-26-April-2006

Saving lifecycle costs through use of mineral cast pumps

Where processing conditions combine both abrasive and corrosive attack, for example in pigment and fertilizer production, specialized materials are required for the manufacture of handling equipment such as pumps. German company Düchting Pumpen Maschinenfabrik has devised a materials solution based on the use of mineral casting – where particles of some mineral (in this case the synthetic ultrahard silicon carbide) are dispersed in a polymer matrix in liquid form and cast. Here, Dr Carsten Düchting discusses the development, characteristics and use of the company’s SICcast® material in pumps for handling such challenging media.

Most processes in the chemical pigment industry are characterized by a combination of corrosive and abrasive attack. Pumping chemically aggressive liquids with solid contents offers two challenges. Firstly, the pump material has to be chemically resistant against liquids like sulphuric acid or hydrogen chloride, which are used for digestion of the ore. As the strength of the acid is usually 80–95% even high alloyed stainless steels can barely resist. Secondly, hydraulic transportation of solids will obviously be accompanied by wear problems. The use of rubber-lined volute casings or plastic pumps helps to withstand chemical attack but offers only limited resistance against the problem of wear. Hard metal alloys provide good abrasion resistance but cannot be used in chemically aggressive applications. Hence, process engineers involved in pigment production report pump lifetimes not exceeding a few weeks.

Improving lifecycle costs

Based on practical experience, avoiding the use of metal in the construction of wetted parts can extend the lifetime of pumps significantly. For example, with clients involved in the production of both white and black pigments based on titanium dioxide (TiO2), a minimum lifetime of six months has been achieved. This represents much higher lifetimes than other technologies with metal pump parts, like impellers or wear-plates. Today’s procurement costs sometimes constitute roughly 10% of total lifetime costs, which shows the outstanding relevance of lifecycle costs. Playing a big role in the overall calculation, wear and maintenance handling have to be examined in more detail.

Erosion–corrosion occurs if metal materials are destroyed by fast flow velocities in combination with chemically aggressive conditions. Metals only offer the opportunity to fully meet one or other of the above-mentioned characteristics. Hastelloy has outstanding qualities towards corrosion but nearly no resistance against abrasion. This material can be used for acids but it is not suitable for fluids with solid content. Materials like duplex stainless steel represent a weak compromise between both but do not show satisfactory results especially not in combination with acids and particles. Ni-Hard 4 is a very hard material that has a good wear resistance but cannot stand acids or corrosive liquids for a prolonged time due to a lack of chrome. Finally, mineral casting offers the whole range of wear and corrosion resistance, which predestines this material for transportation issues in the chemical pigment industry.

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