HYDROMETALLURGY

• Hydrometallurgy – The PAL process
• History
• Development – the 4th generation of PAL
• The process, step by step
• Processing of residues
• Liquid residues
• Solid residues
• Diagram of stages of the process

Vale inco is a world leader in research and development in the mining industry. For nearly 100 years, Inco has used new technologies or perfected existing ones to improve its methods of mining extraction, process technology, production and products.

Inco’s research has two key aims:

• Increased yield
• Improved environmental performance

The development of its PAL hydrometallurgical process is part of this constant search for improvement.

Hydrometallurgy – The PAL process

Hydrometallurgy is a metallurgical process by which metals are extracted from an ore by means of chemical reagents at a high temperature and under pressure, then separated to produce a concentrate or an intermediary product.

Hydrometallurgy recovers a higher percentage of the metal contained in the ore, is more respectful of the environment, and uses less energy than pyrometallurgical processes.

The process

 

History

Hydrometallurgy itself is not new. It has been used since the beginning of the 20th century for zinc, copper and aluminium. But it was not until the 1950s that a hydrometallurgical process using acid was developed to process lateritic nickel ores. The first plant was at Moa Bay, in Cuba. Projects were subsequently carried out in Australia, between the end of the 1990s and the beginning of the 21st century, including at Murrin Murrin.

Development – the 4th generation of PAL

The Vale Inco Nouvelle-Calédonie plant will use PAL, Pressure Acid Leaching. This is the fourth generation of the process: a method which has been perfected by Inco engineers to perform better than the methods used in previous hydrometallurgical projects.

Early research

Inco launched research into the development of its PAL process in 1990.
Laboratory research commenced in 1993 with ore extracted from the Goro Plateau, and a first pilot plant was built the following year.  
After ten major campaigns carried out at this pilot plant, Inco continued its tests on certain stages of the process which were considered critical, and, from 1997 to 1998, built and then operated a pilot installation on the scale of  1/3000th, in its Port Colborne plant in Canada.

Towards a commercial plant

Given the positive results of these tests, Inco decided to create a new company in New Caledonia and to build an integrated pilot plant on a larger scale (1/1000th) on the same site at Goro Plateau. Vale Inco Nouvelle-Calédonie was born.

At the same time, in 1998, the construction of three commercial nickel exploitations was commencing in Western Australia, including that of Murrin Murrin, all using the PAL process. These Australian projects unfortunately met with a series of technical problems, of which Inco took advantage in order to improve the engineering of its own process.
Thus, Inco selected phase technology permitting the identification and solution of technical problems as they appear, at the same time refining and optimising the hydrometallurgical process with a view to the future commercial plant.

The construction of the pilot plant, whose design was enhanced by the Australian experience, was completed at the end of 1999. Its operation over two years has confirmed that the PAL process now mastered by Inco was in a position to work economically on a commercial scale to process the ore from Goro Plateau. Inco thus took the decision to build a commercial plant.

Construction

The construction of the commercial plant commenced in 2002 but had to be suspended several months later, since significant overspends were forecast. The project was reviewed and construction resumed at the beginning of 2005. Production should start progressively in 2008.

The process, step by step

The process essentially consists of subjecting ore reduced to slurry to different treatments, through the addition of water, using an acid solution to extract from it nickel transformed into nickel oxide and cobalt, transformed into cobalt carbonate.

STAGE 1 – PREPARATION OF THE ORE - Turned into slurry

The ore preparation plant is located very close to the mine. The laterite ores (limonite) and saprolite ores (garnierite) are mixed with water, sifted and ground to form a sludge known as slurry. This slurry is piped to the treatment plant.

STAGE 2 - LEACHING

The slurried ore is preheated by steam and injected continuously into an autoclave with sulphuric acid. Leaching once again ‘washes' the ore with sulphuric acid. The role of the acid is to dissolve certain metals which are extracted in this way from the solid ore and transferred into the liquid solution. The high temperature in the autoclave (270°C) permits the acceleration of this extraction and therefore allows a greater quantity of ore to be processed in a small autoclave. However, this high temperature requires operation under high pressure so as to prevent the liquid from boiling.

The ‘leached’ slurry thus obtained contains solids (mainly iron oxide) and a liquid solution containing the dissolved metals including nickel and cobalt but also metals not recoverable for exploitation (magnesium, aluminium, chromium, zinc, copper, etc.). This slurry is then cooled again and depressurised. This operation generates steam which is recycled upstream in the slurry heating circuit before its injection into the autoclave.

STAGE 3 - COUNTER CURRENT DECANTATION

The leached and cooled slurry passes through a decantation circuit designed to separate and wash the solid residues from the liquid solution called ‘mother liquor'. In other words, the solids settle at the base of the decanter from which they are pumped (the underflow), while the excess liquid is collected (overflow).

To wash the solids well, the operation is repeated six times in six successive decanters. By the end of the operation, the mother liquor has recovered 98% of the nickel and cobalt contained in the leached slurry. The solids are sent in the form of a thick paste to the unit for treating solid residues, where they are neutralised.

STAGE 4 - PARTIAL NEUTRALISATION

The mother liquor contains not only cobalt and nickel, but also other metals, present in the original ore (aluminium, iron, chromium, zinc, silica, copper and manganese), considered impurities (since not destined to be recovered and processed), as well as sulphuric acid not used during the leaching process. The acid and some of the metal impurities are eliminated through the addition of limestone and lime to form solid gypsum (plaster), separated from the solution by decantation and filtration operations.

The gypsum and metal hydroxides form a thick paste which is sent to the waste processing plant. Copper is removed by precipitation then by absorption in an ion exchange circuit to remove the last traces. It is, in its turn, sent to the waste processing plant.

STAGE 5 - EXTRACTION BY SOLVENT

Aluminium, iron, chromium, silica and copper have been removed from the mother liquor but, as well as nickel and cobalt, it still contains zinc and manganese, as well as magnesium and calcium, major components of the limestone and lime used to neutralise the acid.

It is injected into a first extraction circuit in which an organic solvent captures the nickel, cobalt and zinc, leaving manganese, magnesium and calcium in the liquor. This solution is sent to the liquid residue processing unit. A second extraction, on contact with a small quantity of hydrochloric acid, releases the nickel, cobalt and zinc. This solution, whose volume is between 20 and 30 times less than that of the mother liquor, gives a concentrate of nickel, cobalt and zinc chlorides. The solvent, with the three metals removed, is introduced again into the extraction cycle.

Passage through a selective resin enables the zinc to be retained. Finally, a concentrated hydrochloric solution is obtained, cleansed of nickel and cobalt.

The last part of this stage consists of the separation of cobalt from the nickel, thanks to another solvent extraction circuit which extracts just the cobalt. Two pure solutions are created in this way: one of nickel chloride and one of cobalt chloride. The solvent, with the nickel, cobalt and zinc removed, is now available for another extraction cycle – recycled in a closed circuit.  

STAGE 6 - END PRODUCTS

The nickel chloride solution is processed in a fluidized bed oven, heated to a high temperature (800°C) through the combustion of a mixture of air and natural gas. The nickel chloride is then broken down into nickel oxide and hydrochloric acid which is reconstituted and recycled for the extraction process.

The particles of nickel oxide thus formed are in the form of spherical granules, comprising successive layers, something like pearls, producing small grey balls, round and solid. The hydrochloric acid is recycled to the first solvent extraction stage.

The cobalt chloride is neutralised by adding sodium carbonate (soda) to form a pulp of cobalt carbonate crystals, recovered, after decantation and filtration, as a purple powder.

Fiche Produit NiO

Fiche Produit COCO3

STAGE 7 - FINAL NEUTRALISATION - Processing of residues

Processing of residues

Throughout the process, despite recycling of some of the process wastes, liquid solutions containing different metal ‘impurities’ and solid residues are produced. The liquid and solid wastes are sent separately to their own processing unit for final neutralisation.

This stage consists of neutralising the acids (sulphuric and hydrochloric) and precipitating the metals still dissolved in the solution in trace quantities, so as to bring their concentration beneath the environmentally acceptable level, as dictated by regulations.

Liquid residues

Some of the hydrochloric acid not recycled in the process and the sulphuric acid remaining at the end of the process must be neutralised through the addition of limestone and lime slurry to produce, principally, gypsum (plaster). The metals precipitated during neutralisation rejoin the paste of solid residues which is, in its turn, neutralised.  

The composition of the liquid waste, thus removed of acids and solid residues, is similar to that of the marine environment: acidity (pH) and the concentration of chemical and metallic elements (calcium, magnesium, chloride and sodium sulphate) are comparable.

This ‘mineralised' water, called marine effluent, is discharged into the sea via a diffuser pipe. This is a 5-kilometre pipe whose final kilometre is fitted with holes to allow the effluent to be dispersed gradually and thus to be diluted rapidly in seawater. The strong current in the Havannah Channel where the diffusion takes place also contributes to rapid dilution. A few metres from the diffuser, it will no longer be possible to detect a difference between discharged water and the water of the lagoon.

Solid residues

Solid residues are neutralised in the same way as liquid residues, through the addition of lime and limestone. What is left is a paste containing metals in quantities equivalent to those present in the natural ore before processing. Gypsum from the neutralisation is added to this. This paste is also known as ore residue.

Diagram of stages of the process

• 1. Leaching

 

• 2. Counter current decantation

 

• 3. Partial neutralisation

 

• 4. Extraction by solvent

 

• 5. Production of end products

 

• 6. Final neutralisation – treatment of residues

It is envisaged that this neutralised mining residue be used to fill mining pits during production, then to cover them with unused laterite and topsoil and proceed to the rehabilitation of vegetation.

For the first pit, it has, however, been necessary to find a place to store the residues, there being no pit available for them.