With the intensification of the energy crisis, coal prices have risen sharply. At the current rate of coal mining, coal reserves are at risk of being exhausted in 50 years. This is especially true for coking coal, which is rare compared to other types of coal. Increasing the number of grades of coal mixtures and preserving non-renewable resources is what should be done for the sake of future generations. At the same time, the extent to which coke enterprises have succeeded in the areas of beneficial use of their by-products, cost reduction, efficiency improvement, energy savings and emission reduction is an indicator of the technical level and strength of the enterprise, and is also an important factor that increases its competitiveness.
The technology of mixing coke dust with coal implies the addition of a by-product produced by the enterprise – coke dust – into coal, followed by its coking (sintering). Selling coke dust (having a high calorific value) at the price of conventional fuel is not only a waste of energy resources, but also leads to environmental pollution. A lot of enterprises sell huge amounts of coke dust produced by them in this way. But coke dust can bring great benefits as part of the technology for mixing coke dust with coal.
Although blending coke dust with coal is not a big enterprise scale project, but today, when attention is paid to every detail, this technology can help reduce costs, responding to environmental and energy-saving trends. At the same time, this project does not require large investments, but significantly increases profits. It cannot be said that the technology of adding coke dust to coal is a very good investment project. And the sooner it is implemented, the sooner the benefits will be received.
Economic benefits of technology for mixing coke dust with coal
Coke dust is a material produced and used by the enterprise that does not require extraction and transportation. In addition, it is not subject to compression during sintering. Adding 1 ton of coke dust increases the amount of coke produced by 1 ton, saving 1.3 tons of coal. Therefore, the economic value of this technology greatly exceeds the economic value of mixing anthracite coal and any kind of lean coal with coal.
The technology of adding coke dust to coal can increase its cost by $45-72 per ton, and also reduce the cost of finished coke to $1.5-2 per ton. Let’s take as an example an enterprise that produces 1 million tons of coke per year, does not add a binder, does not change the basic properties of the mixed coals that form the basis of the coal mixture.
With the addition of 3% coke dust relative to the total amount of coal mixture, the following can be seen:
- With the addition of 40 thousand tons, more than 50 thousand tons of coal are saved.
- The net profit without selling price and processing cost is $1.5-2 million .
- The cost of each ton of finished coke is $1.5-2 less.
With good basic properties of mixed coals, high stickiness, the use of a pressing coke oven or the addition of a binder allows you to increase the proportions of mixed materials, thereby increasing profits.
The cost of processing 1 ton of coke dust, which is $6-7, includes
Energy costs: This refers to the electricity used by the equipment’s engines, 20-25 kWh per ton.
Spare parts and accessories: These are mainly the costs associated with the wear of parts of the dryer, conveyor equipment, as well as the wear of the grinding bodies of the mill.
Wages for workers: Two people per shift.
Investment depreciation costs: Depreciation charge for 20 years.
Administrative costs: Determined according to the situation of the individual enterprise.
Other costs: Determined according to the situation of the individual enterprise.
Technical feasibility and technological requirements
At present, there are more than ten coking enterprises in China that have already begun to apply the technology of adding coke dust, which confirms its technical feasibility.
Coke dust in the coking process itself is an inert substance. Due to the high porosity and large specific surface area, coke dust has a large area of contact with the liquid material of active particles, the connection between which is determined by the adsorption of solid particles in the liquid phase. On the one hand, the coke dust reduces the shrinkage of the char and the amount of volatilization during the solidification stage, reducing material shrinkage in two stages. On the other hand, due to its porous structure, it can reduce the stress generated by the compression of the coke cake and reduce the porosity of the coke. Both of these properties increase the indices M40 and M25 of coke lumpiness and strength by 1-2%. Therefore, in the past, coke dust was often used as a strengthening and emaciating substance.
Average requirements for the technology of mixing coal with coke dust: the size of coke dust should not exceed 0.5 mm, it is best if particles with a size of less than 0.2 mm occupy 80%. The ratio of the added product is usually chosen based on calculations of less than 3% of the total mass of coal.
The use of this technology does not adversely affect the thermal stability of coke. On the one hand, the amount of added dust is not large, on the other hand, coke dust can act as a reinforcing component and reduce material cracking. Therefore, this technology not only does not affect the thermal stability of coke, but even slightly improves it, and also fully meets the requirements for thermal stability of industrial blast furnaces.
It is necessary to carefully monitor and control the size and ratio of the added coke dust. When changing the basic properties of mixed coals, it is necessary to make appropriate changes in the proportions of added coke dust
Research process of coal and coke dust mixing technology
In 2000, in accordance with the requirements of users, Gongbei began to select equipment for coke dust-coal mixing technology and study its technical feasibility. First of all, attention was paid to the proportions of the mixed materials, their particle sizes and their influence on the quality of the coke. Thus, based on the results of mixing coal and coke dust provided by users, as well as the results of tests carried out by coking enterprises, a proof of the technical feasibility of this technology was obtained.
At the second stage, the selection of suitable equipment was carried out and tests were carried out directly at the place of operation. For three years, tests were carried out with equipment such as ball mill, Raymond spring roller mill, roller mill, jet mill with classifier, hammer crusher, vibrating mill, etc. Tests were also carried out on vibrating screens, drum screens, screens with active deck flip-flops, air classifiers and other classification equipment. In addition, attempts were made to classify coke dust particles using a complex of fine grinding mills. And also, in order to identify the influence of humidity on the process, measures were taken to control humidity as part of the experiment. As a result, the data obtained as a result of the tests showed that
In 2003, this technology with vibrating mills was launched at three coking plants in China. All actual operating parameters have reached design targets.
In 2007, in the process of three years of use, the reliability, economy, environmental friendliness and serviceability of not only vibrating mill, but also the technology as a whole were proven, and a state patent was obtained and positive feedback from users. A scientific approach and due diligence over 7 years have made it possible to form a reliable, economical, environmentally friendly, modern and complete technology that brings both economic and social benefits.
System of coke dust and coal mixing
This technology consists of three parts:
- Drying
- Grinding
- Mixing
Feed material → Drying → Grinding → Mixing
I. Drying
The drying unit is mainly composed of raw material hopper, belt feeder, drum dryer, combustion chamber, dust removal device, etc.
1. Why drying
Due to the influence caused by the wet method of coke quenching and the rainy season, the moisture content of coke dust can reach 15-20%, which leads to its sticking, the formation of agglomerates, and as a result adversely affects the subsequent grinding process for the following reasons:
- Blockage: It is difficult to unload the material from the mill, productivity is reduced, the normal operation of the ball mill, air classifier and other equipment is disturbed.
- Adhesion: Adhesion of coke dust to the surface of balls, steel bars and other parts. Reduced equipment performance.
- Clumping: Clumping leads to a decrease in the efficiency of grinding large particles knocked into agglomerates, which entails an excess of the content of large particles in the finished product.
Reducing the moisture in the source material contributes to an increase in productivity by 2–3 times. This point of view has been confirmed in practice in many production areas.
2. Influence of moisture of raw materials on the technological process
When the humidity level of coke dust reaches 15-20%, almost all grinding equipment such as ball mill, Raymond spring roller mill, roller mill and air classifier cannot work normally. At a humidity of 10-15%, the above equipment is also not able to work properly. At the same time, the Bayte mill can continue to operate with an efficiency limit of 50% of the dry grinding efficiency. Grinding efficiency at a moisture content of 5-8% is 70-80% of the efficiency of dry grinding, while other equipment has only 50% of dry grinding. Grinding at less than 3% moisture is similar to dry grinding and does not affect the subsequent grinding process.
3. Dust extractor
Usually, when the dryer is released, it is equipped with a cyclone dust collector and a combustion chamber. It also happens that in order to meet environmental requirements, a dust collector with bag filters or a water dust collector (scrubber) is installed in the circuit. Such dust extractors simultaneously perform both the function of removing dust and the function of air suction, sucking hot air from the combustion chamber into the dryer.
The dust extractor is installed at the end of the dryer near the discharge opening. Due to the large particle size of the raw material, its dust content (-200 mesh) is not high, so the dust removal efficiency is also not high. In connection with the above, it makes sense to consider adding a dust removal function to finished large production lines.
4. System configuration
Most often, the following models of dryers with the corresponding performance indicators are selected. The user can choose similar equipment from other manufacturers.
5. Heat utilization
It is usually believed that after the drying process, the moisture content of coke dust decreases from 15-20% to 3-5%. The required amount of heat per ton is 160,000 – 200,000 kcal.
Heat consumption depends not only on the amount of moisture contained in the material, but also on the form of moisture content inside the material, as well as on the ratio of the amount of moisture contained on the surface and inside the material.
II. Grinding
The grinding stage is the main part of the whole technology, which determines its general characteristics. The choice of grinding equipment is a key point on which the success of the technology implementation depends. To make the right choice, it is necessary to compare the performance characteristics of the various types of mills with the characteristics of the materials to be ground.
1. Selecting the amount of equipment
Usually 2 to 4 mills are needed according to the requirements of the system. You should not choose to use a mill with a high specific productivity for the following reasons:
- In the production process for grinding technology, spare equipment is usually provided. As an example, crushers in a coal supply system, where one device is working, and the other is spare; as well as mills at power plants (from 4 to 10 units). This is mainly due to the presence of wear parts in such equipment, such as: hammers, steel balls, steel rods, etc., which are in direct contact with the material and cannot be stopped during replacement.
- Due to the tense situation with coal deposits, the quality and types of coal mixtures at coke plants often change, which leads to fluctuations in the proportion of added coke dust. Sometimes there are significant fluctuations from 1% to 3%, with a change in the proportion of added dust by a factor of 2. Therefore, installing more equipment increases the efficiency of the entire system. The more equipment, the smoother its operation.
2. Grinding stage
The grinding stage includes vibratory mill, belt feeder (or bucket elevator and screw distributor), vibrating feeder, buffer hopper and discharge screw conveyor.
The buffer hopper contains a certain amount of dry material, which ensures an uninterrupted and uniform supply of material to the mill. The screw conveyor collects the material and sends it to the mixing stage.
The feed and discharge openings of the mill are closed with a soft joint, which provides effective protection against dust leakage.
III. Mixing stage
The mixing stage is mainly represented by the finished product hopper, bucket elevator, hopper cover dust removal device, weighing belt, humidifier and mixer.
The mixing stage works in sync with the belt feeders. The standard capacity of the finished product hopper is usually set up to hold the stock of material required for a day’s work.
The weigh conveyor is hermetically sealed to prevent dust leakage.
If it is necessary to install a humidifier or mixer, the place of their installation is selected based on the requirements of the user and the place where the materials are mixed. Their installation is optional and is done only to reduce the amount of flying dust in the system.
The dust removal device on the lid of the hopper is necessary to remove dust and prevent leakage during material loading into the hopper.
The bucket elevator is completely sealed. To control the conformity of the final product, a sampling hole can be provided in the place of the articulating chute between the bucket elevator and the screw feeder.
Reliability of Vibrating Mill
In the 1980s, the Research Institute of Mining Machinery in Shijiazhuang, within the framework of the project task of the State Fund of the PRC, introduced German technologies and already in 1991 received a state patent, and in 1993 was awarded the national award for scientific and technological progress. The technical level of the medium amplitude vibrating mill series has reached the international advanced level. After 2000, having achieved the above-mentioned results, Gongbei Co., Ltd. has developed a special vibrating coke dust grinding machine, taking into account all the features of coke dust grinding. Patented technologies solve the problem of wear resistance of linings: amplitude, springs, synchronization system and working environment are all patented designs designed according to the characteristics of coke.
1. Vibrator reliability
The vibrator uses patented vibratory bearings and a water cooling system using honeycomb blades, which can reduce the amount of heat generated by the shafts by 30%, and also increase the heat transfer efficiency in the bearing chamber by more than three times, which ensures the smooth operation of bearings in lower temperature conditions and increases the service life of the shafts up to 30,000 hours.
2. Improved durable construction
Through a comprehensive observation of the vibration force, its amplitude and direction, stress at individual points of the entire structure, as well as using finite element analysis and elasticity calculation, a different understanding of the overall strength of the entire body and the strength of its individual parts was determined. Optimal design covering all these aspects results in the highest fatigue strength with the least increase in vibration.
3. Patented fastening mechanism
Mounting the lining on a vibrating body has previously been a great challenge. Now this patented mechanism has not only solved the problem of loose bolts, but has even doubled the life of the lining.
4. Noise reduction and shock absorption
Installing a cushioning system between the lining and the mill drum ensures continuous and uniform force transmission to the material medium and reduces the impact force transmitted from the material to the equipment, which allows to increase the service life of the equipment up to three times. Also, this system is able to reduce the noise level.
5. Improved equipment cushioning system
The operating range of the vibrator and the damping system, consisting of a housing and springs, are a single integral system, the task of which is to ensure the operation of the equipment within its elasticity range. The operating frequency of this system must not be in resonance with the operating frequency of the equipment itself. Thus, it is possible to achieve a guarantee of the service life and reliability of the entire equipment.
6. Quality rubber shock absorbers
Shock absorbers made of imported high quality rubber showed higher fatigue life and reliability than steel springs.
Thanks to the reliability of the above technologies, the vibrating mill has a high operational reliability in industrial environments. In the production practice of foreign countries, the service life of this equipment is more than 20 years, which confirms its reliability on a real example of user operation.
Economic efficiency of coke dust and coal mixing technology
1. Energy saving
When grinding one ton of coke dust, the energy consumption of the vibrating mill is 13-15 kW/h, which is 60% more economical than a ball mill. For more detailed information, please refer to the chapter “Comparison of coke dust grinding performance of Vibratory Mill and Ball Mill”
2. Low lining consumption
The main grinding method in the vibrating mill is impact. Thus, the abrasive wear of linings and other wear parts of this mill is 2–3 times less than that of other grinding equipment. The greater the abrasiveness of the raw material, the more clearly the superiority of this equipment can be seen.
The high utilization rate of the metal rods means that you don’t have to worry about them breaking or leaving the mill with the material like worn balls. Therefore, the rods can be used until their maximum thinning.
The rods used are made of available material, which allows them to be ordered without reference to a specific manufacturer.
3. Low unit cost of grinding material
The cost of processing a ton of coke dust is calculated on the basis of all equipment costs incurred during the entire production process; these are mainly costs associated with the consumption of electricity, labor, consumables, equipment maintenance and depreciation of investments.
For the technology of mixing coke dust with coal, these costs are lower by an average of $0.7-3 per ton.
Environmental friendliness of technology
Leakage of coke dust during its production can lead to serious environmental pollution. Therefore, the hermetic isolation of the production cycle and the absence of dust extractors is both an environmentally friendly and cost-effective solution.
The vibrating mill provides complete airtight sealing at the junctions of the feed and discharge ports with the conveyor equipment, preventing dust from escaping.
This technology requires the installation of dust extractors only at the end of the dryer. Since the particle size of the processed material in this place is quite large, dust removal in these places is not difficult.
Due to the small overall dimensions of the vibrating mill, it is possible to install soundproof walls or other soundproof structures.
The vibrating mill uses water for cooling in a closed circuit. Thus, there is no pollution of the environment and water reserves are not wasted.
Ease of maintenance of vibrating mill
The maximum weight of the smallest individual element is quite small. When changing material environmental conditions, one person is enough to open the end cap and add #45 steel rods. This task can be completed by hand in half an hour.
To replace the lining, it is necessary to open the mill drum flange, remove the worn lining, install a new one and tighten the fixing bolts. This task can be completed by two people within two hours. Wear plates can be turned over after wear to increase their service life and save material.
There are no parts in the vibratin mill that require laborious replacement.
Control system
The conveyor belt of the mixing system must be controlled together with the coal supply conveyor. Other equipment can be manually controlled locally, semi-automatically or centrally via a PLC.
Comparison of Coke Dust Processing Performance of Vibrating Mill and Ball Mill
1. Choice of grinding equipment
To obtain material with a particle size of less than 0.5 mm, it is necessary to select suitable equipment, since such a particle size is a challenge for conventional equipment. The main selection criteria for grinding equipment are as follows:
- The physical properties of materials, namely strength, hardness, brittleness, toughness, moisture content, density and abrasiveness.
- Size requirements
- Mill feed size
- Working conditions
- Production conditions
- Performance and energy consumption
- Auxiliary equipment and construction costs
The choice should be made on the basis of taking into account all the above criteria.
2. Working principle and features of vibrating mill
The main grinding method of this mill is high-frequency vibration impacts. Due to the small working volume and light weight, the equipment can carry out highly efficient material grinding. The high material filling ratio (70-90%) and the high vibration frequency (1000 times per minute) contribute to the highly efficient use of the working chamber. Thus, during vibration, the material is in a suspended resonant state and the particles of the material are crushed due to splitting, that is, tearing along the place where the strength of the particles is the lowest. Therefore, energy saving, high efficiency and compactness are the main advantages of this mill. In addition, due to its small size, easy maintenance,
3. Comparison of vibrating mill and ball mill
1. Energy consumption
The vibrating mill is more than 50% more economical than the ball mill. Compared with the coke dust production line, when producing 120,000 tons of coke dust, the energy saving is about $300,000. The reasons for such a high energy consumption of the ball mill are as follows:
- The efficiency of the transmission system including motor + reducer + gear train is only 90%. That is, 10% disappears during heating and friction. And the vibrating mill uses a direct transmission and as a result the energy loss is much lower.
- The self weight of the ball mill can reach 60-90 tons. For its normal operation, it is required to ensure the normal operation of the entire “weight” of the mill. With all this, the effective volume of its working chamber is only 30 – 50%, and the effective volume of the working chamber of the vibratin mill is 95%. The degree of filling with grinding media for a ball mill corresponds to 30 – 40%, and for a vibrating mill – 80 – 90%. This is the main factor influencing the differences in energy consumption.
- The ball mill is used to produce relatively fine material. Since the impacts of the balls are point and absolutely random, this leads to a large spread in the final size of the material. With coarse grinding, there is a serious overgrinding, which leads to large losses of material. In the vibrating mill, the rod impacts are linear and more structured. The frequency and intensity of impacts received by large particles is many times greater than the frequency and intensity of impacts received by small particles, so the dispersion of material size is minimal. All this leads to energy savings. When grinding coke dust to 0.2 mm, the ball mill consumes about 32-35 kW/t, and the vibrating mill from 13 to 17 kW/t.
2. Power supply location
The ball mill has a high-voltage motor, 380V and 10kV (6kV) work simultaneously in the system, which increases the requirements for the safety of power distribution in the system.
3. Comparison of grinding media consumption
The grinding media in a vibrating mill last as long as possible, but in a ball mill, the grinding media need to be replaced as they wear out. Thus, the steel consumption of the vibrating mill is 30% less than that of the ball mill.
4. Equipment lock
To prevent loss of grinding balls due to their abrasion, the ball mill is unloaded through the grate. Due to the instability of the moisture content of the coke dust and the high surface roughness, blocking of the grate openings and difficulty in unloading the material can occur. The vibrating mill does not have this problem.
5. Installation and maintenance
Due to the heavy weight of the individual parts of the ball mill, sufficient space and bulky equipment are required to lift them. This entails high costs and difficulties associated with the replacement of shafts, linings, metal balls. In this regard, the vibrating mill is easier to use.
6. Adaptability to the material being processed
The final product size and productivity of the vibrating mill can be adjusted by changing the frequency and amplitude of the vibrations. Thus, it is possible to grind both anthracite coals and coke dust with one type of equipment. Since the ball mill is not adjustable, it cannot be adapted for grinding anthracite.
7. The discharge port of the vibrating mill is completely sealed, which also enables vacuum and nitrogen grinding without dust leakage. The ball mill may leak dust, which requires the installation of dust removal devices.
Disadvantages of other technical solutions
1. Air separation
Due to the high porosity of the surface of coke dust, the buoyancy of large and small particles in air is not proportional to the particle size, which leads to mixing of large and small particles during air classification. This causes the failure of the grinding equipment with the air classification function.
2. Roller mill
All double-roll crushing and grinding equipment is based on the opposite rotation of two shafts. The space between two rolls is a key factor in determining the efficiency of grinding. The high abrasiveness of coke dust, even with constant adjustment of the space between the rolls, contributes to an increase in the space between the rolls in a very short period of time. Due to the impossibility of ensuring uniform grinding of the material, cracks appear on the shaft shell, which causes difficulties in adjustment. For this reason, achieving a fineness of 0.2 mm becomes almost impossible.
In addition, coke dust has some special properties, it is easily compressed, which does not favorably affect the uniformity of its subsequent mixing.
3. Hammer crushing
The high linear velocity of the hammers (40-60 m/s) can lead to their rapid abrasion due to the high abrasiveness of the coke dust, which is unprofitable from an economic point of view and makes it difficult to ensure the particle size. It is fundamentally impossible to use a crushing operation to perform grinding tasks.
4. Screening
Classifying 3mm particles is a challenge in the industry, and classifying 0.2mm particles is even more difficult. The main problem is blocking the sieve holes. The use of technology for mixing coke with coal is not possible when adding a large amount of water, as in wet classification.
5. Raymond Mill
- High wear rate and the need for a lot of maintenance.
The high abrasiveness of coke dust contributes to the rapid wear of the mill shafts, the shafts are seriously damaged within ten days, and after a month it becomes necessary to replace them. The service life of the mill rings is only 30 – 50 days. The piping system also wears out quickly under constant exposure to airborne coke dust particles. - Unstable performance.
Performance can drop by 30 to 50% over ten days, and over the next thirty days, performance is only 25% of new equipment, so it is quite difficult to meet performance stability requirements.
The size of the material also becomes unstable due to the rapid wear of the parts.
6. Ball mill
- Requires large space for installation.
- Low efficiency, high power consumption, exceeding the consumption of other equipment by 2-3 times.
- Cumbersome, difficult to maintain.
- Low environmental friendliness, leakage of supply and discharge openings. High demands on dust extractors. High operating costs.
Other Opportunities and Potentials of Coke Dust-Coal Blending Technology
1. Possibility of providing an individual project.
This technology was developed with the expectation of being used by a large number of coking enterprises, so it is quite universal. If your company has any peculiarities related to the operation of the coke oven or production features related to any other aspects, we are ready to provide you with a technical solution developed in accordance with the individual requirements of your company.
2. Ability to work with anthracite coal
The described technology uses a crusher operating in a closed circuit to produce anthracite coal with a particle size of 1 mm at a time. This equipment has a special cleaning mechanism and can crush wet anthracite coal without regrinding it, ensuring that it meets the final product fineness requirements.
3. Mixing with coke residue
Coke oil residue, being a waste product of coke production, and having a high viscosity and ability to foul, presents many problems associated with its storage and transportation. Oily residue can be added to processed coke dust, coke dust that has undergone dry coke quenching, or coal that has undergone a blending stage, thus avoiding fouling of systems caused by its stickiness, as well as making full use of production waste. If your enterprise has a need for such a solution, we are ready to offer you the development of an optimal technical solution in this area.
