Calculating a grinding circuits circulating loads based on Screen Analysis of its slurries. Compared to %Solids or Density basedCirculating load equations, amore precise method of determining grinding circuit tonnages uses the screen size distributions of the pulps instead of the dilution ratios. Pulp samples collected around the ball mill or rod mill and hydrocyclones, screenor classifier (classification system) are screened and the cumulative weight percentage retained is calculated for several mesh sizes to obtain the Ball Mill Circulating Load.
Although the functions of the classifying device and the mill in a grinding circuit are quite different, the performance of each is interrelated and should be viewed as a single unit operation. The performance of the mill is influenced by the quantity and quality of the circulating load being returned to the mill. The performance of the cyclone is influenced by the particle size distribution of the mill discharge. The net result of the unit operation is a reduction in size of the new feed tonnage from a prescribed top size to a desired product size.
It is the above interrelationship that governs both the capacity of the grinding circuit and the desired grind. The capacity of the grinding circuit is dictated by the number of tons of feed coarser than the desired product size that the mill is capable of grinding finer than the desired product size. A the mill produces a diminishing percentage of finished material as the new feed rate increases. To maintain the same product distribution, the coarser mill discharge requires a larger circulating load to return the coarse fraction back to the mill. Therefore, to increase circuit capacity or produce a finer grind, higher circulating loads are required.
As the circulating load increases, typically the cyclone underflow density increases, causing the density and viscosity in the mill to also increase. This can lead to excessive mill viscosity, causing the balls to float, leading to a sharp drop in the power draw. The operator can be misled to conclude that the mill is overloaded due to the higher circulating load. The operator will usually sharply reduce the feed rate, when actually an increase in dilution water at the feed end of the mill will lower the mill viscosity and allow operation at the higher circulating load.
As the circulating load increases to higher levels, it is also not uncommon for the pump or the cyclone to become the capacity bottleneck due to volume constraints. These volume constraints are not always readily apparent, leading to the assumption that the increased circulating load has limited the ball mill capacity. Therefore, changes to the pump and cyclone may be required to handle the higher circulating load caused by an increased new feed rate.
The circulating loads generated in a typical ball mill cyclone circuit contain a small fraction of bypassed fines. The concept that high circulating loads will result in overgrinding can be refuted by regarding increases in circulating load in the same vein as multistage grinding. That is, for every incremental increase in circulating load of 100%, an additional stage of grinding and classification occurs. With high circulating loads, the particle size reduction per pass is proportionately less and the generation of slimes considerably less than with low circulating loads or open circuit grinding. This is due to the lower residence time per pass.
In a properly designed circuit, each finely ground particle of the required product size has ample opportunity to exit the grinding circuit after each pass through the mill. As a result, few -10 micron fines are generated in the grinding circuit. The presence of these fines are likely attributed to the inherent fines in the new mill feed.
When it comes to achieving the best energy consumption, what are the key factors a cement producer needs to address? In this article, extracted from the newly published Cement Plant Environmental Handbook (Second Edition), Lawrie Evans presents a masterclass in understanding and optimising cement plant energy consumption. By Lawrie Evans, EmCem Ltd, UK.
As control of sources, generation, distribution and consumption of energy is central to many current world issues, controlling the industrys energy footprint is a matter of intense interest to governments. This is recognised in such initiatives as ISO 50001, the World Business Council for Sustainable Developments Cement Sustainability Initiative, Energy Star in the USA, PAT in India and CO2 taxes/trading in Europe and in other countries.
For the cement industry, there are three main drivers to energy consumption: electrical power fuel customer demand for high-strength products that require a significant proportion of high-energy clinker as a component.
For the producer, these factors have a significant influence on cost competitiveness, usually accounting for over 50 per cent of total production costs, so that accurately and continuously monitoring energy usage must be a way of life for any producers technical team. The introduction of CO2 taxes in Europe and elsewhere adds a further twist to the story. For major groups, especially, decisions made in balancing maintenance, investments, operations and purchasing requirements all have to take into account the impact on their energy footprint.
Globally a cement major such as Italcementi consumes annually some 6000GWh of power and 35,500,000Gcal of heat for a total of 5Mtpe. This is the same total energy as consumed by approximately 1.6m Italians or 0.6m Americans per year. For fuel-related energy costs, the worldwide industry has largely moved to efficient preheater/precalciner processes and has found many options to switch to cheaper fuels, with the global drive to alternative fuels still proceeding. For electrical energy, options to reduce unitary costs are much more limited in scope. Most countries still have power generation/distribution systems that are effective monopolies and the cement producers cost control capability is usually limited to selecting the appropriate contract and taking opportunities offered in lower-cost off-peak power tariffs, where they exist.
Figure 1 illustrates the wide variation in the cost of power across 14 countries. The average country cost of electrical power at an industrial level varies enormously. When the added complexity of on and off peak power costs, interruption clauses, supply charges versus energy charges, etc, are added, the evaluation of the benefits of energy saving investment can become very complex. Typical cement plant power costs can range from EUR39 to EUR170/MWh.
The most important first step in controlling energy consumption is to be aware of the relative importance of the process areas where most energy is consumed. Figure 2 shows a typical breakdown of electrical energy consumption at a cement plant. The most obvious area for attention is that of grinding, both raw and cement. In either case, grinding is, by design, a very inefficient process.
The ball mill has been the industrys workhorse for over a century and despite its estimated meagre four per cent efficiency, little has changed over the years other than increases in the wear resistance of mill internals and the scale of the equipment. The addition of closed circuiting and progressively higher efficiency separators has improved cement product quality and produced higher outputs for a given mill size, but the case for adding or upgrading separators on energy saving alone has proved to be poor, unless the products are >4000Blaine. Starting from the 1970s, a new generation of mills appeared. Vertical mills (see Figure 3) were common for solid fuel grinding, generally with spring-loaded rollers. The principle of the new generation of vertical mill was to direct higher pressure from the grinding element to the material bed using hydraulic systems. From this approach the roller press, CKP (pre-grind vertical rollers) and Horomill all developed.
The gas-swept vertical mill quickly became the raw mill of choice. Grinding energy was approximately 50 per cent of the ball mill and the drying capabilities allowed direct processing of materials of up to 20 per cent moisture content. The main energy issue was the high power consumption of mill fans, with pressure drops of 100mbar not uncommon with high nozzle ring velocities (>70m/s) and internal mill circulating loads of >1000 per cent. Manufacturers have countered this generally satisfactorily with pressure drops reduced by lower nozzle ring velocities and the addition of external spillage elevator recirculation systems plus higher-efficiency separators.
Better seal designs for mill roller assemblies and pull rods have reduced the inevitable inleaking air issue and its impact on power consumption. However, it remains a design where issues of wear and reliability are more challenging than for ball mills, and these issues have not diminished with increased scale. For raw grinding with relatively dry raw materials, the combination of the roller press and V separator is a viable alternative with far lower mill fan power.
For cement grinding, the technology development away from ball mills has taken a different route. The development of roller presses in the 1980s took advantage of the benefits of higher-pressure grinding and many presses were retrofitted to ball mills as pregrinders. The main benefit was seen at lower Blaines as the first generation of presses suffered from stability problems when attempts were made to grind more finely by recirculating separator rejects. These problems are now largely resolved and the combination of a V and third-generation dynamic classifier separators together with a roller press can produce finished cement with high energy efficiency.
The Horomill and CKP systems have also enjoyed some market success and have provided good energy efficiency levels compared to ball mills. The vertical mill option has been slower to enter the cement grinding market. Grinding bed stability problems offered a challenge which the major manufacturers battled with, until finally a significant number of mills began to be installed in the late 1990s, and this has multiplied in the past decade. However, in pure energy efficiency terms, the benefit of grinding power reduction is countered by the very high power required by mill fans. In addition, the absence of the heat generated in a ball mill and the high volume of air required by the vertical mill have required the provision of waste heat from cooler exhausts and/or auxiliary furnaces to dry raw materials and achieve a limited dehydration of gypsum.
A typical comparison of three competing technologies is given in Table 1, demonstrating that an efficient ball mill/third-generation separator, CKP/ball mill/third-generation separator and vertical mill on a typical 4000Blaine limestone cement show little overall difference in energy consumption. Considering the higher capital cost, and more demanding maintenance and operating regime, there is no clear energy case to favour some of the modern variants.
Even for solid fuel grinding, there has been a minor trend back to ball mills. This is most evident for petcoke grinding, where the demand for very low residues, and the very hard and sometimes abrasive nature of high-sulphur cokes has resulted in ball mill selection.
Many of the grinding design issues, which are still under debate, are usually very clear in other areas of process selection: high-efficiency process fans and low-pressure drop preheaters adequately-sized bag filters for the main exhaust to avoid high pressure drops and poor bag life avoidance of pneumatic transport systems low-energy raw meal homogenisation silos.
The main continued discussions are those of two- or three-fan systems for the raw mill/kiln or single filter for kiln and cooler, precipitator or bag filter for the cooler exhaust and two or three tyre kiln. For a bag filter on a separate cooler the main equipment energy efficiency issue is the air-to-air heat exchanger, but this is often substituted with a water spray in the cooler or more recently, by using a ceramic filter capable of operating at above 400C.
Finally, in design terms, the most difficult decision is to avoid overdesign by applying too many safety factors. Post-commissioning audits often uncover a high contribution to poor energy efficiency from under-run equipment operating where it cannot perform efficiently.
In normal operations maintenance also plays a major part in ensuring energy efficiency. The impact of poor plant reliability upon overall electrical energy consumption is often under-estimated. In the kiln area, 100 short/medium stops (30 minutes to eight hours) per year can cost up to 5kWh/t clinker. The avoidance of inleaking air, correct alignment of motors, stopping compressed air leaks, etc are all part of the value of good maintenance.
In the key area of grinding there are important factors to control. For ball mills, ball charge level, lining and diaphragm condition must be monitored and maintained in near-optimum condition. Mill stops, defined as mill motor off, and measured by mean time between failures (mtbf), are frequently poorly recorded and the resolution of underlying issues is frequently not addressed.
Instability, where ball mill feed is stopped and the mill ground out, is also infrequently recorded or acted upon. When it comes to mill control, operators rarely concentrate on pushing mill production when the kiln is regarded as the key. Expert systems on mills should be universal and well tuned.
Grinding aids can give benefits of 5-15 per cent in production but need to be continuously evaluated for cost effectiveness. Unfortunately, their cost has risen more rapidly than the cost of energy in recent years and the economic balance has to be re-evaluated. The benefit of aids on cement flowability has to be considered, along with the added scope for reduction of cement clinker content with some modern additives. Correct timing on the maintenance of a first chamber cement mill lining and the successful implementation of an expert system on a cement mill both offer benefits in terms of power consumption (see case studies panel). Accurate process measurements are also key to energy saving opportunities. Air compressors are another area for attention. Often, these are multiple units operating on a cycle of on- and off-load. Replacement of one (of three) with a variable-speed type (see Table 2) can provide rapid payback. Even lighting and buildings offer excellent opportunities for power savings. Table 3 shows the 40-80 per cent energy savings that can be achieved by simply replacing old lighting systems. Buildings such as the new Italcementi Group Research and Innovation Centre (i.lab) in Bergamo, Italy, demonstrate that good building design creates significant savings.
A major change has occurred in the last 20 years in the area of in-house generation of electrical energy by cement manufacturers, most significantly using waste heat recovery (WHR) from the pyroprocessing line. Figure 4 shows the areas suited to heat recovery for power generation, and WHR technology is already applied to preheater and cooler exhausts.
The modern technology originated in Japan in the 1980s, where high power prices and large-scale operations combined to produce useful economic returns, with most applications using steam boilers at the preheater exhaust. Little further development happened outside Japan until the turn of the century, when a combination of lower capital cost, Chinese equipment, and the idea to improve recovery by splitting cooler exhausts into higher and lower temperature streams combined to offer the paybacks necessary for the technology to expand, first inside China and then beyond.
The results of WHR have been impressive, eg, with the 19MW net achieved from a combined installation on two five-stage precalciner kilns (5500tpd and 7500tpd) in Thailand being typical. Options for the technology are evolving with other thermodynamic cycles being applied: steam Rankine cycle with various enhancements the most widely applied technology organic Rankine cycle a variety of organic fluids applied and favoured at lower gas temperatures Kalina (ammonia/water) cycle supercritical CO2 cycle.
There are also further developments which can increase the power recovered, including recycling the lower temperature cooler exhaust, meal bypassing preheater stages to boost exit temperatures and the use of alternative fuels and excess air, also to boost preheater exit temperature and energy recovery. Other options for power generation can use the land owned by the cement plant for raw material reserves. These include wind farms photovoltaics, concentrated solar panels or growing and burning biomass either to boost power in a WHR system or for use in an internal, stand-alone power generation plant.
Here is a formula that allows you to calculate thecirculating load ratio around a ball mill and hydrocylone as part of a grinding circuit. For example your ball mill is in closed circuit with a set of cyclones. The grinding mill receives crushed ore feed. The pulp densities around your cyclone are sampled and known over an 8-hour shift, allowing to calculate corresponding to circulating load ratios and circulating load tonnage on tons/day or tons/hour.
If a product all finer than a certain critical size is required, the capacity of the ball mill is increased considerably by using it in closed circuit with a classifier and this increase is made still greater by increasing the circulating load in between the ball mill and the classifier (Fig. 70). In practice, circulating loads between ball mills and classifiers are rarely less than 200 per cent and frequently exceed 700 per cent. It may appear strange at first sight that the circulating load between amill and classifier producing, for example, 100 tons of finished product per day can be as much as 400 tons or more, but this will be made clear by Fig. 71. The trend to the use of higher and higher circulating loads in ball-mill-classifier circuits has resulted in certain mechanical requirements in ball mills, classifiers, and auxiliary equipments.
Presently we use coal from China and South Africa, maintaining volatile matter in fine coal below 32 per cent. We use cooler exhaust gas (ambient) for coal mill drying. We want to use coal from Indonesia having about 42 per cent volatile matter. Is it safe to grind in the coal mill having hot gases from cooler? We have CO2 inertisation system in coal mill circuit. What precautions are required to be taken? How much percentage of high VM coal can be used? How long we can store this type of coal in the yard?
There are well established guidelines for the safety operation of coal grinding equipment. These fall into two broad categories: (i) explosion prevention, and (ii) explosion protection. For prevention the best solution is to ensure that there is less than 12 per cent oxygen in the atmosphere. You cannot do that when cooler exhaust air is used for coal drying. As a general guideline drying with cooler exhaust air is suitable with direct firing systems. Other prevention measures are to ensure there are no fine coal dust accumulations in the system and no possible sources of ignition. These are determined by the design of the coal milling and storage system. With your non-inert coal grinding system you will have to have rigourous explosion protection designed into the equipment. The mill, raw coal feeder and ductwork must be capable to withstand a pressure of 9 bar. The dust filter, any cyclones and fine coal hoppers, must be fitted with explosion relief doors or rip-foil sides. Ducts entering these containers must be have explosion relief ahead of the container is they are longer than 5x their diameter. You can see that there is no simple answer to your question. A full inspection and audit would be required before anyone could certify that it is safe to grind the Indonesian coal in your system. I would recommend modifying the system to dry the coal with inert preheater exhaust gases. Regarding storage it is not desireable to store high volatile coal in large stockpiles. However, you will have to import large shipments to make it economic. The stockpiles need to be compacted to minimise air ingress into the piles. You should also limit the height of such piles. With 42 per cent volatiles the height of the piles should be limited to 4m.
We are having a 1.5Mt six-stage Precalciner kiln operating with South African coal having a VM of 28 per cent. Now we thinking to switch over to Indonesian coal with high VM content (up to 42 per cent) and Chinese coal (VM up to 32 per cent). The coal mill is a VRM having hot gases from cooler. Up to what maximum VM we can go without having explosion problems. With high VM coal what are the other changes to be carried out to regarding coal residue so that we do not get problems in the flame shape.
There are well established guidelines for the safety operation of coal grinding equipment. These fall into two broad categories: (i) explosion prevention, and (ii) explosion protection. For prevention the best solution is to ensure that there is less than 12 per cent oxygen in the atmosphere. You cannot do that when cooler exhaust air is used for coal drying. As a general guideline drying with cooler exhaust air is suitable with direct firing systems. Other prevention measures are to ensure there are no fine coal dust accumulations in the system and no possible sources of ignition. These are determined by the design of the coal milling and storage system.
India is the world's second largest producer of cement and produces more than 8 per cent of global capacity. Due to the rapidly growing demand in various sectors such as defense, housing, commercial and industrial construction, government initiative such as smart cities & PMAY, cement production in India is expected to touch 550600 million tones per annum (MTPA) by the end of year 2025.
With recent growth and success journey there is also a threat approaching to cement industry that its input cost is increasing. Power cost, fuel cost, raw material cost have doubled up in recent years whereas cement price has not hiked in that fashion. Also 40% of the cement production costs are energy costs out of which more than 60% of the total electricity is used in grinding circuits. In order to survive and sustain in the market they need to increase their profitability which can only be achieved by increasing productivity and reducing power consumption. High productivity and low power costs can be achieved by increasing output, lowering breakdowns and optimizing the energy consuming grinding process.
The objective of the study, is to draw attention to the need of Cement grinding process optimization to minimize power consumption and achieve higher productivity. In the study the advantages of vertical roller mill are discussed over ball mills. VRM construction, its process and parameters which affects the performance and productivity of vertical roller mill are discussed. Also the consequences of variations in parameter explained. With proper optimization of these parameters, the productivity of vertical roller mill can be improved and performance stability can be achieved by addressing root causes.. This study can benefit the organizations using VRM and are not able to utilize its full productivity due to some bottlenecks or constraints.
Vertical Roller Mill (VRM) is the most advanced technology in cement production procedure. Bashundhara Cement has adopted VRM from LOESCHE, Germany in both the factories to ensure the best quality. With higher fineness achieved with VRM, it creates stronger and better quality concrete and thus, proves its superiority over other cement brand.
The VRM is a vertically placed large sized machine which is equipped with some rollers used for grinding raw material. It also consists of one table which is rotated by electrical motor & raw materials are feeded on the table. Due to centrifugal force on the table, raw material goes below the rollers (a gap between roller & table has to be maintain) automatically & start getting grinded. The rollers are driven by the friction between table & raw materials. The grinding process is done by exposing a bed of material to a pressure sufficiently high created by rollers which causes fracture on the individual particles in the bed. Due to such grinding, fine particles pass through the separator as the final product to the silo due to huge negative pressure in the mill & course particles falling down (separated by highly efficient separator)on the table for further grinding.
A. The grinding process of Ball mill & Vertical Roller Mill differs fundamentally. The sufficiently high enough to cause fracture of individual particles in the bed & most of the particles on the bed are considerable, smaller than the bed thickness. On the other hand, in a Ball mill, the comminution takes place by impact & attrition.
B. A suitable grinding bed is easily obtained in vertical roller mill with high efficiency separator which ensures high fineness of product. However, in the Ball mill, it is more difficult to get suitable grinding bed.
D. Particle size distribution (PSD) of the finished product obtained from VRM is much better than Ball mill product. High fineness of material ensures more dense concrete with less possibility of chemical attack.