The cement plant of the future

According to the Australian Cement Federation, since 1990 the Australian cement industry has achieved a 35% reduction in fuel used, 15% reduction in power used and 23% reduction in CO2 emissions per tonne of cement. These achievements have been made through investments in technology, the use of alternative fuels and raw materials, and the use of supplementary cementitious materials. This is a great result. But we need to keep improving. So, what does the cement plant of the future look like?

Designing the cement plant of the future

First stop – mineral processing. The basis for quality and cost of the cement starts here. Process engineering research in the cement industry has always aimed at:

- Reducing energy consumption in cement production

- Optimising quality and uniformity of produced cement, and

- Minimising emissions from the cement-production process [1], [2]

The starting point for all these goals is naturally the starting point of the complete process. Redesigning the limestone quarry would have a special focus on:

- Extending the range of sellable product by installing specialised technology in the immediate vicinity of the cement plant, and

- Improving the efficiency of material extraction from natural deposits

From limestone to cement

This task necessitates a conversion analysis for the raw materials within the cement production process, from limestone to cement. As Figure 1 shows, a mixture consisting of limestone (approx. 70 to 80% by mass) and clay/marl (approx. 20 to 30 % by mass) present in native form in many natural deposits for cement raw materials [2] is the starting point for the production of cement. Depending on the chemical composition of the raw material deposit, further components such as iron ore or sand must be added.

Following excavation by means of drilling and blasting or heavy equipment, the material is conveyed to the crushing plant where the run-of-mill (ROM) material is subjected to pre-crushing (using impact or hammer crushers, for example), and is then further homogenised. Grinding to raw meal using vertical roller or ball mills follows in the raw meal plant. The raw meal is then de-acidified in the clinker plant by breaking down the CaCO3 limestone into its CaO and CO2 components. Burning at approx. 1450°C in the rotary kiln then takes place until sintering occurs.

After cooling, the cement clinker is ground in the cement grinding plant while sulphates (e.g. gypsum) are added to yield various grades of fineness (cement qualities). Deliveries of cement take place primarily by means of shipment in silo vehicles or in bags, by water, rail and/or road transportation. The ready-mixed concrete industry (approx. 52 %), and the manufacturers of pre-cast concrete elements (approx. 26 %) are among the main customers for cement. The cement-production process is always dependent on the proximity of a suitable natural raw material deposit.

FIGURE 1: Simplified diagram of cement production clusters

FIGURE 2: 3D computer-generated view and actual view of the Kleinhammer greywacke quarry project, showing integrated armourstone production

FIGURE 3: NIAGARA DS 1600 x 5000 eccentric screening machine

The Cement Plant of the Future makes more products while transforming waste into sellable goods

Limestone sedimentary rock is needed not only for cement production, but is also an important raw material in the building materials, agriculture, water management, steel, glass, fertilisers and paper industries. Stone products are currently produced at and shipped from self-contained facilities (e.g. pre-crushing and gravel plants) in the vicinity of the natural deposits [3]. The concept of additionally using the cement plant’s material deposits more intensively is therefore an obvious step in improving the efficiency of natural deposit exploitation. The Cement Plant of the Future focuses on making use of existing overcapacities in the cement plants’ material deposits, and transforming frequently encountered deposit types which are not suitable for cement production to sellable products by removing impurities. In both cases after the primary and/or secondary crushing stages, a portion of the mineral can be used for producing various stone products such as crushed stone and/or gravel (see Figure 1, Cluster I).

Adding to the product line – mortars

Even today, most cement producers outside of Europe are not ceasing the opportunity of adding mortars to their cement product line. Mortars are relatively simple to make and achieve a significantly higher price per tonne compared to pure cement. The Cement Plant of the Future is equipped to optimally process limestone for the correct mortar additives. Limestone grit, e.g. of the 0.1/1.2 mm fractions, is an optimum raw material for further processing into typical fine-sand fractions that are needed for the production of blended cement or of rendering and dry mortar. Figure 6 shows a system diagram. Downstream from the raw-meal mill the limestone grit is sifted (removal of the ultra-fine particulates fraction <0.09 mm from the material flow) in a classifier and then is fed to the mechanical fines screens for separation into fractions. Fractions of d <2 mm can be used as dry mortar, cement additives, and for flue-gas desulphurisation. It has become apparent in practical application that the use of an advanced screening technology, known as the Fine-Line (see Figure 7), makes it possible to produce limestone grit for dry mortar production at high throughput rates and with precise cut sizes (also see Figure 1, Cluster II).

Adding to the product line – burnt-lime production

An additional application for limestone is the production of burnt lime. Burnt lime (CaO) is a powder produced by burning limestone at approx. 800°C. Burnt lime reacts with heat to form slaked lime (Ca(OH)2) when water is added. Burnt and slaked lime are used by a broad range of industries. For example it is used as an additive for the production of mortar in the construction industry, as agricultural lime in the fertiliser industry, and for de-sulphurisation of “hot metal” (unrefined iron from the blast furnace) in the steel industry. Slaked lime can also be used as an alternative to limestone in fluegas de-sulphurisation in power-generating plants [1], [2], [3]. The Cement Plant of the Future will be outfitted with the required technology to produce burnt lime when and where feasible in order to increase revenues, which will offset the cement production costs.

FIGURE 4: Flow sheet for the preparation facility at Wietersdorfer & Peggauer Zementwerke GmbH’s plant in Peggau

Transforming waste into sellable products by removing impurities

How can we turn waste into sellable product? It utilises innovative washing technology. The Hydro Clean system was designed to wash minerals using high pressure. This combines the highest cleaning power possible with the lowest possible water consumption. The start-up of a low-wear, energy-efficient and resource-conserving high-pressure washing facility for the supply of high-quality limestone fractions for further processing using the Hydro-Clean technology took place at Wietersdorfer & Peggauer Zementwerke GmbH’s Peggau plant near the city of Graz, Austria, in 2009/10 [7]. Figure 4 shows the flow sheet for the material preparation plant. The high-pressure washer shown in Figure 5 was used for cleaning of the heavily fouled deposit material and achieved extremely good cleaning results with a water consumption of only 1.5m³ per metric tonne. The material is exposed to cleaning in the washing chamber (4) for approx. 3 seconds. It is then removed from the washing chamber via a frequency-controlled extraction belt (5). The material’s exposure period can be modified to match the bonding (e.g. bond form, bond type and bond strength) between the contaminant and the product itself by altering the speed of the extraction belt. This makes it possible to react flexibly to fluctuations in the washing chamber (4) for approx. 3 seconds. It is then removed from the washing chamber via a frequency-controlled extraction belt (5). The material’s exposure period can be modified to match the bonding (e.g. bond form, bond type and bond strength) between the contaminant and the product itself by altering the speed of the extraction belt. This makes it possible to react flexibly to fluctuations in deposit material, and so ensure constant uniform cleaning for prolonged periods of operation. A wet screen for production of the 0/5, 5/30 and 30/70 fractions was also used in the material preparation plant, in addition to the installed high-pressure washing system. After washing the coarser 30/70 fraction is routed to the burnt-lime kiln while the finer fractions (0/5 and 5/30) are used for producing dry mortar. The resulting washing water is then cleaned by a water-treatment plant and again fed back into the washing process. Loam yielded in this process is routed as a corrective to a clinker production plant.

FIGURE 5: Diagram in principle of the Hydro-Clean

FIGURE 6: System diagram for grit-fraction preparation

FIGURE 7: Fine-Line HD 1850 x 3750 fine-fraction mechanical screens

FIGURE 8: Diagram in principle of clinker screening with downstream separate grinding

FIGURE 9: Niagara clinker screen with three decks

The Cement Plant of the Future reduces costs

Saving costs using a NIAGARA pre-crushing plant

Most limestone quarries world-wide use a grizzly scalping screen. Some do not pre-treat the material prior to entering the primary crusher at all. This results in the unnecessary processing of materials, creates bottle necks within the crusher and jeopardises the product quality for the subsequent processing steps. Using technology such as the NIAGARA Scalping System, ROM material can be pre-sized prior to entering the crusher, creating a final product at the very first step of the process. The NIAGARA Scalper utilises an eccentric shaft supported by a total of four bearings. This technology guarantees a continuous vibration under all operating circumstances. This keeps the screen surface openings clean and guarantees full removal of all fines, which in return allows for reduced crusher wear and increased overall system performance. Common grizzly screens offer neither of these advantages. A new pre-crushing plant with an integrated armourstone production facility was engineered and constructed at the Kleinhammer greywacke quarry in Germany’s Sauerland region in the 2012/13 period [4]. The scope of supply included not only the NIAGARA Scalping Screen System and armourstone plant, but also all conveying equipment, temporary-storage and dust silos, structural planning and complete installation. An example of flexible production is provided by the armourstone facility, on which the 4 to 40kg and 10 to 60kg weight classes can be produced singly or simultaneously as needed by means of two “NIAGARA” type heavy-duty mechanical screens (see Figure 3). Due to optimal project planning, implementation time for the complete plant from start to commissioning was only some 12 months. The Cement Plant of the Future includes the NIAGARA Scalping System-based pre-crushing plant to optimise product quality and operating costs.

Saving costs by screening clinker downstream

A further example on the road to the Cement Plant of the Future is the screening clinker downstream with separate grinding” project. The starting point for these ideas was the generation of clinker fines as a result of the rapid cooling setting properties of the cement. The cooled clinker is then routed via conveying systems to the clinker storage facility. The cooling and conveying process resulting from the system causes a clinker fines fraction <5 mm of up to 30%.

Saving costs grinding clinker

Ball mills have proven their capabilities for clinker grinding for many years. Single-stage grinding using vertical and horizontal mills and high pressure roller mills have also become popular during recent years. These mills are suitable for the production of standard grades of cement. In granulometric terms, special grades can still be produced to a higher quality by using ball mills. For this and for a number of other market-specific reasons, the Cement Plant of the Future will still contain both alternatives and possible even combination of these mill types. Both ball and vertical roller mills will continue to be state-of the- art for clinker grinding in upcoming years. Based on an energy analysis, it would therefore appear rational to comminute various clinker fractions in separate grinding machines. As is shown in Figure 8, the future cement plant will divide clinker into a coarse and a fine fraction by means of classification using a NIAGARA mechanical screen (see Figure 9). To obtain energy benefits, the coarse fraction is routed to a roller mill for further grinding, and the fine fraction to a ball mill. This achieves an overall increase in specific throughput rate with a simultaneous improvement in the energy-efficiency of the grinding process. The energy savings attained via separate grinding can be as large as 10%. Further advantages include quieter operation and low wear to the vertical roller mill. For the cement plant this innovative solution allows the highly flexible adaptation to the future demands for standard, special, and blended cement products, and for dry mortar.

Conclusion

The Cement Plant of the Future starts in the quarry. It will make more products than before, will turn waste into sellable product and it will be more cost effective than ever before. It will see more intensive networking between cement and concrete production and further processing systems. By locating concrete production and processing in the vicinity of the cement plant will also offer the opportunity to reduce storage and transportation costs and increase the diversity of products available for regional sales. Let the future begin.

Author: Steffen Silge, Head of Sales - Mineral Processing, Haver & Boecker Australia