Factors Influencing Effectiveness of Firewall Designs for Metalliferous Solvent Extraction Plants
The process of metalliferous solvent extraction for the recovery of uranium, copper, nickel and some other precious metals is more than 50 years old; however, the technology is continuously changing and improving.
From 1999 to 2004 the solvent extraction industry suffered four very significant fires in large commercial plants, resulting in high capital and production losses. Many of the implemented fire risk mitigation solutions in metalliferous solvent extraction plants have been a "knee jerk‟ reaction to these devastating fires. Many of these solutions have been derived from misguided information and limited technical and fire risk knowledge.
Metalliferous solvent extraction plants, like many industrial processes, are most efficient if the process equipment is installed close together to reduce piping lengths. Shorter piping lengths will reduce frictional losses, thermal losses and the head required to pump liquids, which in turns reduces the energy required for the process and the capital costs. However, the reduction in plant footprint means a reduction in the separation distances between fire hazards and therefore a potential fire could spread throughout the entire plant. Firewalls are needed to provide a positive isolation between settlers.
The purpose of this study is to investigate some key factors that influence the effectiveness of firewall designs in metalliferous solvent extraction plants. The study will then derive a simplified design correlation for engineers and designers to estimate the firewall height required to adequately isolate settlers from each other and achieve separation between fire zones in solvent extraction plants in the conceptual and feasibility phases of a project.
A series of computational fluid dynamics simulations involving a theoretical solvent extraction plant with varying settler sizes, firewall heights, separation distances between the settlers and the firewall and varying wind velocities were modeled. The Mudan equation for the thermal radiation intensity experienced by an element outside the flame envelop under wind and no wind conditions was then used to compare the results from the simulations.The following factors were studied and found to influence the effectiveness of firewall designs for metalliferous solvent extraction plants:
- The Settler on fire surface area had minimal effect;
- Increasing surface area by 125% (400m2 to 900m2);
- Increased the maximum radiant heat transmitted to Settler 2 by 0.4kW/m2 and the average radiant heat by 0.1kW/m2;
- The separation distance between the settlers and the firewall had significant effect;
- Increasing the separation distance from 0m – 1m, reduced the radiant heat transmitted to Settler 2 by 40% – 80%;
- Further increasing the separation distance from 1m – 2m, reduced radiant heat by an additional 25% – 40%;
- The prevailing wind had significant effect;
- Increasing wind velocity from 0km/hr – 10km/hr, increased the radiant heat transmitted to Settler 2 by 180% – 380%;
- Increasing wind velocity from 10km/hr – 20km/hr, increased the radiant heat by an additional 170% – 300%; and
- Increasing the wind velocity beyond 20km/hr appeared to not increase radiant heat flux on average, but peak radiant heat continued to increase.