Agitator Failure Investigation (Ghana)

In the BIOX® process, large agitators are used to mix gold ore slurry and distribute bacteria that accelerate the oxidation of the sulphides in the ore and free the gold. Spargers are used to pump large volumes of air through the slurry which supply the bacteria with enough oxygen and carbon dioxide for the oxidation reactions and cellular growth.
Fatigue crack on an Agitator shaft.

Golden Star Resources' Bogoso/Prestea gold mine near Bogoso, Ghana, is one of the largest gold mines that use the BIOX® process. In 2009, AXIS Mechanical Design was contracted to find the cause of frequent shaft failures of their agitators. These shafts are made from 11 meter high 20" NB steel pipe and failed, as a result of fatigue, every second to third month which is much less than the designed lifetime.

Experimental tests were done on agitator shafts in the smaller reactors at Fairview mine in Baberton, South Africa, to familiarise the team with the conditions at these plants and to ensure a smooth and well planned trip to Bogoso.

Measurements had to be taken on the agitator shafts while they were in operation, which meant that customised WIFI data acquisitioning systems and specialised computer programs had to be prepared for this project. Strain and acceleration measurements were taken on different parts of the shaft and blades by using strain gauges and accelerometers. The gauges were protected against water and the low, 1.2 pH, slurry.

Mechanical Engineers from the AXIS Mechanical Design team travelled to Bogoso and performed the same tests on the agitator shafts there. Scaffoldings were constructed to make parts of the agitator shafts more accessible inside the 15 meters high and 12 meters diameter reactor tanks.
FEA performed on the Reactor.

The data obtained from the reactors in Ghana were taken back to South Africa and evaluated. Several data analysis tools and methods such as FFT's and Rainflow Counting were performed on the data to determine the nature and origin of the stresses inside the shafts and to recalculate the fatigue lifetime of the shafts. Complete Finite Element Analyses were also performed on several 3D solid and 3D shell models of the top structure and the agitator shaft to compare these results to the test results.

The team at AXIS Mechanical Design used the results found to identify problems in the design and maintenance in parts of the reactor. AXIS Mechanical Design supplied the client with a complete report on the test results and recommendations for the redesign and maintenance of the reactors.