Platinum group metals (PGM) have unique chemical and thermal properties, yet are extremely rare and have a high intrinsic value. As a result, PGM containing feedstocks are traded as part of a well established circular economy; and establishing an accurate value for each type of feedstock is essential. Wet chemical assay is a reliable method for determining this and requires highly chemical and thermally resistant crucibles during the conversion stage of hexachloroplatinate to platinum metal. The material choices for crucibles are discussed as well as their manufacturing methods and their impact on crucible lifetime and contamination risk.
Platinum Group Metals
PGMs are a group of metals that include Pt, Rh, Pd, Ir, Ru and are renowned for their unique properties including high melting point, low chemical reactivity, and their ability to catalyse reactions. As well as these properties they are also extremely rare, difficult to process and have a high intrinsic value.
Due to their price premium and unique properties, PGM containing feedstocks are often traded for subsequent refining and reuse. Their value means it is essential to accurately determine the quantity of PGMs in the whole feedstock for a fair transaction between the seller and the buyer.
Wet Chemical Assay Method
The wet chemical assay method is an extremely reliable method for determining the quantity of PGMs in a feedstock. In this method, the material is dissolved using a sequence of aqua regia, peroxide and hydroxide rinses. The solution is then evaporated and redissolved using hydrochloric acid. Base metals are precipitated off and a solution of hexachloroplatinate ions is formed. A precipitate of ammonium hexachloroplatinate can then be formed and filtered from the solution. In the final stage this precipitate is converted to platinum by heating in a heat resistant crucible to drive off the chlorides and nitride species. Hexachloroplatinate has a number of hazards including sensitisation, toxicity and is an oxidiser.
Typical temperatures during conversion are 600°C to 900°C. The temperature itself is demanding for most materials, but since highly aggressive species of chlorides and nitrides evolve at these temperatures the environment is particularly aggressive. The choice of crucible material is therefore important in order to achieve accurate measurements and to ensure crucible life.
The type of feedstocks can also affect the material choice, as this influences the chemical environment and temperatures required. This paper discusses the crucible materials used, and their manufacturing processes to maximise their lifetime.
Material Choices
Typical materials used for metal assay crucibles are porcelain and alumina. Porcelain crucibles are typically used if lower temperature conversion is needed (below 800°C), in this situation the feedstock is relatively clean and the environment is less chemically aggressive. However, if larger amounts of chlorination are required, or higher temperatures are needed (>800°C) more chemically inert crucibles are needed. The ideal material for this is fully dense, high purity alumina. Typically the alumina content should be >99.5%, and the minimum density should be >3.89g/cm3.
When considering the microstructure of the crucible, it is relatively easily to see why these requirements are necessary, and typical high purity alumina microstructure is shown in figure 1. Impurities in alumina tend to migrate to the grain boundary and combine in a glassy form. This glass is key weakness in the microstructure of lower purity aluminas, and is an opportunity for reactive species to attack. As the purity of the alumina decreases, the amount of grain boundary phases increase until an interconnected glassy phase is formed. This gives a continuous route for the species to attack and dissolve the material. Thus, keeping the alumina content above 99.5% significantly increases the resistance to chemical attack.
Figure 1 a typical high purity (99.5%) alumina microstructure
The theoretical density of 99.5% alumina is around 3.93g/cm3. For a consistent composition, a drop in the density is a result of increased porosity. Such porosity, or roughness to the surface, leaves opportunity for reactive species to preferentially attack. Therefore, improved chemical resistance is seen when the density of the alumina crucibles is above 3.89g/cm3
Crucible Manufacturing Methods
Both Porcelain and alumina crucibles can be slip cast. Here, the required raw materials are mixed with water, milled to the correct particle size to form a low viscosity slip. The slip is then poured into a plaster mould of the required shape. The water is drawn by capillary action into the plaster mould, and builds up a layer of ceramic particles on the wall of the mould. Once a sufficient thickness is built up, the remaining slip is removed. The mould and crucible can then be dried ready for firing. An illustration of this is shown in figure 2.
Figure 2 an illustrative explanation of slip casting
Slip casting is well established and ideal for crucibles of this size. The crucibles and the moulds dry relatively quickly without causing the capacity and footprint issues of larger, slower drying crucibles which reduce production capacity and increase cost.
An alternative route for manufacturing alumina crucibles is to use isostatic pressing. In this case, tooling consists of a metal mandrel with a rubber ‘bag’ exterior. A dry powder, containing the precursor ceramic and a binder is used to fill the cavity between them. The assembly is then placed in the chamber of the press, where the assembly is pressurised via a hydrostatic fluid such as water or oil. The hydrostatic fluid means that the powder is pressed from all directions minimising density variations, which can lead to shrinkage issues and cracking. This is illustrated in figure 3.
Figure 3 a illustration of iso-static pressing
The resulting crucibles tend to have a very smooth internal surface, and a more uniform density than many other ceramic processing methods. As a result, in application testing has shown that iso‑pressed crucibles tend to last around 5 times longer than alternative production methods.
Bespoke Design for Metal Assay
By contacting crucible manufactures directly it is possible to have bespoke crucibles manufactured to customer requirements. For example, one such customer contacted Almath Crucibles requesting design changes to the assay crucibles they were using. The high intrinsic value of PGMs meant they wanted to remove as much product as possible from the crucible after use. Furthermore, the customer focus on health and safety meant they were keen to maintain a flat bottom on the base of the crucible to prevent them from toppling over whilst containing the hazardous assay chemicals. A image of the bespoke crucible is shown in figure 4. Isostatic pressing was used to form a round interior, for ease of product removal; whilst creating a flat exterior base to avoid toppling. These crucibles now provide a longer life, easier product removal and improved safety.
Figure 4 image showing the bespoke crucible
Conclusion
Wet chemical assay of PGM feedstocks is an essential process that must be done accurately and repeatedly whenever PGM feedstocks are traded.
The selection of crucible materials and processing method is important to maintain crucible life in an aggressive chemical and thermal environment.
Porcelain and alumina crucibles are the most common materials used in the conversion process of hexachloroplatinate to platinum metal. Porcelain crucibles are used when the environment in less aggressive, but when high chlorination has been used alumina crucibles are necessary.
Slip casting and isostatic pressing are two production methods used for producing these crucibles and it has been found isostatic pressing offers an extended lifetime.
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