Proactive rolling bias test applied on sample stations

Abstract

The most basic concept of sampling theory is that “a sample is part of a lot”, where the sample collected needs to be representative to the lot sampled. On sample stations, the lot to be sampled is represented by the material transported by the conveyor belt, while the sample is collected and further subsampled via cutters until the final sample collection point. Current normal practices to evaluate the operation of sample stations that support processing, metallurgical balance, reconciliation, and final port shipments are typically based on visual inspections: Material build-up on cutters, samplespillage, reflux while sampling, pegging on sizing screens, worn cutter lips are all observations that indicate issues. Being subjective observations, these do not allow the quantification of the sample’s representivity, and the risks for mining businesses due to a positive or negative bias being incorporated during sample collection stage. Bias Tests are mentioned in several International Standards across commodities (ISO3082 for Iron, ISO 13909-8 for Coal and ISO12743 for Copper, Lead, Zinc and Nickel, for example) to compare the sample obtained against the material it is supposed to represent at the control point. The current methodology and strategy used in the industry requires the interruption of the regular production process multiple times in a row for extended periods of time, to manually extract the material from the conveyor belt (also including manual handling and safety considerations). For this reason, bias tests are not very popular in industry (“we lose a lot of money and time having to interrupt our process many times”) - and are therefore usually performed only very reluctantly, or not at all, exposing mining companies to higher production and financial risks than necessary, hence it is simply assumed that the processes involved are not affected by bias. This paper is presenting a proactive approach to perform a Bias Test, developed at Hay Point Port Coal, a Rolling Bias concept has been developed, switching the current reactive, time consuming and manual process task, to a more proactive and frequent methodology that allows for trending analysis of the sample station. Quarterly planned maintenance stops are used to perform the bias test, where a vacuum system developed and tested by ALS Laboratory and BHP Coal, performs the collection of the material from the conveyor belt drastically reducing the time required to perform the task manually, but more importantly reducing the exposure of people to safety and manual handling risks. This approach enables Hay Point Port to have quarterly performance data of the sample station, converting this process to a more objective, proactive, and sustainable approach where data, every quarter, has been monitored since 2019.