Avinash Mathur elaborates on operational improvements that can help manufacturers reduce the production cost and improve environment quality, leading to cement sustainability.
The cement industry is a ?post-mature? industry – an old industry where change is slow and marginal. In such an industry, the only way for any cement company to be the Number One is to be a cost leader. Thus, it is imperative to focus on the potential opportunities in operation improvement right from limestone mining to packing plant.
Mining
For a few cement manufacturers using surface miner in limestone mining, it is generally drilling and blasting which is used in mining limestone – a natural mineral resource and key raw material to produce clinker. Raw material quality is the challenge in clinker production. Kiln is the heart of the plant. Clinker is made from limestone. First pre-requisite of stable kiln operation is kiln feed homogeneity. Control of kiln feed homogeneity starts from the limestone quality at mines, less the variation in mines, less variation in plant?s kiln feed. High variation in limestone quality cannot be brought under control by raw mill weigh feeder and CF silo as these also have their own limitation to control.
In the national and global cement, major variation in limestone quality is controlled by PGNAA (Prompt Gamma Neutron Activation Analyzer) system. PGNAA helps reduce variation in material quality. Consistent stockpile and raw mix chemistry is the key to ensuring smooth kiln operation and flexible, controlled quarry operations. The PGNAA online elemental analyser monitors the elemental composition of limestone in real time to troubleshoot issues in pre-blending stockpile control/quarry management, raw mix proportioning control and material sorting. The result is precise control of the raw mix, chemical uniformity of kiln feed, increased kiln efficiency and ultimately superior performance of cement. Cement manufacturers who have so far not installed PGNAA on line elemental analyser, may thus focus on this opportunity to improve mining operations. Present limestone production is by drilling and blasting. It is not eco-friendly as underground water level goes down and rehabilitation of abandoned mines is a problem. Since limestone rock has now developed high fracture due to blasting, it is recommended that for sustainable limestone mining plant should use blast-free surface miner. Single machine can mine over 200 tonne per hr in rock UCS (Ultimate Compressive Strength) of 100 MPa. Under fractured rock of the plant mines at present, it could mine 400 tonne per hr.
In an Indonesian major dry process cement plant, where drilling and blasting is not allowed by government, surface miner is already working. Cement manufacturers may visit this plant. Surface miner is safer, less noisy, lower liability, eco-friendly, clean, and even cut up to 4.5 m wide and 1.1 m deep, usable without the need of primary crusher. There are no big chunks or large boulders. It produces manageable debris easily hauled away by trucks. Surface miner can also be used in collecting leftover limestone in abandoned mines, thus, increasing the life of existing mines. In this process, abandoned mines would automatically be get rehabilitated. Land so reclaimed can be converted into agricultural land by filling the overburden material. It can also be developed into artificial lake, fish pond, water reservoir etc. By selling the reclaimed land, plant may earn additional revenue and will recover the old cost of land purchasing. A comparison between surface miner and conventional mining is given in Table 1.
Thus, drilling and blasting is not the appropriate technique for limestone mining. Thus, there is a need to focus and switch over to blast-free surface miner which is highly cost-effective.
Cost benefits of surface miner
Considering limestone-to-clinker factor as 1.6, in a million tonne clinker plant, limestone requirement would be 1.6 million tonne. Cement manufacturers would require one surface miner of 400 tonne per hr costing $2.5 million (Rs 15 crore). Effective working is 5,000 hr per year, while production capacity is 2 million tonne and production cost $1.5 (Rs 100/t).
- Annual limestone cost = Rs 100 x 1.6 million t = Rs 160 million
- Considering conventional mining, limestone cost = Rs 200/tonne
- Cost saving through surface miner = Rs 100/tonne.
- Annual cost saving = Rs 16 crore.
- Pay back = less than a year.
Other benefits would be consistent limestone quality from mines for assuring stable kiln operation and elimination of the need of installation of cross belt PGNAA analyser in limestone production.
Rotary kilns
Kiln is the heart of a cement plant. Kiln operation targets are stable kiln operation, good clinker quality, higher clinker production and less CO2 emissions. Stable kiln operation is the key to long refractory life, high fuel efficiency and uniform quality clinker. For stable kiln operation, kiln feed must be homogenous.
Kiln dust in pre-heater exit gases is called dust loss which is collected in kiln ESP and recycled by mixing it with kiln feed from homogenising silo. Inevitably, the dust which is returned from the kiln ESP is of a different composition to that of the original feed, due to segregation of the finest, least dense material from the denser, coarser material.
The Pre-heater dust losses for kilns range in between 6 per cent and 9 per cent as it depends upon cyclone efficiency. As soon as dust losses are mixed with homogenising silo feed, it becomes heterogeneous. This dust quality and quantity returned to the kiln feed is the variable which is beyond the control of Central Control Room (CCR) operators. Homogenising factor ranges in between 10 and 30. In majority of the plants, homogenising silo is working as storage silo.
Kiln feed homogeneity ensures stable kiln operation. It should not contain high per cent of quartz grains coarser than 32 ?m and high per cent of calcite grains coarser than 90 ?m for ease of burning in the kilns. Standard deviation of CaO should be <0.2. As soon as kiln ESP dust rich in high alkali partially calcined and partially un-calcined with high concentration of fine particles is mixed with silo feed homogeneity ends and heterogeneity starts. To avoid this present location of mixing, kiln ESP dust needs to be changed. Cement manufacturers should focus on appropriately feeding the dust above kiln flame as shown in Figure 1.
Heat flux above the kiln is as good as below the kiln flame. Presently, this heat is absorbed by refractory lining, leading to increase in radiation loss through kiln shell and goes as a waste through kiln exit gases. Potential benefits which could be accrued would be direct solid-to-liquid reaction, clinker synthesis at lower temperature, fine crystal formation, higher hydraulic reactivity, saving in heat consumption and increase in clinker production.
Free lime in clinker is kept below one per cent, which needs to be increased in increment of 0.2 per cent to 2 per cent as at this free lime level expansion does not occur in mortar and concrete. Higher free lime will have appreciable positive influence in reduction of specific heat consumption (SHC), i.e., kcal/kg clinker.
Cement mill
During clinker grinding, the traditional mills working on compaction and impact as grinding force, generates electrostatic charges over the surface of cement particles. Reverse polarity electrostatic charges attract each other whereas similar polarity electro-static charges repel each other. Finer particles are agglomerated into coarser particles and are rejected by a classifier back to mill, causing over-grinding of particles and leading to increase in specific power consumption and decrease in tonne per hour.
These electrostatic charges need to be neutralised. Current industry practice is to use grinding aid at the cement mill to partially neutralise electrostatic charges but its recurring expenditure as an additional cost is not compensated by highly competitive cement market. Grinding aid consumption in a million tonne cement plant costs over Rs 30 million.
Plant can solve this electrostatic problem upon installing an ioniser at the inlet of mill and classifier as given in Figure 2. It generates ions which neutralise the electrostatic surface charges on cement particles to zero volts. Ioniser keeps ground particles in dispersion, improves material flow through the mill, improves tonne per hour, reduces power consumption and improves cement quality. By installing ioniser, plant can produce high performance cement.
Cement packing
Weight variation allowed in 50 kg bag is ?1 per cent. If the cement manufacturer?s bags weigh more than one pre cent variation, the company suffers in revenue and if bags weigh less than one per cent variation, the company suffers in brand image. The focus should be on striking a balance in between. During the packing process, static electricity on the surface of the cement particles causes cement to adhere to the dispensing nozzles/spouts. This prevents the correct cement supply in bags, resulting into weight variation. Ioniser should be installed in each roto packer as shown in Figure 3.
Installation of ioniser in roto-packer may bring down weight variation in bags from ?1 per cent to ?0.3 per cent, leading to a saving of 7,000 tonne cement in million tonne cement plant. Unit cost of ioniser is $ 8,000 (Rs 5 lakh). Payback is less than a month. In addition to this, post packing, there would be improvement in early and late compressive strengths of cement due to zero volt surface energy on the dispersed cement particles. Upon focusing on the above opportunities in operational improvement, cement manufacturers may reduce the production cost and improve environment quality leading to cement sustainability.
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