Stainless Steel Awards 2008

OVERALL WINNER & WINNER PRODUCT CATEGORY



Impala Platinum Holdings
Product : Autoclaves made from Duplex Stainless Steel instead of carbon steel

Impala Platinum scoops this year’s overall prize at the Stainless Steel Awards 2008 for its Second Stage Leach Autoclave. It also takes top prize in the product category for astounding the industry and the judges with its revolutionary vessel that is lighter, safer and has drastically improved efficiency. 

This unit was manufactured and installed as a replacement to the conventional brick and lead lined autoclave that required extensive maintenance and repairs. The new duplex stainless steel (SAF 2205) unit has been in operation for the past 18 months and its performance has exceeded the highest expectations.

“The use of cutting-edge technology both in the material application as well as the manufacturing of the vessel provided a leading benchmark in terms of high-level quality aspects resulting in improved benefits already being realised,” says Joseph Roux, the engineering manager of Impala Base Metal Refineries.

The Impala team with the assistance from Engineering Systems and Facet Consulting designed the autoclave.
Metso ND Engineering was awarded the contract for manufacturing.“This project exemplifies true innovation – a daring redesign, combined with the novel use of duplex stainless steel in an environment where lined carbon steel has been the industry norm for decades, leading to a technologically advanced product with significantly lower maintenance costs and reduced downtime,” says Prof Madeleine du Toit
from Facet Consulting.

This exciting and pioneering design, the first of its kind in the world, has reshaped the industry. “Impala has set a superior industry standard, in both its quality of welding and craftsmanship, as well its fine attention to detail in producing this remarkable autoclave,” says David Shklaz from Engineering Systems.

What is an autoclave? An autoclave is a vessel used for conducting chemical reactions under high pressure. All pressure vessels need to be designed and constructed in accordance with the Occupational Health and Safety Act. The new product is a breakthrough in the leaching process for the mining and refining industry. 

Autoclaves are used by Impala Platinum Refineries for the recovery of metals by means of a leaching process. This means soluble and other constituents are removed through the action of a percolating liquid.

The original autoclave design has been in operation for a number of years in the mining and refining industry and consists of a heavy wall carbon steel shell with a lead lining and two layers of acid bricks. The acid bricks act as a wear-resistant material and ensure low surface temperature at the skin of the lead lining. The lead lining acts as a corrosion barrier for the carbon steel shell envelope. Compartments created by partition walls of brick create buffer zones for different stages of the process. These require frequent maintenance due to
the nature of the operation which includes abrasive wear, vibration harmonics and turbulence that cause brick spoiling.

The problems with the old autoclave design caused reduced productivity and efficiency problems. Major concerns were raised over high maintenance requirements as the bricks had problems. Major concerns were raised over high maintenance requirements as the bricks had a tendency to collapse and caused leakage and contamination.
The resulting high costs and lengthy maintenance times were compounded by the heavy installation. Working conditions in brick lined autoclaves are hazardous.

Disposal of a decommissioned vessel presented major problems. The unit was too large and heavy to lift and the dangers of working with lead meant a specialised workforce needed to be tasked with the removal. At the same time the brick had a tendency to swell, crack and collapse further complicating the task.  

The engineers operating the autoclave realised a new design was needed to reduce the high costs of manufacturing, installation and operation. A safer working environment was also identified as crucial, as was the reduction in downtime due to maintenance and a more ergonomic way to dispose of the old unit.  

The team identified that the material of construction had to be readily available and have extremely high mechanical properties and easy workability. It also had to be resistant to high temperatures and provide corrosion resistance.  The replacement material for the brick lining also needed to be corrosion resistant in order to protect the steel from the chemicals used in the refineries. 

The team investigated various high alloy materials for the application but superior performance was obtained from stainless steel duplex SAF 2205.

No evidence of wear, chemical attack or pitting corrosion was measured during the respective test works. It resisted localised corrosion including inter-granular, pitting and crevice corrosion and has a higher mechanical strength than carbon steel.  The carbon steel shell was replaced with the duplex SAF 2205 in the new design. It is welded in circular sections to form an elongated impermeable cylindrical body. It has several agitator nozzles and compartments.

The new autoclave design has marked benefits. Due to the elimination of acid bricks the larger volume provides the benefit of increased throughput combined with improved leaching efficiencies. The elimination of the brick lining means no bricks can crack, fall-apart and allow contamination of the product. 

The design was so successful that a world patent has been registered. The patent covers aspects such as the use of SAF 2205, the duplex shell, the cylindrical body, the compartment and agitators. The major improvements of the new stainless steel design include the increased throughput, improved performance and less down time as well as a safer working environment. Lead has been eliminated and replaced with a healthier and more environmentally safe material. The total lifespan of the autoclave has also been lengthened.

The autoclave project has also made significant developments in the progression and advancement in the field of welding and optimised the integrity of welding quality standards in South Africa.