Cement plant quarries typically provide approximately 90% of the raw materials needed to make cement, but because they account for only about 10% of total operating costs, they are easily neglected. Even today, many operators only pay attention to the quarry when problems arise with raw material quantity or quality. This article shows how all operators can better understand their natural resources through block modeling and mine planning.
To better understand the quality and quantity of 'ore,' cement plant operators often create a block model that represents their deposit. A block model is a collection of 3D blocks that contain chemical and geological data based upon drilling information. The 3D model can be colourised by chemistry, location and / or rock type attributes.
In addition, varying degrees of confidence can be assigned to an individual block’s data based on the block’s distance to drilling, faults, and other geological features. Blocks with high confidence in their respective chemistry and geological data can potentially become Measured Resources. Medium confidence blocks have the potential to become Indicated Resources, and low confidence blocks have the potential to become Inferred Resources. Figure 1 provides an example of a block model that contains each resource type.
Block models are generated by drilling campaigns. Although a gridded design is preferred, other approaches can be used. In addition to the drilling campaign layout, designing the campaign to fully penetrate the permitted pit floor is essential. Data collection and accurate core logging are also vital because any model is only as good as the data on which it is based.
Core drilling is the preferred drilling method to develop a resource block model for reporting resources and reserves. Depending on the deposit and industry, reverse circulation drilling is acceptable under certain circumstances. Other forms of drilling (e.g., blast drilling and auger drilling) can be used to help verify a resource block model but should not be used to report resources or reserves. Understanding the regional and local geologic bedding, faulting, and groundwater surface is critical when designing a drilling campaign. Figure 2 illustrates the complexity of modeling some quarries because of faults while ensuring exploration drilling is adequate to define each reserve block.
The data
| Cumulative MgO Cut-off (%) | Incremental Volume (Mt) | Cumulative Volume (Mt) | Volume Above Average Grade (Mt) | Quarry Life (Years @ 1.9Mt/yr) |
| 1.5 | 67.61 | 67.61 | 11.78 | 35.6 |
| 1.75 | 8.2 | 75.81 | 3.58 | 39.9 |
| 2 | 3.48 | 79.29 | 0.1 | 41.7 |
Above - Table 1: Magnesium oxide (MgO) sensitivity analysis.
After a block model has been generated, knowing the type of data that can be extracted is essential. The most important data provided are the quantity and quality of the ores. Resource reports take several forms, and the most important report is the confidence level, as already discussed. Block models can also analyse the sensitivity of the ore deposit to varying chemical cut-off constraints or other parameters that are deemed important. Table 1 shows an example of a cement plant quarry’s sensitivity toward magnesium oxide (MgO) concentrations in the raw feed to the plant. As the maximum MgO cut-off limit increases, more resources are available within the quarry, increasing its remaining life.
Block models can also create maps that help quarry managers to visualise deposits. Vertical slices, known as cross sections, are often used. As shown in Figure 3, these cross sections can display chemical parameters with varying concentration ranges, geological bedding or the confidence levels of a block. The same information can be shown as a horizontal slice through the block model, known as a grade map. As shown in Figure 4, grade maps can group blocks to represent an entire bench, a specific elevation range, or an individual geological bed. Cross sections and grade maps are critical outputs that help identify chemistry 'hotspots' in advance of any potential problems for blending purposes; thus, an appropriate plan can be developed.
Developing a mine plan
After a block model has been created for a quarry, operations will develop a mine plan based on the block model. Mine plans can range from a sequential design showing where to mine from time period to time period, to guidance for the total available material remaining in the quarry, or even the valuation of a property for investment (or lawsuit) purposes.
Mining operations come in various forms, such as surface-to-underground, open-pit truck-and-shovel, and underground longwall mining. When creating a mine plan, the mining method is one of the most significant factors to consider. Open-pit truck-and-shovel, contour mining and dragline strip mining are some of the usual mining methods involved in surface operations, which are most often used in the cement sector. Room-and-pillar, longwall, and long-hole stoping are mining methods typically used in underground mining. Properly selecting and considering the mining method significantly impacts the mine design, equipment selection, production targets and overall mining costs.
In addition to mining the deposit and block model resource estimation, mine plans must consider several other considerations and disciplines, such as hydrological and geotechnical engineering, environmental engineering, financial analysis, taxes and permitting. These disciplines are usually included in a mine plan by understanding the ultimate mining limits, access roads and ramps, mining and development of waste material, operational and capital costs and reclamation obligations.
What mine plans tell operators
The average limestone cement quarry requires a sequential design of mine plans that show where to mine within a given period, typically annually or quarterly. For sequential mine plan designs, mine progression maps similar to Figure 5 are one of the most critical outputs of the mine plan. Progression figures provide the mine operator with information on where to mine, which bench to mine, when to mine (annually or quarterly), and what can be expected from the block in terms of tonnages and chemical qualities. Progression maps can be created on a by-year or by-bench basis. In addition to the progression figures, renders of how the topography will change by the time the mine plan is complete can help operators visualise how the quarry will develop over time.
Why your quarry matters
The most important assets an operation gains by understanding its quarry are knowledge of expansion opportunities, problematic areas, and the expected chemistry of raw feed coming into the plant. This knowledge allows the quarry and plant to proactively tackle upcoming challenges by planning and budgeting for appropriate solutions. A plant can also plan to receive raw material that consistently achieves a targeted range of chemical concentrations instead of a raw feed that potentially has large swings in chemical concentrations from week-to-week (or even from day-to-day).
Figure 6 shows a quarry nearing the end of its life that had approximately 10 years of quarry life remaining in the baseline scenario of the 'Permit Area (Bench 5 Excluded).' Based on the block model, the quarry conducted additional explorative drilling and identified material immediately adjacent to the quarry that had better chemical concentrations than the quarry’s permitted area. With this additional exploration drilling and modeling, the operator identified several expansion opportunities that could improve the remaining material life of the active quarry at a significantly reduced cost.
Figure 7 shows a quarry that purposefully did not extract material that was considered too poor in chemical quality to blend into the raw feed. Leaving these areas of 'unblendable' material in the quarry hindered the quarry’s development by creating choke points and longer hauling distances. As a result, fuel costs increased and production levels were lowered. After the quarry was block modeled, the operator realised that, while those unextracted areas were of lower chemical quality, the material was not 'unblendable.' With proper mine planning, the quarry is now developing through the problematic areas. The result is increased resources and lifespan of the quarry, shorter haul distances and less wasted material.
Concluding remarks
Methodical block modeling and mine planning in quarries offer several ways for cement producers to fully optimise their existing natural resources. Less variable raw material feeds, greater operational foresight, lower operating costs and longer-lasting resources are just a few examples of optimisation.
