March/April 2011

An incredible journey

The design, manufacture and global transportation of the world’s largest autoclaves

By Heather Ednie

Each 780-tonne autoclave took 18 days to be transported just over 120 kilometres from the port at Samana, Dominican Republic, to the Puerto Viejo operation | Photo courtesy of Hatch

The Pueblo Viejo gold operation in the Dominican Republic is on track for its first gold pour in the first quarter of 2012. But getting to this point has not exactly been easy sailing. Because the ore is double refractory, in order to access it, the project had to commission the largest autoclaves ever built and transport them halfway across the world from Malaysia to the Caribbean.

Accessing gold

Pueblo Viejo, a joint venture between Barrick Gold Corporation and Goldcorp Inc., boasts gold reserves of approximately 23.7 million ounces. Barrick owns 60 per cent and is the managing partner. The sulphidic refractory gold deposit is being constructed to a 24,000 tonne-per-day design capacity. In the first five years, it will produce an average of 1.042 to 1.125 million ounces annually (100 per cent basis).

Hatch Autoclave Technology Group has been on the project since 2006 with the engineering, procurement, construction management (EPCM) contract for the core part of the refractory process, namely, the autoclaving facility and supporting oxygen plant. “It’s a double refractory gold ore, associated with two mineralogical issues: sulphides, in the form of pyrite and minor base metals such as silver, copper and zinc, which prevent cyanidation from directly recovering the gold; and a natural carbon component to the ore, which causes issues with the recovery under a conventional system,” explains Hatch project manager Kevin S. Fraser. The incorporation of autoclaving in the processing of the ore addresses these challenges.

In the autoclaves, all sulphides are oxidized, producing sulphuric acid as a by-product. The iron, copper and zinc dissolve into the solution; then the acid is washed out in a counter-current decantation (CCD) circuit leaving free microscopic gold to allow for leaching in cyanide. The underflow slurry is pushed to a carbon-in-leach circuit. Also in the autoclaves, the carbon contained in the sedimentary materials is oxidized with the combination of oxygen and high temperatures.

Within the autoclaves, oxygen is the primary reagent. During design of the autoclaves, Hatch included a heat recovery circuit that, combined with the nature of the exothermic reactions that produce heat within the autoclaves, makes the heat autogenous by design. “Without the heat recovery system, we would need some external heat source,” Fraser adds. “This way, we keep the energy needs down.”

The Pueblo Viejo autoclaves are the largest, by weight, refractory-lined autoclaves built to date globally, with an on-hook weight of 780 tonnes when shipped lead lined. The massive structures are each 5.6 metres by 34.8 metres, and will nominally operate at 230 degrees Celsius. There are four in total.

The grand voyage

Hatch designed the autoclaves and provided quality assurance during fabrication. They were built by KNM Process Systems Sdn Bhd in Gebeng, Malaysia. “We had two full-time quality assurance inspectors on site throughout the 33-month fabrication,” Fraser recalls.

The autoclaves were then shipped approximately 10 kilometres to the Kuantan Port using a 24-line, self-propelled modular transporter (SPMT) heavy-haul unit. At the port, they were loaded on the Beluga Bremen, a P2-class ship owned by Beluga Charters out of Germany, for its maiden voyage. The autoclaves were transported in two shipments of two autoclaves each on the Bremen, which used both of its 750-tonne cranes and needed six hours to load the autoclaves from the SPMT into the cargo hold.

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