Process Simulation Pans Out for Goldmine

Christopher Columbus was more on target than he realized—but technologically more than 500 years too late. In fact, the first large island he and his fellow crew landed on in 1492, Hispaniola, had lots of the gold they were seeking, along with a western passage and spices from Asia.

However, then and in the centuries since, gold-bearing deposits on the island that includes the Dominican Republic and Haiti were too thinly and widely spread, and so were not worth trying to recover. However, at today's $1,700-per-ounce prices, that calculation is beginning to change, and simulation software is enabling training and optimization to optimize large-scale mining and recovery.

For instance, Barrick Gold Corp., based in Toronto, Ontario, Canada, has been building its huge, new Pueblo Viejo gold and silver mine on the 11-km-square site of a former mine that's been closed for about 10 years, and is located in the middle of the Dominican Republic about 100 km northwest of the capital of Santo Domingo. Barrick operates and owns 60% of the mine, while Goldcorp owns the rest (Figure 1). Meanwhile, Fluor Corp. and Hatch provided engineering, procurement and construction management (EPCM) services for the project.  

The mine and its processing facilities cost about $3.7 billion to build., and with proven and probable gold reserves of 25.3 million ounces, they're expected to process about 24,000 metric tons of ore daily, and annually produce about 1 million ounces of gold, 5 million ounces of silver and 6100 tons of copper and generate about $1 billion in annual revenue. During construction of the plant, more than 16.4 million metric tons of ore—representing approximately 1.9 million contained gold ounces—were stockpiled. And, Barrick reported on Aug. 14 that Pueblo Viejo had achieved the first gold production with ore now being processed through the first two of its four autoclaves. 

Managing Multiple Processes

Though much of the site's ore is located close to the surface, Pueblo Viejo still requires about 20 diverse sub-processes to extract gold, silver and copper from it. These include crushing, rough grinding, high-pressure oxidizing, counter-current decantation (CCD) washing, carbon-in-leach (CIL), sulfide precipitation and many others, all of which require various operator skills and experience.

For example, the autoclave circuit at Pueblo Viejo houses four of the world's largest vessels for the pressure oxidation of sulfide gold ores (Figure 2). Steam and oxygen are injected into the autoclaves to oxidize the ore. Each autoclave is almost 6 x 40 meters, and ore is retained in them for 60 to 75 minutes at 230 °C and 3450 kilopascals. This process for improving autoclaving was pioneered earlier at Barrick's Goldstrike mine in Nevada, where it helped increase gold recovery from 35% to 85%.

Pueblo Viejo's many ore-processing applications are controlled by approximately 11,000 I/O points or device signal tags (DSTs), which are managed by a DeltaV distributed control system (DCS) from Emerson Process Management and PLCs from Rockwell Automation.

To help optimize Pueblo Viejo's many processes beginning at start-up, process systems engineer Paul Yaroshak reports that Barrick decided to simulate about 10 processes to help train its operators before the facility began processing ore. However, because of economic and time constraints, Pueblo Viejo couldn't simulate all of its processes, so it decided to focus on those with appropriate complexity, criticality and similarity to other processes, such as limestone crushing, which is representative of other crushing processes at the site. Other simulated processes include copper recovery, pressure oxidation in the autoclaves, iron precipitaion, cyanide destruction, acid wash/stripping and the CIL circuit (Figure 3).

Most of the operators were new to mining and Pueblo Viejo's equipment, and there were also some language barriers among the staff. As a result, Pueblo Viejo's Ready for Start-Up (RFSU) program was developed, and included training on process fundamentals using computer-based interactive modules and simulator-based training, as well as field demonstrations and hands-on practice.

"Though the site's overall deposit was exploited previously, we expanded it, so this is a new mine and a totally new processing plant. Our management wanted a safe and secure transition from project construction to regular operations," says Yaroshak. "So the main driver for this simulation system was to prepare our operators for start up with unified, hands-on training before the plant was up and running."

Securing Simulations

Consequently, the dynamic process simulation system, MiMiC from Mynah Technologies, was supplied and implemented jointly by Lakeside Process Controls, an Emerson distributor that also provided project management support, and the Pueblo Viejo process group. MiMiC was picked because it provided required low-, medium- and high-fidelity levels as needed; integrated seamlessly with DeltaV; was already being used by the EPCMs for checking I/O; was intuitive; and allowed easy change for model and control system enhancements.

Pueblo Viejo's MiMiC simulations consist of 10% low fidelity with basic I/O tiebacks and simple tuning; 60% medium fidelity with engineering first principles and conservation of energy and mass; and 30% high fidelity with mass/energy transfer equations, differential equations and empirical models. Models for the each of the plant's 10 processes to be simulated were built using an iterative process, which begins with a first-principle Excel model; adds the DeltaV configuration; uses MiMiC to perform simulation conversion and I/O tie-backs; creates medium- and high-level models; builds trainee screens; and finally conducts an initial review, makes changes, and completes a final review.