Modern quarry planning requires modern tools

: 01 April 2019

Figure 1: The limestone quarry supplies most of the material to produce white and grey cement. A textbook example of a safe quarry design and well-managed quarry.
Figure 2: Block model for the limestone deposit. Distribution of MgO (%).
Figure 3: The flysch formation in the clay pit is the cause of the high variability of its chemical composition.
Figure 4: Distribution of SiO2 in the clay deposit (%). Note that the blocks are much smaller than the blocks in the limestone model.
Figure 5: Distribution of Al2O3 in the clay deposit (%).
Figure 6: Distribution of Fe2O3 in the clay deposit (%).
Figure 7: Yearly plan for the limestone quarry. The blocks represent the material to be extracted during 2019.
Figure 8: Yearly plan for the clay deposit split into quarterly steps. Blue blocks are proposed for extraction in the first quarter, green blocks in the second quarter, yellow blocks in the third quarter and the red blocks in the fourth quarter. This respects the right balance between the clayey and sandy materials at all times.
Figure 9: The use of the clay deposit with 1.5% of purchased sand is about 60% (top picture) and is 85% without any purchased sand (bottom picture). Red blocks are proposed for extraction.

The ‘good old days’ where you could build pre-blending piles by mixing X trucks of ‘grey rock,’ Y trucks of ‘yellow stone’ and Z trucks of ‘brown stuff,’ are long gone, due to the increasing complexity of the deposits available for cement manufacturing, more stringent restrictions on emissions and the use of alternative fuels. In 2019, modern quarry managers have to operate according to these new limitations. A paradigm shift with respect to quarry management and planning is well overdue...

A cement plant in Europe decided to adopt a new approach to its raw material management, in which it would fully integrate its quarries into the cement manufacturing process. The new extraction plan had to comply with the quality targets of the raw mix, while reducing the use of purchased correctives and maintaining safe quarry layouts. To strictly follow its chosen methodology and review its mining strategy, the plant decided to use the AthosGEO Blend software, developed by cobo GmbH.

Analysis of the quarries

The plant operates two deposits. The limestone quarry supplies the main raw mix component for cement (See Figure 1). The second pit provides a mix of clay and sandstone that is used as a second raw mix component for grey cement. Both deposits present operational and quality challenges.

The variability and the content of magnesia (MgO) in the extracted limestone had been increasing throughout 2018. Figure 2 shows the distribution of MgO in the limestone deposit.

The clay deposit is very heterogeneous (See Figure 3). Achieving the correct balance of clayey and sandy materials for the second component is key for controlling the quality of the raw mix (See Figures 4-6). These materials contain magnesia, sulphur, alkalis and other elements that must be predicted and controlled to produce an acceptable raw mix.

New plan targets

With the new plan, the plant wanted to:

Predict the magnesia content of the raw mix and keep it below 2%;
Predict and control the balance of the clayey and sandy materials in the clay pit;
Predict and control the sulphur / alkali content in the materials extracted from the clay pit;
Reduce the consumption of purchased sand.

Methodology

This is where apbp consulting’s holistic raw material management approach combined with AthosGEO Blend came in. The approach taken by abpb consists of integrating the deposits into the clinker manufacturing process. The idea is to optimise their use and establish extraction schedules based on raw mix criteria, by including all parameters that influence raw mix production and quarry operations into the deposit evaluation process. AthosGEO blend is the tool that enables abpb to follow this methodology.

In this case, apbp and the plant needed a block model for each of the deposits to be considered. These data are fed to AthosGEO Blend as .csv files.

Next, the correctives and alternative materials that were to be evaluated were added. For this, abpb and the plant needed to know the chemistry of each material to be assessed by AthosGEO Blend. It is also possible to include the cost (or revenue) of each material and the software will estimate the cost (or revenue) of the raw mix preparation process.

Then the chemical quality requirements of the raw meal must be defined. For raw mix, you would define ranges for lime saturation factor, silica ratio, alumina-to-iron ratio and any other relevant quality parameter available from the block models - for example MgO, Na₂+ equivalent, SO₃). By playing with these raw mix criteria, it is then possible to assess the influence of alternative fuels on the deposits.

A plant can define the amount of purchased corrective it is prepared to or ready to use, try to force compensated alternative materials into the mix, or let the software find the optimum amount of each material. It is also possible to optimise deposits for up to two products simultaneously, for instance for raw mix and mineral component, by defining quality criteria and the quantity needed of each product.

Finally, mining restrictions are brought in. This adds all physical constraints (like permit limits, properties, buffer zones, infrastructure, protected areas, rehabilitated areas) to AthosGEO Blend to realistically restrict quarry operations. It is also possible to include the final pits that will be considered, define the configuration of the working face and even the preferred mining direction.

Setting up AthosGEO Blend takes half a day and the establishment of a full extraction schedule in terms of raw mix should not exceed three days.

Back to the case-study

apbp consulting established a yearly plan with the quarry team to do without using any purchased sand (See Figures 4 and 8). In fact, the global optimisation of the deposits demonstrated that using no sand increases the use of the clay deposit: with 1.5% of sand only 60% of the deposit can be extracted. However, without sand 85% of the deposit can be consumed (See Figure 9). The savings on sand paid back the investment into the software several times over. The plan also honoured the quarry team’s intention of developing the bottom bench of the limestone quarry, and pushing back the east and north walls, according to deforestation requirements (See Figure 4). Finally, the extraction schedule respected the raw mix requirements at all time, especially the 2% constraint on MgO. It consistently delivered the right mix of clayey and sandy material from the clay pit (See Figure 8).

To summarise...

AthosGEO Blend can be used to apply apbp consulting’s holistic raw materials management approach for clinker production. This typically results in up to 20% of savings on raw mix preparation costs. Nevertheless, this methodology carries benefits that go beyond cost reductions and optimising the use of correctives. It allows the plant management to:

Anticipate, which avoids nasty and expensive surprises. For instance, knowing the quarry development plan well in advance is the key to proper land and quarry fleet management;
Comply with emission limits by controlling detrimental elements directly within the quarry.1 Being able to predict the expected emissions also help select appropriate mitigation equipment;
Improve the homogeneity of the raw mix, which improves the downstream processes by reducing the thermal energy costs, decreasing the number of blockages and diminishing the amount of off-spec clinker.

1. Bláha, P.; Pujol, A.; ‘Emissions control starts in the quarry,’ in Global Cement Magazine, October 2018, pp. 12 - 15.