QA/QC IN MINING REALITY OR FANTASY? M. Sc. Samuel Canchaya Exploration Senior Geologist Cía. de Minas Buenaventura S. A. A. - PERU

Transcription

1 QA/QC IN MINING REALITY OR FANTASY? M. Sc. Samuel Canchaya Exploration Senior Geologist Cía. de Minas Buenaventura S. A. A. - PERU

2 Introduction The importance of SAMPLING is well understood All aspire to get QUALITY QA/QC procedures can help to achieve and control quality It is good to know that today, QA/QC procedures are widely applied However, currently these procedures are being applied inappropriately: focused mainly on secondary sampling and analytical procedures, neglecting the primary stage of sampling Primary Sampling Stage is not only the most important stage, but the main source of errors. 2

3 General structure of sampling errors Errors aditivity: GE = PSE + SSE + AE GE = Global Error PSE = Error in the Primary Sampling Stage SSE = Error in the Secondary Sampling Stage AE = Analytical Error PSE >> SSE > EA 3

4 General structure of sampling errors Bias % 1000 Activity Primary sampling Activity % Relative error Sampling 100 a 1,000 Transport and storage 1 a 100 QA/QC 50 Secundary sampling Volumen reduction and comminution Chemical preparation: digestion and acid dissolution 10 a a 20 QA/QC 0.1 a 1 Chemical Analysis After: Gy (1999:10) Chemical Análisis 0.1 a 5 Compiled after Gy (1999:10); Gy & Francois-Bongarson (1999) and Paski (2006)

5 Max The common practice of QA/QC Quality Assurance (QA) means the set of systematic and preventive actions to ensure the quality and reliability in the sample, in order to satisfy optimal quality requirements Twin samples Min Quality Control (QC) is the set of regular activities whose purpose is to ensure that all quality requirements are being met More widespread action of the QA/QC, is the properly insert of control samples within each sampling batch to be analyzed Internationally accepted ratio of control samples is about 20 % 5

6 Overall insertion rates of control samples Sample type % Twin samples 2 Coarse duplicates 2 Pulp duplicates 2 Low value standards 2 Intermediate value standards 2 High value standards 2 Coarse blanks 2 Pulp blanks 2 Check samples in secondary lab 4 TOTAL: 20 Modified after Simon (2007) 6

7 Primary sampling: the most neglected stage of mining industry QA/QC procedures are mainly applied to the stages of chemical analysis and the prior mechanical preparation Equipment and chemical analysis are becoming more and more sophisticated, with increasingly powerful processing and better performance. On the contrary, by the side of sampling, it is still difficult to convince managers to approve the purchase of sampling equipment, despite they are less sophisticated and cheaper The number of publications related to chemical analysis is several times greater than those related to sampling 7

8 Primary sampling: the most neglected stage of mining industry In many countries sample-specific courses are almost non-existent in the universities, while courses in chemical analysis are regular The staff responsible for chemical analysis are specialists with a high expertise while those in charge of sampling are considered of lower category and usually are not asked for exigent specific qualifications or experience Labour force involved in sampling are generally frowned upon and belongs the lower scale of salary 8

9 Conclusions: QA/QC procedures mainly applied to the phase of chemical analysis and the prior mechanical preparation Primary stage of sampling is not only important, but it generates the largest proportion of bias and errors However, primary sampling remains being the most neglected stage of the mining activity 9

10 Recommendations: Any company, unit of production, exploration project or prospect should have Sampling Protocols according to the nature of the targets Spare no effort or money to achieve representative samples, preserved and unpolluted Implementation of a Safety Sampling Chain, which distributes and transmits the responsibility for sampling and sample integrity, among all those involved during the sampling process Minimize errors in the primary sampling phase, especially: Choosing the most appropriate type of sampling for each case and circumstance Selecting optimal sampling grids or sampling distances, using geostatistical procedures Monitoring deviation of holes during drilling Defining sample weights, volume and particle size of the increments Homogenising prior to any exercise of volume reduction 10

11 more Recommendations: Avoid contamination during all stages of sampling Control and minimize loss of fines (which often contain precious metals) especially during drilling, washing and cutting of witnesses. Quantity, weight and grain size of increments of the samples should be determined with specialized procedures Sampling equipment must meet TOS (Theory of Sampling) specifications Always use the best equipment and sampling tools; and systematically verify that they are in good condition: check lists Perform surface sampling of rocks and structures using portable diamond blade cutters 11

12 more Recommendations: Appropriate scheduling of: Tests of repeatability and reproducibility (R&R) Sampling annual audits, preferably made by outside companies Tests of heterogeneity and bias, especially at the beginning of each sampling campaign ERR % Recommended use of: Disposable gloves (specially when using spray paint) Disposable wipes for cleaning hands and sampling tools Water sprays instead of brushes to wet cores during logging Rotary riffles (instead of splitters or quartering blankets) Sampling stations at minerallurgical flows and belts, must meet TOS specifications and preferably should be installed at the end of the belts 12

13 more Recommendations: Conform Sampling Team with the best and most qualified human resources, with good pay, permanent training and qualification Implement use of bar code, allowing direct recording of measurements and laboratory data with automated systems Banish bad practices in the handling of cores, uncovered stacking boxes or walk on them, use a brush to moist them, bad transportation and poor storage, etc. 13

14 Sampling procedures with portable diamond blade cutters Portable cutter Traditional way of sampling Freshly cut slab of rock Portable cutter in operation

15 Recommended tools: Water spray Disposable gloves Plattner Mortar Homogenizer Clean hand towels 15

16 Quartering blanket Not recommended: Porous gloves Paint brush Splitters 16

17 Rotary riffles: OK 17

18 Reduction of sample volume: Quartering blankets Method DER Recomendation Quartering blankets Splitter 1 10 Not recomendable Only with proper feed Rifle 0.1 Good reduction Splitters with feed hopper Riffle Relative standard deviation (RED) after Paski (2006) 18

19 Storage of cores Improper way Proper way 19

20 THANKS M. Sc. Samuel Canchaya Moya Tel: Cel