Grade Control and Reconciliation of an Ore and Waste Rock from Mine Production

MINING & SOCIAL ISSUES with Ignatius Kamwanje
The Author is a Consulting Geoscientist with experience
in Mineral Exploration, Mining Geology, ESIA, Ground
Water Resources and Occupational Safety, Health and
Environment
.

(An Overview)

Grade control is an essential part of the mining process that maximizes profits/value and also reduces the economic risk. It requires that some tonnes of materials that are mined should be delivered to the processing plant at an optimum grade through definintion of an ore or waste material according to the cut-off grade. In grade control, the metal content being extracted from the ground is never constant and with technological advancement, it has made mining easier.

1. Importance of Grade Control

One of the most important roles played by the geologist is the supervision of grade control. In an open-pit operation, grade control involves the sampling of blast-hole materials produced by down-the-hole hammer drills and the classification of bench/flitch reserves into ore or waste material, or into various metal types. In some cases, grade control may also involve the sampling of truck loads to ensure that rocks are assigned to the correct stockpile or waste dump. In underground operations, grade control may involve the mapping and sampling of stope, sampling of muckpiles or tram car loads, jackhammer cuttings or diamond drill cores. It is the geologist’s job to ensure that mining is closely following the mineralized zone and that overbreak during stoping is kept to a minimum. The main purpose of grade control is thus to ensure that material being fed to the process plant is of economic grade and that large fluctuations in grade are minimized by blending ores from different flitches/benches/cuts or from different stopes. It is essential that this is undertaken so that modifications can be made to sampling practice or to the methods or parameters used to calculate grade, tonnage or contained metal.

(a) Mineral Economics in Grade Control

The grade of the mineral can vary dramatically due to depth. Geologists will use this information to define the economic limits, to ensure there is return on investment when mining the mineral. At one point in an open pit operation, mining becomes unprofitable because the grade of the metal is too low than the cut- off grade (the value of the metal in the ore is less than what it costs to mine it). Therefore grade control is important in this case because it stops mining business from running at a loss. A grade control geologist is therefore tasked with maintaining the grade going to the process plant by designing careful blending techniques/ ratios so that the throughput should be kept to the required daily production level. A grade control geologist should try to be careful with dilution that can be a major setback to poor grade resulting from blast movement. This may lead to significant ore losses.

(b) Profitability in Grade Control

Grade control technology improves efficiency, increasing the productivity and profitability of an operation. Grade control systems allow machinery to operate at a continuous pace, rather than having to stop for an Ore Spotter to come and measure the excavation, as with conventional equipment. The overall benefit is that it saves time and money), without compromising on accuracy. Grade control drilling consists of a number of drill holes in close proximity, usually in the ore body. It has room for constant spacing and drill holes depths are designed depending on the Rl(relief). Blast hole/Grade Control drilling. allows the geologists to determine the ore grade and its changes as well as helping locate the boundaries of the mine pits as to whether it is an ore or waste material The geological spatial controls/distribution to mineralization is fundamental to pit optimization, n resource estimation, geotechnical engineering, and mineral processing. It enables improvement in exploration targeting, resource estimation confidence, geotechnical mine design support, short- and long-term mine planning, grade control, mine reconciliation, and in the long run,profitability. In addition, there is need to have expertise who are needed to analyse and apply grade control data in a way that minimizes differences between planned and actual production, optimizes business performance, and reduces risks of misdirected ore and unintentional dilution. In this scenario, experts use highly proficient and advanced mining software packages that produce robust grade, geological, Ore combination models by running simulations based on histograms, variograms derived from distributuios e.g. Gaussian(normal) ,Exponential etc, from a Smallest Mining Unit(SMU) upon which the entire mining process is based.

2. The impact of blast movement on grade control

Grade control geologists spend many hours modelling the distribution of minerals throughout a rock mass based on data from drilling and assaying a significant investment and the in situ rock is blasted so that it can be efficiently handled in the process plant.
Failure to accurately account for blast movement results in ore loss, dilution, or mis-• classification (low grade to high grade ore or other contaminates). The financial consequence of getting this wrong is significant and diminishes the ge-• ological QA/QC done in defining the ore body.

3. Mine to Mill/Process Plant Reconciliation

The significance of ore grade reconciliation is to identify, analyse and manage variance between planned and actual results in a way that highlights opportunities from mining of an ore in SNUs. A consistent record of poor reconciliation is an indication that the variance has not been adequately managed. This ultimately results in significant economic impact on mining companies. The Mine to Mill Reconciliation indicator contains information about the metals in the reserves and in the plant feed. In essence, it relates that which was planned to feed the plant to that which was actually fed to the plant. Fluctuations in periodic reconciliations between the mine and the mill are an expected and common experience in mining and processing operations. These fluctuations are caused in part by errors in the estimates of tonnage and grade of the mined material as well as errors in the mill’s assessment of the material processed. Such problems can result from a poorly-conceived geological/resource model of the mineralization which affects both the spatial distribution of the ore as well as the tonnage. Poorly conceived models of the mineral resources and ore distribution can arise as a result of insufficient and inadequate drilling and sampling, inappropriate application of geological constraints in the estimation process and the use of inappropriate methods to estimate the block grade distribution. Reconciliation estimates, detects and corrects problems in all stages of mine operations from resource estimation to metal production. If used correctly, reconciliation helps to better predict the life of mine (LOM), improves the adherence of production plans to the respective budget, and allows for effective control of the mining and milling processes. However, the accuracy of reconciliation results requires that all input data must be generated in accordance with the principles of sampling procedures Reconciliation serves as a tool that tests for internal consistency in the quality of data, and it is able to reveal discrepancies between predictions and actual outcomes, which go a long way to help improve future predictions. Reconciliation studies may provide early warnings or signs for problems that can occur either in the predictions or subsequent production steps.

4. Practical and Theoretical Approaches to Reconciliation

There are a variety of techniques in ore /waste reconciliation. The most commonly used include;

(a) Reconciliation of resources with grade control

This technique involves the use of grade control data in estimating the total in-situ mineralization above a particular and calculated cut-off grade. Comparison is thereafter made with prior estimated mineral resources from that same area so that the resource geologist is able to determine if the mineralisation is indeed present in the quantity and quality estimated at the mineral resource estimation stage. The advantage of using this method lies in the ability to produce a total mass balance comparison high grade ore, marginal, low-grade ore mineralisation and waste.

(ii) Reconciliations between ore reserves, grade control and production

It assumes that both grade control and ore reserves estimates should be based with respect to bulk densities of mineralisation, block sizes, waste material as well as appropriate assumptions on dilution and mining losses. The advantage of this is that management is able to undertake optimized approach for effective operations.

(iii) Depletion by mining

The availability of grade control and production data from a defined part of the orebody makes it possible to extract the ore reserve estimated for that part of an orebody from the total mineral resource estimates. The geologist is able to conduct direct comparison with grade control, mine output/ore feed and process plant results.

(iv). Reconciliation factors (Mine Call Factors).

Mine Call Factors (MCFs) have been extensively applied in performing production reconciliations. They are calculated with the aim of separately evaluating the model estimates to daily grade control estimates and to process head-grade estimates. This calculation relies heavily on comparison of tonnages, grades and metal content of the block model grade control model, and also tonnes, grades and metal content as produced by the mine for the period. One assigns the daily production/short term figures based on grade, tonnage and metal content to the overall block model

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