Tonnages And Grade On The Leach Pad example essay topic

UTOPIA MINING COMPANY RESOURCE - RESERVE RECONCILIATION PROCEDURES -MINE MASS BALANCING- 1 Introduction It must be stated at the outset that there is no single procedure that can be termed THE procedure and the following notes are not intended to be the bible and only manual of the procedure. Various mining operations have their own peculiar intricacies demanding their own tailor-made procedures. The following notes should thus be taken as a guide for the process and initiate debate to refine and suit them for the type of operation at Utopia mine. The notes are drawn largely from the author's experience in similar operations in the last twenty years. All reconciliation procedures have measured and accepted start and finish resource positions and an operational position in between. Reconciliation is the marrying of the start and finish positions to what happened in the course of the operation.

2 Objective The objective of any resource / reserve reconciliation process is to track the movement of the commodities and afford management with historical control tools for the management of the same resource. These tools become invaluable in the management of the mine as more resources are extracted. Reconciliation allows management to use history to manage the future. The key objective of a reconciliation process is to track the movement of resources in a given time period and to reconcile the movement with accepted benchmarks in the operation and produce a material mass balance for the operation. This is an iterative process involving adjustment of various empirical mine factors and ratios so that they tie in with resource measurements at various stages of the operation. 3 Definitions It is necessary at the outset to have accepted definitions of the various categories and types of resource found at the mine.

Preferably these definitions should be internationally acceptable. If they are not, they should be appropriately qualified by persons competent in the type of resource handled by the operations. As an example, it is common to see a glossary of terms at the back of most mining companies' annual reports. This could be a starting point. Obvious resource categories like the following or any that will be mentioned in the reconciliation should be defined. Resource, Measured Resource, Reserve, Ore, High Grade, Stockpile, Heap leach, PLS, BLS, Fine gold, Gold sales, etc.

4 Gathering the Information The next stage will be gathering the information necessary for the reconciliation from the monthly management reports or other sources that management uses. This information provides insight into resource movement in the course of time. Perhaps in tandem or earlier, the process of compiling the resource / reserve book at the end of the period of operation will have been completed. This should establish by measurement and not subtraction, the finish position of the resource / reserve position. A similar and start position will have been established, again by measurement, at the start of the period under review. 5 Reconciliation In the process of gathering information from the management reports, it will become apparent that various ore types have been mined and thus the start position of the ore resource / reserve should be reduced by that much and arithmetically should equal the finish position.

This normally does not happen hence the iterative process of revisiting various mining factors used to arrive at the ore mined in the period under review. Noteworthy here is the fact that the final resource / reserve position is tonnes, grade and metal in the ground, on stockpiles (including the heap leach pad), barren and pregnant ponds, lockup and other forms of tailings. In the course of collating data from the management reports it may become apparent that more ore has been mined than was previously defined in the original resource / reserve statement for the locations that were mined. Explanations for this pleasant position are thus called for and may come from ore added by exploration, mining outside previously planned positions (which can be confirmed by survey), ore discovered in the course of development and hence called Not In Reserve ore (NIR) or ore gained by downward adjustment of cut-offs. It is not unknown that less ore than previously defined or anticipated from budgets is realised in the period under review. This alarming position again needs to be explained.

Reasons like adverse changes in the economics of mining and hence grade cut-offs, "geologists over-estimated the ore and it was simply not there", adverse and unavoidable and permanent changes in mining methods, incorrect mining factors like densities or truck factors and / or survey measurements. Situations like the foregoing result in writing off of quantities of ore resource / reserve. Although it is alarming, it is an acknowledgement of the negative factors that management should be aware of. 6 For Example In the example of Utopia Mine, attention should be paid to but not limited to the following. 6.1 Starting Position There is an opening ore resource / reserve position statement. This statement has certain definitions attached to it like pay limit, resource / reserve definitions and a pit plan.

It is thus necessary to see how operations proceeded in relation to this set-off plan e.g. actual pit outline versus planned outline, actual ore grade and tonnage versus planned ore grade and tonnage. 6.2 The operation 6.2. 1 The Pit The pit is the equivalent of a stoppe in underground mining parlance. By definition, all material above cut-off is measured resource and proved ore.

These two types of material are numerically equal and are the basis of the design of the pit or stoppe. Quite often the "waste" dump is not far from the mill or processing plant, to the extent that the marginal costs of trucking material of medium grade and low-grade quality to the plant are small. When there is spare processing capacity, these two material types are often processed instead of keeping the plant idle. It is then necessary to acknowledge that the definition of "actual ore" includes such ore reclaimed from "waste", low grade and medium grade hence an "operational pay limit" should be compared to the planned pay limit that was used is the reserve definition. This positive variance in the end will show up as the NIR at higher tonnage and lower grade than the "ore reserve" statement. 6.2. 2 Exploration Recent exploration resulted in reclassification of resource material.

Ore resource moved into the reserve category and / or lower confidence reserve material was upgraded. This resource movement should reflect in the reduction of the appropriate resource categories and increase of the appropriate reserve category as the Added by Exploration category. Invariably exploration results in a write off of portions of the resource or lower confidence reserve. For example new drilling may indicate that previously Inferred or Indicated Resource has shorter strike or dip continuity than previously expected. Thus the resource is written off and similarly the corresponding reserve category is also written off. If the write-off is more than "added-by-exploration" then there is a net Written Off tonnage.

Happily this is not the case at Utopia and what we have is an "Added by Exploration" category. 6.2. 3 From the Pit The setup at Utopia is that quantities of the various categories of ore mined in the pit are arrived at by survey and these are standard and acceptable benchmarks for subsequent measurements and reconciliations. Material accounting after the pit is by way of truck counts except stockpiles that are surveyed every month end. Trucked tonnages are arrived at by applying load factors to truck counts.

This is an area of potential reconciliation problems as all tonnages onto the heap leach pad from the pit and stockpiles are based on truck counts. Load factors can be misleading. The tendency is to err on the upper side of tonnage because operators are more likely to under fill than overfill trucks more so when the product size distribution is very varied like ROM ore. Moreover, truck factors vary according to product size distribution. The result is that more trucks are counted and more tonnage is declared as having been trucked than actual. If management is aware of these pitfalls then steps can be taken to ensure that errors are minimised and correct tonnages are declared. 6.2.

4 The leach Pad All things being equal, tonnages and grade on the leach pad are the same as what left the pit less stockpiled material. The Leach Pad mass balance is quite involving because of the element of time in the leaching process and measuring the tonnage on the pad. As the pad grows, management would have to ascertain pad bulk densities at various and representative locations by way of measured pits. These pits may be lined with waterproof plastic and filled with water and then densities are calculated. These densities would then be applied to a leach pad survey volume and tonnages calculated. Furthermore, management should have derived time leach curves for the ore type and these curves should show how long it takes to leach a unit of ore.

As new ore is loaded onto the pad and sprayed management knows how much has been leached and how much is remaining. Various ages of ore can thus be appropriately "depleted" of their load of gold in time. This "depleted" gold can be "worked back" into the tonnage loaded onto the pad and a theoretical head grade is calculated and reconciled with the actual head grade as sampled at loading. This is further reconciled with the grade and tonnage from the pit and stockpiles.

The aim here is to do a mass balance between the leach pad, the pit and stockpiles against fixed benchmarks of surveyed tonnage (and grade) from the pit and gold recovered from the gold plant. 6.3 After the Leach Pad Metallurgical accounting after the Leach Pad is much simpler because of precise measurements at various stages in the material flow. Issues of gold lock-up in solution and in carbon and gold sales are ascertained with accuracy. The mass balance can be carried out with confidence and reconciled with the leach pad mass balance. 7 Summary of recommended reconciliation steps The steps narrated above can be enumerated as below a) Define clearly the various material types handled by the operation. b) Sum up quantities and quality of the materials that were handled at various stages of the operation. This summation comes from monthly management reports. c) For the stage between the pit and the processing plant (i.e. stockpiles, crushing plant and leach pad), compare the summed tonnages and grade against what was budgeted to come from the pit, for the various categories of material.

Summations should be carefully carried out to follow material paths between the source (pit) and the three destinations of stockpiles, crushing plant and leach pad. Seek explanations for any differences. Possible explanations of variances are as noted in 5, 6.2. 1 6.2.

2 and 6.2. 3 above. The variances between the opening and closing positions of the pit reserves as compared to what went to the three destinations and explanations of them will constitute an ore reserve reconciliation. Naturally this has to tie in with the goings on after the three destinations. d) As part of the pre-startup testwork, metallurgical testwork was carried out including leaching tests. These tests were largely bench tests in controlled environments. They may or may not bear relationship with reality on a leach pad but are a starting point.

The reconciliation or mass balancing of the leach pad will answer this uncertainty. Paragraph 6.2. 4 above highlights a recommended practice in whose absence, as is the case, the time leach curves derived in the initial testwork are a fallback position. Once the tonnage and grade that left the pit are established and the tonnage and grade that left the various stockpiles, crusher and ROM are also established with acceptable confidence, then the tonnage that was put onto the Leach Pad can be said to be established. The head tonnage and grade are now known. This is then compared or reconciled with similar management report summations of material loaded onto the pad.

Explanations for any differences between the two sets of figures are sought. Truck counting and truck load factors are common sources of error and these will need to be "re-calibrated" to be effective management control tools as noted in 6.2. 3. Point 6.2. 4 suggests an alternative way of ascertaining pad load. e) Point 6.3 above is the final checkpoint where everything (tonnage and grade) is established by reliable measurement without empirical factors coming into play. Volumes and grades of liquor in the barren, pregnant ponds can be ascertained with extreme confidence.

The same is the case with gold in carbon and elsewhere in the gold winning circuit until the point of sale. All this liquor and gold came from the leach pad and can thus be built back to find the "recovered" or leached grade of the pad. With the knowledge of the "recovered" grade and prior knowledge of the pad head grade, pad lock up over the period of time can be calculated and reconciled with the "empirical" leach curves. It is now possible to work out leach pad dynamics from the tail and head ends and afford management with powerful control tools for the operation. 8 Conclusion In operations where material flow is measured at "frequent' positions of the stream, reconciliations can be made fully between any two stages. However in the Utopia situation, as is typical of heap leach situations, the leach pad presents a hurdle, although not an insurmountable hurdle.

It presents a weak link in the material balance process involving the mine and processing plant. With respect to the mine end of the process, a reconciliation of the resource / reserve is possible and should result in a reconciliation statement to accompany the ore reserve statement. The reconciliation statement should in summary state where applicable, for both geological resource and ore reserve, the parameters listed in the table below. Geological resource Ore Reserve Measured Indicated Inferred Proved Probable Opening Added by exploration Ex NIR Written off Depleted by mining Closing The reconciliation should enable management to come up with various mine factors that assist in the management and control of the operation.