Greg: Mining may well have been the second of humankind's earliest endeavors – granted that agriculture was the first. The two industries together were the primary industries of early civilization. Milestones in human history are marked by mankind's ability to mine new minerals, and use them to advance technology.
Stan: Today, mining is prevalent in more than a 100 countries and directly employs 3.7 million people and even more indirectly. The global mining industry contributes about 45% to the world Gross Domestic Product (GDP) on a direct and indirect basis, from iron ore and copper that serve as building blocks of modern cities, to coal and uranium that keep the world's power plants running, to potash, an essential ingredient in fertilizer manufacturing for the agricultural industry that feeds the world's population. Everything around us is either mined or grown in some way. For the source of this data and more information see miningfacts.org, "How many jobs depend upon the mining industry," and the July 2012 Mining Weekly article, "Global mining drives 45%-plus of the world GDP."
Greg: We are long overdue for a close look at this industry, so this is the first of a series of columns to catch up. I have asked Michael Schaffer, president of Portage Technologies, to provide us a solid understanding of the characteristics and uniqueness of this industry and the advances that have been made. Portage has operations in Canada, Mexico, Chile and South Africa.
Stan: What makes mineral processing difficult to start with?
Michael: The raw material (ore) hardness, grade and rock size vary greatly. There is no homogenization of the feed and no direct measurement of raw material characteristics. This results in fast and frequent unmeasured disturbances. Variability is minute by minute. There can be a huge change in 30 seconds and the operators have to stay on their toes to keep on top of them. Some applications provide the extra challenge of being in difficult to reach places. An issue with a cyclone can take even the best operator 10 to 15 minutes to evaluate and respond. The operator is constantly playing catch up. What they can see by way of measurements is sketchy at best. It is like they had on a pair of glasses with one lens missing and the other lens with the wrong prescription.
Greg: In most of the plants where I worked there were well-mixed volumes that attenuated the changes in the process. Feed tanks blended out variations in raw materials that were minimal to begin with, and storage tanks blended out variations in the product. The attenuation was so significant that in some cases it was difficult to justify better automation to reduce variability. For the most part, the true step load disturbance seen in the control literature did not exist when control loops replaced on-off actions. The exception was the plants that used the extruders and sheet lines for the final product (e.g., clear plastic interlayer for safety glass). Fast disturbances seen in chemical processes can be slowed down by better automation system design, as outlined in the 10/18/2013 Control Talk Blog, "Disturbance Dynamics Recommendations Tips".
Stan: What are some of the challenges within the process?
Michael: It doesn't matter how small you grind a particle. Each is unique. You almost always have two phases (solids and water) and in many cases there is a third phase with air creating foam and froth. All fluids are non-Newtonian. The mineral content can double in a matter of minutes, changing the flow characteristics dramatically. The processes are complex, with many circulating loads. The operator not only has to think of what is coming, but also what is coming back. Grinding typically has two recirculating streams and flotation can have as many as 10. There can be a building up of inventory within the circuit that forces a cutback in production. A greater recirculating load ultimately means less fresh feed as the capacity is consumed by the circulating load. The problem escalates as the material keeps coming back at you. The best operators recognize when the inventory is building and find a way to pull it out.
Greg: Recirculating or recycle streams presented a challenge in even less difficult processes due to a "snowballing effect" where an imbalance in material and contaminants build up. The solution almost always requires some added intelligence, with optimization often offering a greater opportunity than seen in once-through processes. How do you stop a problem from the start?