Joanne Frankham describes Johnson Matthey’s AquaCat technology for the recovery of precious metals

Precious metal recovery: Joanne Frankham describes Johnson Matthey’s AquaCat technology for the recovery of precious metals

Joanne Frankham

In Brief

* Precious metal catalysts have a long life and are cost-effective if the metal is recovered

* Traditionally, spent catalysts are incinerated to recover the metal

* The AquaCat process is an environmentally friendly metal recovery technique

* The AquaCat system reduces physical handling and fuel consumption

Precious metals are used extensively in the preparation of catalysts with application in many industries, particularly the pharmaceuticals industry. They are used in either a homogeneous form or, more commonly, they are heterogeneous, where they are fixed to a solid support to make handling easier.

In many applications the precious metal catalysts are the only option to provide the speed and selectivity required for the reaction. Additionally, the long life of these catalysts makes them the most cost-effective solution.

However, precious metals, for example platinum and palladium, do constitute a high initial investment. A key factor in the economics of the process is the recovery of the precious metal from the spent catalyst, so that it can be used again. To achieve this, fast and efficient removal of the metal is very important. However, spent process catalysts are contaminated typically with organic materials that were present in the reaction mixture. Often, these organics are hazardous, and have to be removed along with any carbon from the support, before the complex process of metal recovery can begin.

In the traditional method of refining, the spent catalysts are incinerated to destroy the organic content of the material. An ash suitable for sampling is produced. This ash is then evaluated and only after this is the precious metal content determined, before chemical treatment of the metal oxides that remain.

Peace of mind

Johnson Matthey, based in Royston, UK, has developed a new process, AquaCat, for the recovery of precious metals from spent catalysts. The first stage in the process is to determine the precious metal content of the material, before any processing takes place, using new patented direct sampling technology. The second stage of the process uses supercritical water oxidation. This patented process developed in collaboration with Chematur Engineering, Sweden, removes all hydrocarbons by converting them to carbon dioxide and water, leaving the precious metal in its oxide form.

AquaCat is suitable for many applications, particularly spent catalysts from industries such as pharmaceuticals, active pharmaceutical ingredients and process catalyst manufacturers, agrochemicals, flavour and fragrances and any other company using platinum group metals as a liquid-phase catalyst. The majority of these are heterogeneous, In addition, some solid materials can be made into a slurry in the sampling vessel, The slurry has a known mass of water and surfactant added to create a homogeneous suspension. These liquids are mixed repeatedly in an extensively validated process to ensure homogeneity. The liquid or suspension is then sampled from the recirculating loop to determine its precious-metal content before any processing takes place, giving the customer total peace of mind.

Johnson Matthey has validated the sampling process and compared it with traditional recovery methods. The quantity of metal found, when comparing the direct sampling and traditional incineration routes, shows that the two sampling techniques are equivalent (see Figure 1). Following analysis, the liquid is pumped to the AquaCat supercritical phase. The customer benefits from having the sampling earlier in the process, as results are available much sooner. This assists with future catalyst purchase planning.

[FIGURE 1 OMITTED]

At a temperature greater than 374[degrees]C and pressure higher than 221bar, the water in the AquaCat process reaches the supercritical phase. However, these parameters are varied, as required, for different feedstocks. In the supercritical phase the physical properties of water change. Most importantly, the solubility of gases and organic compounds are increased to almost 100%, while inorganic compounds become insoluble.

When oxygen is added to organic contaminants under these conditions, a very rapid reaction occurs aided by the presence of the catalyst, resulting in the ‘burning’ and almost complete destruction of the organics.

The product from the reactor then passes back through the economiser to preheat the incoming feed, followed by a steam boiler, where approximately 1t/hour of steam is generated and fed into the main site steam. The product is then cooled down to ambient temperature and pressure, before the oxidation gases are separated off.

The only gaseous emissions from this process are carbon dioxide and nitrogen at room temperature, leaving clean water containing a fine particulate phase of precious-metal oxides. The solid precious-metal oxides are then separated from the water, ready for refining and reuse as fresh catalysts.

Supercritical

There are a number of environmental benefits to using the AquaCat process. There is a reduction in the amount of physical handling of the customer material, as it goes directly into the vessel from the container in which it is supplied by the customer. The AquaCat system is autothermal–the energy produced is used in other parts of the process, leading to lower fuel consumption, During the supercritical reaction phase, carbon and all organics are completely oxidised to carbon dioxide and there are no carbon monoxide emissions.

As the process takes place at lower temperatures than the traditional process there are no dioxans, furans or nitrogen oxides (NOx) produced. Additionally, sulphur, phosphorus and halides are all contained in the system and not emitted to the atmosphere. The AquaCat process is well ahead of current environmental legislation standards. It enables companies to exceed the duty of care obligations that they are required to meet, with regard to spent catalyst recovery.

As a result of the supercritical nature of the process, AquaCat is able to treat materials that are unsuitable for conventional incineration. This includes residues that have a high-phosphorus, high-sulphur or high-organic content, or materials that have a high calorific value or are flammable,

Joanne Frankham is marketing and publicity officer at Johnson Matthey in Royston, UK.

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