The Broken Stocks Saga

What follows is a series of short articles on mine scheduling. Like any good posts, they should include some storytelling and so I’m going to kick off with a story that was pivotal in many of my learnings about mine scheduling.

For a period in the 90’s, I was Grade Control Superintendent at an iron ore mine in West Australia. This position leads a team charged with producing weekly stockpiles of lump and fines ore. Between these two stockpiles, we had a total of 8 quality targets with very tight bands that we had to achieve. Broken stocks (material already blasted) are critical in this weekly ore stockpile construction, as they’re typically all there is available to mine from within that time frame.

Mine Planning Department were responsible for short term schedules of all mining equipment over a 3 month time frame. In that schedule, whilst drill, blast and excavation were all scheduled, the primary driver was scheduling the excavators to mine the quantity and quality of product required.

We came into a situation where we had very high phosphorus in our broken stocks, which made it nearly impossible to build weekly stockpiles to our quality targets. The problem was that Mine Planning in scheduling the diggers to mine specification product, were effectively scheduling specification product to be added at the far end of the schedule. However, in the next week we were taking a specification batch away from the inventory. So the inventory (broken stocks) was not being corrected and for a sustained period of nearly 2 months, the broken stocks continued at high phos levels. During that entire period, (so about 9 stockpile builds) we really struggled to produce specification weekly stockpiles. This was very much a saga for me, as I drew a lot of heat for railing off spec product!!

This situation was a catalyst for many learnings and the first of those was about broken stocks management.

Broken Stocks Management

My first learning related to the broken stocks and how critical they are to achieving target qualities for each stockpile construction, which cannot be understated. If the grade controllers are provided with a set of broken stocks that are close to quality specs, it is relatively easy for them to hit their target. On the other hand, if they are provided with broken stocks which have one or more qualities a long way from target, it is made extremely difficult.

Similarly, provide a large quantity of broken stocks and you provide more choice, provide a small quantity and even though they might average out at the required specification, the lack of choice (and therefore scheduling flexibility) may still make it difficult to achieve target. Now there is whole article in setting inventory targets and the subsequent trade-off between risk and cost, but I will leave that for another article.

So whereas the short term planning group were scheduling the shovel excavation for the next month, I believe a much better solution would have been for them to be managing the broken stocks instead. The short term mine planning process should have involved determining what the starting broken stocks were (quantity and quality) and then assessing which blocks need to be added to this, to either maintain stocks within target bands, or restore them to within target bands.

In this way, scheduling is effectively an inventory management process, which is both a critical part of mine scheduling and one that I see as typically under-utilised. Inventory management will be discussed in further detail in an article to come.

Author: Mark Bowater