It seems to me that the metallurgists have a bit of a skate at feasibility time. Most of their effort seems directed to figuring out factors to use in their design rather than going to first principles like the miners do. But perhaps I am biased again.

A previous section described the work required to obtain a process flow diagram (PFD) and this is the critical document for the process/metallurgical engineers. This document uses colored lines to show where all the bits go and an amazing amount of information is contained on this drawing. That is why it forms the basis of almost all process plant designs. Each line represents the flow of some type of material and the important elements of that material flow are included. For example, line A is the secondary ball mill (what a silly name for a piece of equipment) recycle line. It must be designed to handle 300 tonnes per hour of solids, 500 tonnes per hour of water and the maximum temperature will be 50 degrees Celcius.  When the process flow diagram is fixed then the plant guys size the equipment that is required and they usually do this by calling their friends who sell the equipment and get them to do it for them. Not only that but then they are given all manner of ballcaps, pens, calculators and other graft! It is amazing really. “Thank you! Thank you for the privilege of doing your work for you. Here! Take this expensive calculator as a momento of my gratitude!” Perverse actually.

When they have figured out the number of crushers, semi-autogenous mills, regrind ball mills, pumps, tanks, filters, reagent systems and process control stuff they then bring in their “topgun” layout guy to create a floor plan that works. Making floor plans seem sort of a girlie thing but if the plant is designed such that the operator bangs his head on a pipe every time he goes to check the regrind mill then a hatred is quickly kindled which will lead to violence unless the layout guy is big and burly.

The last job for the plant designers is to create a couple of P&ID’s - piping and instrumentation drawings. At this point the plant designers bring in their helpers - the electrical and civil guys. They figure out how much concrete and steel beams are needed to support all the plant equipment as well as where the switches and plugins go plus how many feet of wire goes to run the pumps etc. The electrical guys call these single line drawings and for the life of me I don’t understand why they take so long to draw.

When it is time to calculate the cost of building all this stuff the plant guys call in more support in the form of professional cost engineers. They have factors for everything and somehow they come out with realistic cost estimates for the plant as designed. It has to do with counting the cubic meters of concrete required, the number of steel girders put in place, the miles of pipes and the number of times the wiring can be stretched around the earth at the equator and putting a cost to all this.

One final job for the plant guys is to design the placement of all the left over stuff. Remember that in most mines the good stuff occupies only a very small amount of the rock that is sent for processing. In a copper mill, for example, you are over the moon to collect 20 pounds of copper for every tonne that is processed. So what happens to the other 2220 pounds of rock that is thrown away? Depending on the type of plant, it may be ground up to resemble fine sand and sent to a tailings pond where the water is separated from the sand and reused in the plant. The sand is left in a pile and some young university student is photographed trying to get a tree to grow on it. The picture is used in the company’s annual report and the student switches to law so he or she can work in the city rather than the boondocks. I suppose that is a bit cynical.

But enough of the plant part of the feasibility... what about the other parts of the mine?