The 4 Ws of dilution in open pit planning

If there is one common issue between projects and mines, it is the contentious issue of mining dilution. Dilution is one of the important factors that may significantly impact the economy of a mining project. This article seeks to address the 4Ws of dilution: What, Why, Which and When.

So yes, what is dilution?

In simple terms, dilution refers to the waste material that is not separated from the ore during the stages of mining and is sent to the processing plant (Ebrahimi, 2013). During operation, there are two types of dilutions that you may find in a mining block: internal dilution and external dilution. Internal dilution refers to waste or even low-grade ore (ore below cut-off) within a mining block that cannot be mined discretely and is eventually mined with the ore in the mining block. External dilution refers to waste outside of the orebody that is mined with the ore within the mining block

Figure 2: Internal vs external dilution in a mining block

Why is it important to have a good grip on dilution?

Most people who are involved in mine planning are aware of the significant economic impacts of dilution depending on what dilution factor is applied and how it is actually applied to the block model. To bring everyone to speed, one of the main consequences of dilution is the increase in ore tonnage sent to the plant with a reduction of mill feed grade. Lower feed grade translates to lower mill recovery, and this accompanied by increasing processing costs translates to less income. Dilution also increases the economic cut-off of the project. For marginal grade ore, dilution may reduce the grades in a block to a degree that it becomes uneconomic to be processed. This is precisely why it is important to think carefully about the approach that you are going to use to model dilution on your block model.

Which dilution options are available to dilute your block model?

There are numerous dilution techniques that are available to dilute a block model. The answer to the question “which approach to use” is dependent on the type of deposit in question as well as the operational aspects.

As an open pit planning consultant, the most dominant dilution modelling technique that I have encountered in projects involves choosing a flat dilution value that sounds appropriate for the deposit in question and applying it globally on the block model instead of quantifying it. In some cases, the dilution value is based on historical information, some based on rules of thumb while in some cases there might not even be a solid technical basis behind the selected dilution value.

Fortunately, there are certain techniques that are available in Datamine’s Studio products to model block model dilution for mine planning purposes. The following techniques can be used:

REBLOCK

For the purpose of pit optimisation, the size of the block model cells should be representative of the degree of selectivity that can be achieved during mining operations. It is always advisable to represent the actual degrees of selectivity (SMUs) possible in practice. The REBLOCK process in Datamine Studio can be used to create block model cells matching the SMUs and re-calculating the block model attributes (e.g. grades, contaminants etc.) according to user-defined criteria.

Figure 2: SMU compositing with re-calculated block model attributes

During the REBLOCK process some ore will be diluted with waste material. Upon application of the cut-off grade, some ore blocks may possibly be converted to waste blocks. Some engineers may apply additional dilution beyond the REBLOCK process while others will consider the block model fully diluted at the end of this process.

Figure 3: Original model vs reblocked block model

DILUTMOD

The DILUTMOD process is used to dilute grades in a parent cell by a user-defined dilution width. The block model grades are adjusted in parent cells which have adjacent cells of a different rock type.

Figure 4: Different rock types in a block model

So, whenever the rock type changes between adjacent cells, the grades are adjusted by addition of material from the adjacent cell and the loss of the same volume of material to the adjacent cell. The adjusted grade of the parent cell will then be calculated using tonnage weighting.

Figure 5: Block model with in-situ grades versus diluted grades after running DILUTMOD

With reference to Figure 5 above, it is visible that the grades of the yellow blocks (undiluted ore) and the red blocks (undiluted waste) remaines the same before and after running Dilutmod. This is because these blocks have adjacent blocks of the same rock type (in a 3D space). The grades of the green blocks (diluted waste) and orange blocks (diluted ore) change because these blocks had adjacent blocks of a different rock type (in 3D space)

USING MINEABLE SHAPES OPTIMISER (MSO)

Datamine’s Mineable Shapes Optimiser tool, the underground stope optimising tool, can be used to model dilution in a narrow-vein deposit.

Figure 6: Block model showing a narrow-vein deposit

By considering an SMU as an underground stope, MSO can be used to create wireframes for each mining unit over the entire deposit. Using these stope wireframes, the block model can be sub-blocked (using MODSPLIT function) and assigned as either “ore “ (inside the stope wireframe) or “waste” (outside the stope wireframe” prior to optimisation. The material within the MSO generated stopes is specified as ore and is inclusive of planned dilution (on the footwall, hangingwall or both)

Figure 7: MSO wireframes representing the SMUs and showing dilution and ore loss areas

There are other techniques that are available to dilute the model. It is important to note that diluting a block model is not a one-glove-fits-all phenomena. Dilution varies in different mines based on the characteristics of the deposit in question. So before deciding on an approach, it is important to understand the characteristics of your deposit as well as the operational aspects.

When should you apply the cut-off grade?

Another interesting thing to think about is whether you apply the cut-off grade before or after diluting your block model. Well, the answer depends on which dilution technique you are utilising. When you incorporate dilution into the block model itself (e.g., Reblock, MSO approach), then the cut-off grade is most likely applied to the final, diluted blocks. When dilution is added to the final production schedule for example, then the cut-off grade would have been applied to the undiluted material. The timing of when to apply the cut-off grade will have an impact on the ore tonnages and head grades being reported.

Dilution during pit optimisations

The last important thing I want to discuss is dilution in pit optimisations. It is important to ensure that your optimisation incorporates dilution into the whole picture. Most pit optimisation solutions (Whittle, Studio NPVS) have input fields for a flat dilution factor and mining recovery (ore loss) factor. During optimisation, some engineers will apply global dilution at this point via the input fields on Studio NPVS. When dilution is applied using a parameter on Studio NPVS, the following effects can be observed:

• Unit processing costs are increased;
• The economic cut-off grade is increased;
• The processing recovery will be affected if it is a function of grade;
• Block values ($) will be reduced;
• Recovered products will be affected

However, there are benefits of applying the dilution factor on Studio NPVS. This includes:

• Dilution is easy to apply and vary for different scenarios; and
• It is relatively easy to test different cut-off grades and economic parameters

Other engineers prefer preparing the block model beforehand, so that blocks already have dilution embedded into them and they leave the dilution factor on the optimiser at 0. To be honest, there are pros and cons for either approach. The advantages of incorporating dilution into the block model include:

• Dilution can be model in greater detail and better accuracy;
• Can account for variations by location, lithology etc.; and
• Directly reports accurate or quantities as supplied to the processing plant

Disadvantages of incorporating dilution into the block model:

• May be more difficult to test different cut-off grades and economic scenarios; and
• May be more difficult to plot original in-situ grades on plans and sections