Metal Accounting for Mines and Processing Plants: An Introduction

In order to ensure that there are proper controls and sufficient reliable information available to determine the sources and the amount of metal that can be accounted for mines normally institute a formal system of metal accounting. In the absence of such a system shareholders will not have the assurance that the reserves and resources that are declared in the company annual reports are accurate and reflects reality.

According to Janish (1973) metal or ore accounting is the activities relate to the systematic recording and presentation of ore valuation, grade, and ore treatment statistics of a mine.

For the purpose of this article the term metal accounting is taken to mean the systematic process that is followed to track the metal in mines from the in-situ metal through the mining process to the final product. The data pertaining to metals that a company will process in order to attempt to recover is normally collected from different sources. These measurements includes sampling and survey measurements in the mine and analysis of mass and grade information that is translated into standardized report format that is required to meet specified reporting requirements.

In this article the reader will be introduced to the term "Mine Call Factor" and the concept of metal accounting through the use of a proposed ore flow balance sheet.

Mine Call Factor

In most leading gold mines, one of the primary measures of the efficiency of production of metal is the so called "Mine Call Factor". This factor compares the anticipated gold in situ as calculated by the surveyors, based on actual sampling values with the amount of gold actually extracted from the ore by the plant with allowance for lock ups and losses to tailings. The recovery in the plant utilized a calculation that is based on the mass of the feed into the plant and an analysis of the inputs and outputs to the plant to determine the gold called for. The factor is usually expressed as the ratio between the "gold called for" and the "gold accounted for", as a percentage.

Typically, the mine call factor is calculated monthly and form a useful means of monitoring discrepancies between mine output and metal produced.

Ore Flow Balance Sheet

In order to ensure that all the sources of the metal such as gold are correctly explained and taken into account and that all the recoveries and losses are accurately accounted for it is standard practice to generate an ore flow balance sheet for every accounting period, mostly on a monthly basis.

The metal accounting balance sheet consists of two sides namely, the sources of metal that is called for by the on the left hand side and the metal counted for on the right hand side.

To generate suitably accurate information contained in the sources of metal that is called for (left hand side) is the responsibility of the survey or geology department on the mine. This department will compile an ore valuation report that includes the mined grade or mineral content and other ore treatment statistics. They accurately determine the quantity of rock mined from the various sources by exact measurements of the volume produced and converting it into tons by applying the density factor of the rock for the area.

The quality of the ore is expressed in terms of the grade and is normally articulated in a gold mine in terms of grams of gold per ton of rock. This measure is obtained by conducting suitable and sufficient sampling of all areas mined. The quantity and quality of the rock mined and disposed of inside or outside the mine is also considered in order to arrive at the mass of the metal bearing material supplied to the plant.

The ore flow balance sheet will start at the top of the left hand side of the ore flow balance sheet with the rock broken in the mine and concludes with the metal recovered from the smelter in the reduction plant on the right hand side.

The ore balance sheet normally contains a portrayal of the sources of rock such as the grade and tons of rock mined from the ore reserve, not from the ore reserve, development section and other sources. This then result in a clear and unambiguous number of tons that is generated from the mine that is linked to a predicted metal content based on the sampling values obtained during the reporting period. The metal content is accurately declared by making sure that a weighted average is used during the calculation of this value.

The ore balance sheet will also reflect the quantity and quality of the ore that is sent to the stockpile and the plant respectively. A shortfall is calculated by deducting the tons generated by the mine as well as the tons sent to the stockpile from the tons sent to the plant. The shortfall in tons is normally reflected at a zero metal content value.

On the other side of the ore balance sheet, recovered metal are accurately accounted for by working backwards from the recovery and residue data. The tons and value that is retrieved from the un-milled stockpile into the plant are also accounted for.

The calculation is continued by considering the value and tons of any hand sorted waste as well as recoveries made from other sources and reducing this by the residue value of this reduction or contribution. A balance is achieved in tonnage at the entry to the reduction plant by calculating the shortfall as explained above.

Based on the weighted average of the contribution of the various sources of metal the content are carried through the calculation on both sides of the ore flow balance sheet to determine the mine call factor. The ideal is that the mine call calculated factor comes as close as practically possible to 100%. In reality the mine call factor can be below 100% one month and above it the next month. This can normally be ascribed to fluctuations in the throughput velocity in the plant. If it is found to be other factors such as the accuracy of measurement anywhere along the accounting trail, then the problem should be dealt with swiftly and decisively to ensure that the information can withstand the rigor of standard auditing practices.

Conclusion

The ore flow balance sheet is a mathematical illustration of the flow of ore and its mineral content from the in-situe reserves to the final product in the smelt house.

The key to the successful interpretation of the result is in evaluating the trends and the accumulative results.

Mass measurement and sampling are normally relatively complex across the various points required to be measured in order to comply with the stringent accuracy requirements of compiling a useful ore flow balance sheet. It often requires specialist equipment and advanced in-line sampling and analysis techniques.

If the calculations based on the ore flow balance sheet does not provide answers, it implies that some information is missing or incorrectly reflected and should be rectified without delay.

© Carl Marx