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Apr-2014

Recovering tungsten from spent hydrocracking catalysts

When catalyst is no longer serviceable for hydrotreating, it takes a metallurgical route to supply tungsten for steelmakers

JEAN-PIERRE DUFOUR and SOPHIE COMTE
Valdi

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Article Summary

Valdi, located near Limoges in France, specialises in the recycling of industrial waste, more specifically stainless steel scrap along with metal hydroxide sludges and spent catalysts generated by oil refineries in its hydrotreating processes.

In partnership with Eurecat and L’Electrolyse, Valdi has developed an efficient recycling process to recover tungsten from spent 
nickel-tungsten catalysts (NiW catalysts). Among the spent catalysts which are treated, those containing nickel and tungsten are used to provide secondary tungsten for the European market in the form of a concentrated tungsten salt, as opposed to the primary tungsten extracted from mines.
All of these operations have been running for more than 10 years in conformity with European regulations including the Seveso directives.

Why recycling?
The European market for spent catalysts amounts to 20 000 tonnes of material. Of this total, only 1500 tonnes are spent NiW catalysts. Nonetheless, tungsten is a strategic metal as defined by Bureau de Recherches Géologiques et Minières (BRGM), the French geological survey (see Figure 1). Only two countries in the world are net exporters of tungsten and can easily control the price of ore.
 
NiW catalyst composition
Refiners use NiW catalysts for hydrocracking processes. The catalysts are placed in a column into which the feedstock is introduced under pressure with hydrogen. Different classes of catalysts are stacked in the column, one above the other, often separated by ceramic beds (see Figure 2). In most instances, NiW catalysts are positioned just under a layer of molybdenum-nickel catalyst (NiMo catalysts). This arrangement explains the formation of mixtures of NiW spent catalysts and NiMo spent catalysts. The entire recycling process begins with spent catalysts being classified and stored according to the characteristics of the material.
 
Recovering the tungsten
The NiW catalysts are sorted into two sub-categories:
• NiW catalysts without molybdenum
• NiW catalysts with a molybdenum content higher than 0.8%.

Valdi is able to treat NiW catalysts and the mixtures containing molybdenum. Table 1 shows the average composition of NiW catalysts received in 2013. Depending on the composition of each catalyst sample, the process recovers either molybdenum together with 
tungsten or just tungsten. In the first case (including the presence of molybdenum), the recycling process is completed by a further step involving the removal of phosphorus and arsenic.

Recovery of both molybdenum and tungsten from spent catalysts involves a combination of pyrometallurgical and hydrometallurgical processes (see Figure 3).
 
The roasting step
First of all, a new roaster that began operations at Valdi in 2011 is used to produce calcine, which provides feedstock for the following step. This roaster is specific; it was 
developed by Valdi to deliver highly effective desulphurisation and decarbonation, good productivity and very low cost of treatment. It was designed to be capable of treating all categories of spent catalyst. The conditions of roasting are controlled and the gas treatment is also closely monitored. Screening at the head of the furnace and at its output enables the extraction of ceramic materials found together with spent catalyst. In the roaster, the temperature and rate of air injection are adjusted in order to remove sulphur (as sulphur dioxide), to burn hydrocarbons, and to oxidise the metallic elements.

A stream of compressed air sends the gas into a post-combustion chamber which treats it for oxides of carbon. The fumes pass through three levels of filtration; the gas rejected is devoid of particles and does not exceed 50 mg/m3/hr of sulphur dioxide. These three stages of filtration are performed by sieves with different cut-off points: the first stage retains sublimated metal particles, the second and third stages treat the gas. Dust captured in the first stage of filtration is treated at the hydrometallurgy stage with the roasted calcine. The dust obtained after the second and third filtration stages is sold to the paper industry, so the process delivers zero waste. The roaster and its filtration system are the subject of patents.

The hydrometallurgy process
The calcine that was produced during the roasting process is treated by hydrometallurgy in three steps: first calcine is leached to produce a slurry containing tungsten and molybdenum, depending on the composition of the catalysts and their contaminants (which are mainly phosphorus and arsenic). In a step using decantation and filtration, a solid concentrate containing alumina and silica, along with oxides of nickel, is produced. This material is reserved for the pyrometallurgical process.

The soluble fraction of the slurry is purified to remove phosphorus and arsenic. These contaminants are not recycled and are disposed of as hazardous waste; they represent less than 1% of the starting material. Tungsten and molybdenum are present in the purified solution which is mixed with reagents to precipitate calcium tungstate or a mixture of calcium molybdate and tungstate. The yield of tungsten recovered in one shot is more than 85%. These metallic concentrates are calcined and sold to steelmakers, more particularly to companies looking for tungsten units, for example the EuroW company which produces tungsten carbide and cemented carbides, or Erasteel which produces steel and tungsten alloys. The hydrometallurgical process is protected by a patent.

The pyrometallurgy process
The leaching residues are melted at a high temperature in a submerged electrode arc furnace. Nickel alloys are obtained from this operation; they are sold to steelmakers while the silico-aluminate slag produced is used in roadworks, while dust produced from the melting stage is recycled.

Conclusion
The development of a recycling process for NiW spent catalysts by Eurecat, L’Electrolyse and Valdi was subsidised by the European Union and was awarded ‘Best of the Best’ project status in 2009. After several step developments in the process, in 2013 Valdi successfully treated more than 800 tonnes of spent NiW catalysts. New investments made in 2011 (a new roaster) and 2013 (hydrometallurgy capacity) enabled the company to reach 3000 tonnes capacity. With its pyrometallurgical and hydrometallurgical process, the company is able to recycle a large range of spent materials, including NiW, NiMo, NiCoMo and CoMo catalysts.


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