The heavy vessel for conducting chemical reactions under high pressure was installed to replace the conventional brick- and lead-lined autoclave that had been causing reduced productivity levels and efficiency during its period of operation.
The stainless steel autoclave, which embodies the promise of reduced downtime, increased batch time and subsequently increased working volume, was put into operation 18 months ago and has since exceeded the expectations that were placed upon it.
The product has been patented globally, in addition to a registration deed to use duplex materials without the need for an additional brick layer, which would normally be a necessary precaution against corrosion in a traditional autoclave.
Implat's traditional autoclave, made of a design which has been in use for many years in the mining industry, comprised a layer of 40-mm carbon steel, a layer of lead lining to prevent acidic corrosion from affecting the carbon steel layer, as well as layers of bricks and vertical partitions between the layers.
High maintenance was required on the old autoclave, as bricks would collapse at the nozzles and partition walls. This necessitated frequent maintenance to avoid deterioration on the main shell and underperformance as a result. The frequent failure of the brick lining also caused contamination to the final product, thereby further reducing productivity.
Implats says that, among the engineers who operated the autoclave for many years, it was clear that a solution to the problems of the traditional autoclave design had to be sought out. The solution had to be an innovation, as no such solution yet existed, with a view to providing a safer environment for those who need to work around the apparatus, while also to reducing the costs involved in the manufacturing, installation, operation, downtime for maintenance and environment-sound disposal of the unit after its life has been depleted.
"Cost savings in both the manufacturing as well as in operating thereof are being realized, all at increased throughput and higher leaching efficiencies," says Implats engineering manager Joseph Roux.
Further, the innovative solution needed to be compatible with the platinum being manufactured in the plant.
The material of which the solution was to be fabricated had to be readily available and possess extremely high mechanical properties. It also had to be resistant to the high temperature and simultaneously provide the corrosion resistance required during the manufacturing process.
The material of construction had to be easy to work with, throughout the manufacturing processes, including rolling, forging and welding, and it needed to be used in the partition walls while remaining resistant to vibrations during the agitation process in the autoclave.
The construction material also needed to have similar properties to the bricks used before, in terms of corrosion resistance, so as to protect the steel material from the chemical substances used at the Implats refineries.
Samples of various alloys had been placed in the autoclave as part of a test, and they showed deterioration. The SAF 2205 material did not show any signs of wear or chemical attack by the chemicals used by Implats. Also, no pitting corrosion signs appeared on the duplex material.
An autoclave is a vessel used for conducting chemical reactions under high pressure thus requiring material suitable to withstand these conditions.
Traditionally, in the platinum industry, bricks are the material used to protect the carbon steel from corrosion. In the event of a failure or crack in the brickwork, an additional layer of lead is installed between the inner shell diameter and the brick liner. The total thickness of the lead lining and two layers of bricks would normally be between 200 mm and 250 mm.
The new stainless steel autoclave has allowed for the elimination of the use of lead, a substance that makes for a dangerous and harmful working environment. It is also a difficult material to dispose of safely without damaging the environment.
The environmental point of view has provided further reason for replacing the use of toxic lead substances with the use of the more environmentally sustainable duplex materials.
The duplex material makes up most of the shell, which consists of stainless steel plates formed in circular sections, assembled in abutting end-to-end composition, which are then welded together so as to form an elongated impermeable cylindrical body. This array of stepped and staggered welded plates increases the rigidity and helps avoid any continuous circumferential welds.
A series of manholes and nozzle openings were included for access into the interior of the vessel and for process connections to connect to the process piping as required.
A number of internal baffles are fixed at certain locations inside the body in order to create compartments for different stages of the process.
Agitators are also provided on the inside of the autoclave to ensure proper mixing of the acidic substances used for the leaching process. These agitators must be properly supported and secured to manholes to reduce vibrations as far as possible, thus reducing stresses and avoiding fatigue of the components.
A finite element method analysis is to be carried out on the components subjected to cyclic stresses to demonstrate the integrity of the elements and to prove that the walls and other elements do not exceed the maximum permissible stresses allowable for the materials construction.
In terms of the Occupational Health and Safety Act, the autoclave is defined as a pressure vessel, therefore, strict regulations in the Act had to be adhered to when designing and constructing the apparatus.
"The safety of the operators is the first priority. Compliance with the regulations requires that all operators exposed to hazardous substances are protected adequately and that any substance used, such as lead, which may be dangerous to any person, will be applied using suitable protective equipment," says Roux.
"Every precaution is taken to ensure that the safety of operators and any other personnel is not compromised when working with such hazardous substances for manufacturing or in operation".
Meanwhile, a patent application for the stainless steel autoclave has been filed, and a world patent registered to use the duplex material without the need for additional lead lining and brick layers, which acted as a corrosion barrier before.