

This chapter deals with upgradation of laterite ores. The upgraded ore is smelted to ferronickel or processed to nickel in hydrometallurgical refinery. It describes the mineralogy and profile of laterite ores, the process of upgrading, its benefits as well as various layers of this ore and their properties. The upgrading methods and the extents of upgrading at four laterite mines are given in a table. All upgrading of laterites is based on the principle that laterized nickel minerals, such as goethite and garnierite Upgrading this material requires that the weathered skin be broken into small pieces without breaking up the nickel-lean core. All laterite ore bodies are different. For this reason, every ore must be thoroughly tested to determine the extent to which it can be upgraded. Laterite ores are always upgraded before smelting or leaching. Upgrading of these ores involves - gently crushing and/or grinding the ore and separating the resulting small, soft, low-density nickel-rich laterized mineral particles from the large, hard, dense, nickel-lean, unlaterized precursor rock and by-product laterization products, such as quartz. Upgrading minimizes the amount of material that has to be transported, smelted and/or leached per tonne of product nickel. It thereby conserves energy, minimizes the usage of reagent and equipment requirements.

Refining of the Platinum-Group Metals

The feeds to the precious metals refineries are highly concentrated, containing between 15% and 70% in platinum-group metals. There are three broad categories of process for the refining of PGMs based on the technique used for the separation of platinum and palladium: (i) precipitation processes, used by Lonmin Platinum and Krastsvetmet; (ii) solvent extraction processes, used by Johnson Matthey, Anglo-American Platinum, Vale, and Heraeus; and, (iii) ion-exchange process, used by Impala Platinum. This classification is based on the method of separating platinum and palladium, the two most abundant of the PGMs. While each of these processes is different, they are all based on the same broad principles. The first of these is that the separation of any metal has three areas of activity: primary separation, secondary purification, and reduction to metal.

Recycling of Nickel, Cobalt and Platinum-Group Metals

Nickel, cobalt, and platinum-group metals are recycled extensively. Examples discussed are the recycling of nickel-containing stainless steel, cobalt-containing batteries, and platinum-, palladium-, and rhodium-containing catalytic converters for automobile. The first process in the recycle plant is most often smelting and converting, which makes a final product alloy, for example, stainless steel, or concentrates the recycle metal in sulfide matte or metal alloy ready for hydrometallurgical refining. The purity of the refined recycle metals or chemicals is equal to that of the primary products (that is., products from the ore).

Nickel Production, Price, and Extraction Costs

Nickel imparts corrosion resistance, workability, high-temperature strength, and attractiveness to most of its applications. It is mainly used in alloys, especially stainless steel. Nickel is produced from laterite and sulfide ores. Laterite ores are mainly mined in tropical islands and tropical South America, while sulfide ores are mainly mined in northern Canada and northern Siberia. Approximately 2 million tons of nickel are used each year. About two-thirds of this comes from ore while the remaining one-third is from the recycling of end-of-use scrap.

Separation of the Platinum-Group Metals from Base Metal Sulfides, and the Refining of Nickel, Copper and Cobalt

The primary purpose of the base metals refinery is to produce a PGM concentrate that contains about 60% platinum-group metals. This concentrate is the feed to the PGM refinery. There are two different routes to meet this primary objective. They are as follows: (i) the slow cooling of matte, followed by magnetic separation and (ii) the whole matte leaching process. The magnetic concentrate process has the advantage that it separates the main PGM production from the refining of base metals. The secondary objective is to produce nickel, copper, and cobalt products for sale. The copper and nickel are separated from one another by metathesis reactions in the first leaching stage, where nickel dissolves and copper precipitates. The solution from the first stage is purified to remove, lead, arsenic, iron, and cobalt, amongst others. The nickel in solution after purification is either crystallized, reduced using hydrogen to nickel powder, or electrowon. The residue from the first stage contains mainly copper, which is dissolved in the second leaching stage. The copper solution is purified and copper recovered by electrowinning.

Smelting of Nickel Sulfide Concentrates by Roasting and Electric Furnace Smelting

Roasting and electric furnace smelting accounts for about a quarter of nickel sulfide smelting. The other three quarters are done by flash smelting. The main advantage of roasting and smelting is the high recovery of nickel, copper, cobalt, and by-product precious metals. Its main disadvantage is its large consumption of electricity. Other advantages of electric smelting are (i) its ability to attain and control high slag temperatures, which is critical when the calcine feed contains considerable MgO, which has a high melting point, and (ii) its ability to efficiently smelt metal-rich scrap and other recycle materials.