Enhanced methods for nickel recovery from low-grade ores and bleed streams

Way back in elementary school when you thought of nickel, the first image that popped into your head was probably that nice shiny 5¢ coin. Even though we hardly carry change purses anymore, nickel is still ever present in our day-to-day lives. From the shiny stainless steel trim on your building, to the knives in your kitchen, nickel is found everywhere.With nickel becoming favoured for use in transport, power generation, cell phones, food preparation, and medical equipment, it leads one to think about where all of this nickel is coming from. 

The truth is, high-grade nickel reserves are being readily depleted, and mining companies are looking for new ways to recover nickel, this highly desired base metal. I’d like to dig a little deeper into the recovery of nickel, specifically from low-grade nickel ores on the Earth, and from bleed solutions from the electrorefining of other metals.

The Importance of Nickel Recovery

Nickel is the fifth most abundant element on Earth, at an average bulk of 2.7%. However, the average nickel content of the crust is estimated at 0.008 percent. With such low content, it makes it obvious why nickel mining is such an important industry, and why recovery of nickel and nickel recycling is becoming increasingly significant in our consumer-based society. 

Nickel has many important properties that contribute to its widespread use in industry, these properties include: corrosion resistance, conductive and magnetic properties, and its capability for electromagnetic shielding. Conductive nickel pastes are often used in capacitors: these thin layers of pastes are produced by using powders with very small spherical particles, to give tight packed layers that help increase conductivity. Nickel coated graphic particles are used for electromagnetic shielding in seals, gaskets, and consumer electronic products such as laptops and phones. The breakdown of primary nickel consumption in the US in 2012 is shown in Figure 1; the end-uses of nickel are shown in Figure 2.

Fig. 1: Primary Nickel Consumption in the US in 2012

Fig. 2: End Use Nickel Consumption in the US in 2012

Worldwide, approximately two million tonnes of nickel are used each year: 2/3 from primary sources and 1/3 from the recycling of nickel from end-of-use scrap. In the US it was reported that 99 000 tons of nickel were recovered from scrap in 2011, up from the 63 500 tons of nickel recovered from scrap in 2009, indicating from recycling is on the rise. 

Production of Nickel from Lateritic and Sulfide Ores 

In 2009, the world’s top nickel producers were Russia, Indonesia, Canada, Australia, and New Caledonia. Nickel is produced primarily from laterite and magmatic sulfide deposits. Laterites comprise approximately 70% of the world’s nickel deposits, while only contributing to about 40% of global production. Laterite is rock formed in tropical and subtropical climates, over long periods of erosion, and nickel-bearing laterites have an average content of 1-1.6% Ni.

At present, sulfide nickel deposits are the primary source of mined nickel. Sulfide ore grades range from 0.15-8% Ni, with 93% of known deposits ranging from 0.2-2% Ni.

All nickel ores have relatively low nickel content, the classification is as follows: high-grade nickel ore has Ni content greater than 1.8%, middle-grade nickel ore has Ni content between 1.3-1.7%, low-grade nickel ore has Ni content between 0.6-1.2%. Nickel is continuously mined, even at these low percentages of ores, because it is in high demand.

The variation in nickel concentration depends on the type of rock deposit that is weathered - ie. peridotite, dunite, pyroxenite, or serpentinite. Laterite formed from the weathering of serpentinite, for example, contains 45-55% iron and about 1% nickel, this is known as nickeliferous iron laterite. The other type of nickeliferous laterite is known as nickel silicate, which contains less than 30% iron, 30% silicon dioxide, and about 1.6% nickel. In general, laterite has high concentrations of iron, titanium, and aluminum oxides. 

With these low percentages, not all nickel recovery from laterite mining is economically feasible. In 2004, Dalvi, Bacon, and Osborne has indicated the following cut-off grades for laterite projects: smelting projects require ores greater than 1.7% Ni, and leaching projects require ores greater than 1.3% Ni, in order to be economically viable. They also stated that nickel sulfide ores mined for nickel need a grade of greater than 3% Ni for underground mining, and greater than 1% Ni for surface mining, in order to be profitable.

Nickel Recovery and Techniques of Mining Nickel from Lateritic Deposits

I’d first like to focus on the processing techniques of mining nickel from lateritic deposits, which can be a challenge to execute efficiently. Lateric ores are frequently treated by pyrometallurgical processes - using a furnace to smelt the dried ore and carbon as a reducing agent. Sulfur can be added if a matte is required, and further refining can be used to produce ferro-nickel or matte.

Limonitic (oxide) lateric ores are usually treated by hydrometallurgical methods: the Caron Process and Pressure Acid Leaching (PAL), Figure 3. Ores with high levels of magnesium are treated by the Caron process: using selective reduction of the ore, and ammonia leaching. This method is more energy intensive, and produces lower metal recovery.

The method of PAL involves preheating the slurried ore, and leaching with concentrated sulfuric acid at high temperature and pressure. Nickel and cobalt are converted to soluble sulfate salts, and are recovered from the slurry in a counter-current decantation circuit (CCD). CCD involves washing the residue and recovering soluble nickel and cobalt.The remaining acid is neutralized using a limestone slurry, which produces a gypsum precipitate. Hydrogen sulfide is injection to precipitate nickel and other sulfides. From here, there is further leaching to remove iron and copper, and finally precipitation of nickel by addition of ammonia, ammonium sulfate, and hydrogen. 

An excellent example the use of PAL on low-grade lateritic nickel ores is the Ambatovy mine in Madagascar, a joint venture between Sherritt, Sumitomo, and Korea Resources. This is one of the largest lateritic nickel mines in the world, and has the capacity to produce 60 000 tonnes of refined nickel a year, for at least 29 years. This mine also can produce 5 600 tonnes of refined cobalt, and 210 000 tonnes of ammonium sulfate. 

Fig. 3: Flow Sheet of the Processing of Lateritic Ores (Natural Environment Research Council, 2008)

Recovery of Nickel and Processing of Magmatic Sulfide Ores

The processing of magmatic sulfide ores differs from laterites: sulfide ores are crushed in multiple steps to separate the ore minerals from the gangue. At each stage the ore is separated by size, using vibrating screens, as well as magnetic separation of iron-rich pyrrhotite. After crushing, the ore is mixed with water to create a slurry and is ground to a powder. Water is then added again, to produce a suspension, and air is blown upwards through the tanks.

Here, chemical separation is used: chemicals are added to make some minerals repel water, allowing the minerals to float to the surface, this froth is then removed – designed to remove copper concentrate. The second stage of froth flotation produces a nickel concentrate of 10-20% Ni, along with other by-products and gangue. 

Smelting is the next stage, designed to recover as much metal as possible. Flash smelting is a common method: dry concentrates are fed into a furnace and heated to a liquid matte and slag. From this step, sulfide matte can be recovered, which contains cobalt, and nickel (about 70%).  Iron is recovered in the slag, and sulfur as sulfur dioxide. At this stage, pyrometallurgy and hydrometallurgy processes can be used to refine the metals. In pyrometallurgy, metals are separated from the contents of the matte by using heat – to separate based on chemical and physical characteristics such as melting point and density.

In hydrometallurgy, metals are separated based on difference in solubility as well as electrochemical properties. A common method for nickel leaching from the matte is by using ammonia under high pressure, producing a nickel-bearing solution.  This solution is heated to remove copper, and the use of hydrogen gas under high pressure precipitates nickel metal; remaining nickel in the solution can be precipitated by adding hydrogen sulfide. The process steps of the two methods of magmatic sulfide ores can be found in Figure 4. Chlorine and acid leaches can also be used to separate and concentrate the nickel, with the final step of hydrometallurgy being electrowinning.

Fig. 4: Flow Sheet of the Processing of Magmatic Sulfide Ores (Natural Environment Research Council, 2008)

Enhanced Methods of Nickel Recovery

New processes are being developed in order to be more cost-effective and environmentally sound. One example of this is Activox®, which removes the smelting step, and uses ultra fine grinding and pressure oxidation of the concentrate in an autoclave, prior to solvent extraction and metal precipitation.

Bioleaching is another process that is being looked at for low-grade ores, ideal for reworking waste dumps. This method, under atmospheric pressure, is being used in the Talvivaara project in Finland, a sulfide mineral resource of only 0.27% Ni.

Low-grade ores, like nickel sulfide, are abundant on Earth. Nickel sulfide deposits contain high levels of magnesium silicate gangue minerals (MgO) and can be difficult to process with conventional flotation and smelting technologies. MgO is hydrophilic, which interferes with flotation of sulfide minerals, and reduces flotation recovery. In China, the Jinchuan Group Go. Ltd has 400 Mt of low-grade nickel sulfide mineral ores, and uses bioleaching for nickel recovery. This method is interesting, because traditional techniques using sulfuric acid are disfavoured, since magnesium silicate minerals are so reactive in acidic media - consuming large amounts of acid, which increases operational costs.

High-grade nickel ores are almost exhausted, since they are so heavily exploited and less abundant than the low-grade ores. High-grade nickel oxides are produced by fluid-bed roasting and chlorine-hydrogen reduction of nickel matte. High-purity nickel pellets are produced by using vapour processes, an example of this is the carbonyl process: the copper and precious metals remain as a pyrophoric residue, which requires separate treatment.

Whether high-grade or low-grade, nickel ores need to be processed after mining, in order to upgrade their nickel content from 1-4% Ni to 10-20%. Concentrating the nickel ores usually takes place close to the mine site, and involves chemical and physical processes to crush the ore and separate the nickel-bearing and gangue minerals. 

Mining nickel directly is the obvious route to obtaining pure nickel, however, nickel can also be recovered as a byproduct of other metals. For example, copper electrolysis requires a bleed to eliminate the buildup of impurities that accumulate over time as a result of the electrorefining of impure copper anode to produce pure metal cathode. A bleed is required when certain impurity concentrations become too high in the electrolyte thereby adversely affecting the electrorefining performance.  The bleed stream is continuously withdrawn and replaced with fresh acid. The amount of bleed and the amount of replacement acid is controlled in order to keep the sulfuric acid content below 10%. This bleed stream is then further treated typically by neutralization to precipitate the contained metals.

Nickel can be recovered from the bleed stream of a copper electrorefinery. Classical methods for nickel recovery are precipitation, oxidation, and crystallization. Nickel and sulfate tend to build up in the copper refinery electrolyte and can be removed by crystallization as NiSO₄·6H₂O crystals from the bleed stream. They are then separated from the mother liquor in a centrifuge, and dried and bagged for shipment. 

In most copper refinery electrolytes the concentration of copper is between 35-60 g/L Cu (typically between 40 and 50 g/L), 120-200 g/L H₂SO₄ (typically between 150 and 200 g/L), and 0.3-25 g/L Ni. Soluble anode impurities continuously dissolve into the electrolyte, which is why they must be continuously removed from the bleed stream to prevent build up. Nickel is not the only metal removed from bleed streams, so are arsenic, bismuth, cobalt, iron, antimony, and 1-2% of the copper (owing to the fact that the corrosion of copper at the anode is faster than the plating at the cathode).

Nickel Extraction from Bleed Streams with SX

Another method for extraction of nickel from the bleed stream is the use of solvent extraction (SX). Copper-selective solvents can be chosen, and many steps are followed to get nickel and copper powder from the bleed. The following block flow diagram was shown in a paper by A. Agrawal et al. in 2012, demonstrating the steps of SX for nickel and copper recovery, Figure 5. 

Fig. 5: Flow diagram for the treatment of copper bleed stream by solvent extraction route (A. Agrawal et al., 2012.) 

SX is a tool used in the processing of complex and secondary non-ferrous metal resources due to ease of metal separation, purification, enrichment, and analysis. Similarly to the process shown above, certain solvents can be used to first extract copper, then adjust the pH to 9-10 for extraction of nickel. Copper SX plants are common, and they are a closed loop operation such that they use leaching and electrolysis to produce metal, while recycling the reagents in the system. For example, SX can be used to extract and separate nickel and cobalt from chloride, ammoniacal, and sulfate solutions. Queensland Nickel uses a SX route where cobalt is removed as a sulfide by a reagent, then the loaded nickel is stripped with a high concentration of ammonia-ammonium carbonate solution, and the stripped solution is used to produce basic nickel carbonate. 

Ammoniacal solutions are used in hydrometallurgical extraction of base metals because iron and manganese are rejected into the residue; solutions without these impurities can undergo metal separation by SX. Some examples of ammoniacal solutions include ocean nodules, nickel laterites, sulfide concentrates, super-alloy scrap and other waste materials, and sulfate solutions.

Sulfuric acid is the most common medium for hydrometallurgical metal extraction; SX processes often report the separation of nickel and cobalt from dilute sulfuric acid solution. This separation is carried out by cation-exchange type reagents. Ion exchange (IX) is used frequently in metal finishing industries for nickel recovery. Some copper refinery tankhouses also use IX for acid recovery which involves treatment of the refinery bleed stream from liberator cells with short bed IX systems to produce a concentrated acid strip for recycle to the tankhouse, and an acid-free byproduct stream. This acid-free stream contains impurity metals, as well as valuable nickel salts which can be further treated for nickel recovery, using electrowinning to produce nickel metal, or evaporation/crystallization to produce nickel sulfate crystals. 

Nickel and Copper Powder Production through Hydrogen Reduction

Another method of copper and nickel powder production, reported in 2008 by A. Agrawal et al., uses hydrogen reduction. This method takes the copper bleed and uses hydrogen gas to produce copper powder. The mother liquor with nickel sulfate solution then undergoes precipitation of copper with sodium sulfide to remove the remaining copper, followed by evaporation and crystallization to produce nickel sulfate crystals. The nickel is releached with ammonia followed by hydrogen reduction to produce nickel powder, Figure 6. Reduction of aqueous metal ions with hydrogen at high temperature and pressure can produce spherical particles, and hydrogen reduction of nickel can be done from different aqueous solutions.

Fig. 6: Flow diagram for the treatment of copper bleed stream by hydrogen reduction (A. Agrawal et al., 2008.)

Overall, I have only just scratched the surface when it comes to methods of nickel recovery. Since high-grade nickel ores are far and few between these days, fluid-bed roasting and chlorine-hydrogen reduction of nickel matte are becoming less popular techniques. The recovery of nickel from low-grade ores is increasingly more important, from new techniques of pressure oxidation of the concentrate in an autoclave to bioleaching of the nickel in the ore, it is evident that time and money are being put into researching these processes. Solvent extraction and ion exchange are also used in the recovery of nickel from various bleed streams from the electrorefining of other metals such as copper. The unique properties of nickel such as corrosion resistance, conductive and magnetic properties, and the capability for electromagnetic shielding, are what make nickel such a valuable resource. In the future, more recovery and recycling of nickel from secondary sources and low-grade ores will be even more important as ore grades decline and primary resources are depleted.

References

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Agrawal, A., Kumari, S., Parveen, M. and Sahu, K. (2012). Exploitation of Copper Bleed Stream for the Extraction and Recovery of Copper and Nickel by Bis(2,4,4-trimethylpentyl)phosphinic Acid. Mineral Processing and Extractive Metallurgy Review, 33(5), pp.339-351.

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