Prof.Dr. Hakan BENZER 1, Onur ATAKAY 2, Doç.Dr. Hakan DÜNDAR 1, Prof.Dr. Oktay ŞAHBAZ 3, Volkan CEBECİ 2
1Hacettepe University, Mining Engineering Department – 2Minerva Engineering Mühendislik A.Ş. – 3Dumlupınar University, Mining Engineering Department
As a sector where energy is used intensively, the cement industry consumes an average energy of 110 kWh per ton of cement in the process from raw material preparation to the final grinding stage, and more than 60% of this energy is spent for raw material and cement grinding. Considering that most of the energy used is consumed in grinding circuits, it is a fact that possible improvements to increase energy efficiency in grinding circuits will provide a significant contribution to reducing operating costs. In this context, energy savings can be achieved depending on the optimization of operational parameters, and energy savings can be taken to a higher level with the integration of new grinding technologies.
While approximately 4.2 billion tons of cement was produced across the world in 2020 (1), 73 million tons of cement was produced in our country in the same year and approximately 79 million tons of cement was produced in 2021 (2). New investments are being made to meet the increasing need for cement while reducing the carbon footprint is on the agenda all around the world in the upcoming period. Although the average unit energy consumption for cement grinding in Turkey in 2020 was 40.22 kWh, this value varies depending on a multitude of factors ranging from the admixtures that are used to the type of cement produced. The necessity of a finer grinding to increase product quality causes this value to increase even more especially in plants where production is carried out using conventional methods. Despite their low performance in energy utilization, the ball mills have often been preferred by the industry due to their ability to carry out mass production. Ball mills by design are designed to carry out production between 2 mm in feed size and 100 microns in terms of product grain size. This grinding range has been extended in terms of both the coarse grain size and the fine product grain size as a result of the industrial conditions. As a natural consequence of this, efficiency loss have occurred in grinding energy utilization. Energy inefficiency has been eliminated by using pressurized systems in the coarse grain size section.
Existing circuits are retrofitted with additional new technologies, as well as flow charts consisting of equipment with high energy efficiency are implemented with new investments in order to reduce energy costs in grinding circuits. In the process that started with highperformance air separators, a significant capacity increase was achieved in the existing grinding circuits through the pre-crushing systems that were developed and the energy consumed per ton of cement was reduced significantly.
High-pressure roller presses were developed in 1984 after it was demonstrated that crushing under pressure was more energy efficient than other conventional grinding methods (3; 4). Roller presses provide ease of grinding of the material thanks to the micro-cracked structure by crushing the ball mill feed material under pressure cause an increase in the capacity of the ball mill through use of a pre-crusher before the ball mill. The unit energy consumption in the ball mill decreases due to the increased capacity and energy saving are achieved per ton of cement. The energy losses that occur during a grain size reduction process carried out in a conventional grinding circuit using a ball mill are less than the one during the grinding process carried out with a roller press. As the material is crushed under pressure in form of a bed, load is exerted on each grain by the other grains in contact with it and the fragmentation process takes place with less energy loss. Roller presses operate in a more energy efficient manner than ball mills due to this fragmentation principle. Different flow schemes were developed and a certain part of the grinding process in ball mills was shifted to roller presses in order to make use of roller presses better. The different circuit configurations that are created in this context are provided in Figure 1. It has been demonstrated by industrial data that unit energy consumption decreases between 10 and 50% depending on the circuit configuration as a result upon integrating roller presses to ball mill circuits (5; 6).
Figure 1. Circuit configurations with roller press
a) open circuit pre-grinding
b) hybrid grinding
c) semi-final grinding
d) closed circuit pre-grinding
Circuitry designs where the roller press is used alone in cement grinding have been implemented it has proven itself as a pre-crusher in terms of energy consumption. Systems that are operated in closed circuit with air classifiers and intended directly for the production of finished products have taken their place in cement plants. In parallel with this development, vertical roller mills, which have been widely and effectively used in raw meal grinding for many years, have also achieved successful applications in clinker grinding. Finer grinding is required in order to achieve quality as the product grain size distribution in both roller presses and vertical roller mills used in clinker grinding has a narrower range than in ball mills. Also, quality goals can be achieved through use of a number of chemical admixtures.
The high return quantities produced in roller press and vertical roller mills leads to a steeper product size distribution and consequently results in a relatively lower quality product than the product of the same fineness processed using ball mill. Closed-circuit roller press applications and vertical roller mill applications, which are expected to replace ball mills, are less advantageous in terms of quality unlike their energy efficiency. On the other hand, considering that the effective grinding fineness offered by the ball mill is limited to 100 microns, the need for finer grinding depending on product quality (average product grain size is around 15 microns in industrial practice) makes the energy use in ball mills even more inefficient.
Minerva Engineering Company has developed flow charts that are planned to be implemented in the coming period as a solution to the quality issue that occurs in roller presses and vertical roller mills and the lack of energy efficiency in ball mills. The decrease occurring in effective energy utilization in ball mills in return of the increase in grinding fineness leads to the conclusion that the use of ball mills in very fine grains is not economically feasible. A more efficient energy utilization has been achieved in finer grain sizes in cement grinding and successful results have been obtained in terms of quality with the development of the dry stirred mill, which effectively performs grinding to finer grain sizes. Developed in conjunction with pressurized grinding systems, the integrated use of this dry stirred mill will be the beginning of a new era in the cement industry in terms of both energy and quality. Considering that grinding fineness is one of the most important parameters in terms of quality, it is obvious that any investment to maximize energy efficiency in fine grinding will create a difference and provide a competitive environment.
The flow charts that have been proven by laboratory and pilot-scale tests to offer greater advantages in terms of both energy and quality in cement grinding are provided in Figure 2. It was aimed in the flow charts proposed by Minerva Engineering Company, to achieve the target fineness in a more energy efficient way in the dry stirred mill by feeding the classified product output by the vertical roller mill or roller press directly into the dry stirred mill. At minimum of 20% reduction in energy consumption per ton is achieved with no compromise to quality compared to existing pre-crusher ball mill systems when the low energy consumption rates of the vertical roller mill and roller press are combined with the efficient fine grinding performance of the dry stirred mill.
Figure 2. Flow charts of dry stirred mill with pre-crusher
a) roller vertical mill+dry stirred mill b) roller press+dry stirred mill
Minerva Engineering Company has commissioned one of the two pilot plants that have been installed in the world for dry stirred mill technology (Pamir) where samples of roller press circuit product or roller mill circuit product samples can be tested for the proposed flow charts. The system design comprises a compact grinding system consisting of a mill driven by an 18 kW motor and a mesh fan group as auxiliary equipment. System optimization can be achieved in the pilot plant by conducting trials under different operating conditions. Figure 3 shows the images of the mill employed during the pilot test studies.
Figure 3. Photos of pamir mill pilot plant
Offering positive results in terms of energy cost upon being integrated into an existing ball mill circuit, the possibility to apply the dry stirred mill directly after the pre-crusher stands out as a great advantage. The final product of the desired quality can be produced in a more energy efficient manner by grinding in the dry mixing mill, the product which will be obtained around 200 microns as output by the pre-crusher closed circuit systems. Although the findings from the pilot plant studies show that they are known to be higher than the grinding system operating on an industrial scale, that is in support of this hypothesis. Below provides the difference of product and feed Blaine versus specific energy consumption when the V separator fine material is milled in the roller press V separator closed circuit system within the pilot plant (Figure 4).
Figure 4. Blaine gain versus specific energy consumption as a result of Pamir pilot plant application
The first industrial sized Pamir mill was integrated into the existing mill process at Bursa Cement Plant and various sampling studies were carried out from various points such as end product, separator return (separator coarse grain) and mill filter return. Then, various feed grain size distributions were made subject to testing and mill performance was evaluated. The sampling studies were carried out when the mill became fully stationary and the stationary state of the mill was determined by monitoring the electricity consumption and production rate. After both parameters became fixed, samples were taken and subjected to characterization studies. Size distribution, surface area and strength properties were determined in terms of characterization. Additionally, operating conditions were measured and evaluated by the technical personnel of Bursa Cement.
This study aims to summarize the development stages of the 1000 L mill and to share the results obtained from the grinding circuit in which the mill was commissioned. Initial results which were attained by realizing an energy saving rate of 18% and capacity increase rate of 22% through grinding of the cement received from the separator inlet of the ball mill were promising. An improvement of 10% in mortar and concrete quality was achieved in addition to energy savings, compared to baseline measurements . The operating conditions were also scrutinized in the study in terms of convenience and difficulties. A simulation regarding the advantages expected to be offered to the cement industry by the new mill was also carried out. The findings show that the advantages are not limited to what has been already achieved and that additional benefits can be achieved for sustainable cement production (7)
While energy use in cement grinding circuits is becoming increasingly important depending on the increasing energy costs , the obligation to reduce the carbon footprint in cement production makes it essential to use admixtures in production to substitute for clinker. It is an undeniable fact that grinding fineness rates will become finer as a result of the increase in the amount of admixtures. Energy efficiency will become even more important in cement grinding lines in the following 10- year period. Enabling efficient energy utilization, it is obvious that Pamir technology will play an important role in this process.
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