In recently years, the breakthrough of China dense medium research has stimulated quickly development of coal preparation technology .The intrinsic change and rush improvement of coal preparation technology thanks to the popularizing of various dense medium technology and steel structure module layout. The established project has the traits of environmental protection, beauty and durable which breaking through the traditional view of dirty and chaos. But there are also some points waiting to improve:

1.    The files of optional pilots are not complete and are lack of representative.

On the basis of the lack of data representative of the formulation and screening coal preparation technology, is designed to mislead, the consequences will be serious.

So a new award article clause 5.1.6(boldface mandatory provisions)  explicitly requests “ the feasibility study and preliminary design must review to the representative of screening and float & sink test data. When screening and sink experiment data representation is insufficient, should be adjusted according to the provisions, make it close to actual production". This is one of the main changes of the new code and old code . This requires must be combined with the mining condition and the Honi exploit technology to make reasonable adjustments to the loan data, then increasing the representation of data.

At present, China temporarily have not unified standard to adjusting data. Only be adjusted in accordance with the provisions of clause 5.1.6 principles. on the adjustment of design specifications  is according to : 1).Adjacent coal mines, coal preparation plant according to the actual production situation 2).According to the coal geological report, coal mining method, transportation etc. to adjust. When adjust the data, needing to pay special attention to the following aspects:

（1）    To make reasonable predictions for the production of coal ash;

（2） To point to small size kiln raw coal screening data composition to make reasonable adjustments, in view of the steam coal separation is adopted by coal gangue or size separation. So for steam coal separation, the accuracy and the representative particle size of raw coal screening data is particularly important.

If there is no good washability analysis or reasonable prediction, the actual production of coal preparation plant after the completion will encounter many problems, even unable to deal with on site. In all the problems, coal slurry system caused by imprecise ash content prediction and poor handling capacity of coal water system is a prominent problem.

[Case]One Sichuan Coal Group has a coal preparation plant project which designed by German in Huayingshan a coal preparation plant. Because the design of coal washability don’t have enough data and presided over the design engineers do not have coal experience on Southwest coal washery projects ,directly derive experience from the north coal preparation plant design. After the completion of the coal preparation plant, coal slurry system often brings a lot of production and environmental protection problems due to a serious shortage of processing capacity.

The reason that caused serious shortage of coal slurry system is Southwest China's coal is easier sliming than the northern coal and secondary slime content is also higher. Another reason is that changes in exploiting mining .In the early period , supporting the coal preparation plant of coal mine uses the high-grade general mining, during the coal preparation plant design and construction process, coal mining method upgrades to long arm of low coal seam mining (Comprehensive Mechanized Mining). Upgrade to comprehensive mechanized, roof and floor DUNNBASS rate increased significantly. Because of the coal enterprise management system caused the coal mining and coal preparation two team lack of communication  and total contract price has been set, the general contractor did not immediately adjust the design scheme. Therefore, after the completion of the coal preparation plant, coal slurry system frequently shows problem because of lack of capacity.

The coal preparation plant project becomes the coal group's heartache, since then, the group never implement the coal preparation plant project, and high-level group have a prejudice against modern coal preparation technology.

2.    Can not correctly handle when the coal preparation plant infuses the second coal.

If the selected raw coal from coal preparation plant comes from the multiple mine or the multiple coal seam of same mine (in fact most of the design is with this case), then you need to consider multi coal should adopt the mixed selected or the group selected .

Clause 5.1.5 on requirements "when the various coal seam in the same density separation conditions.The optional, elementary ash difference away from large, clean coal sulfur larger or coal which is appropriate to separate separately". Then, underground mining separate exploit , separate transport, ground storage and are selected in the most reasonable way;

If coal medium conditions permit the implementation of mixed coal washing method, also must solve the problem of how to ensure the realization of each layer of coal mixed in the proportion . At this time, the best solution is to take the underground mining, transportation, ground storage according to the proportion of coal to be  blended.

However, underground mining adopting transportation and exploiting separately is  difficult, generally do not have such conditions of mine. But other way, can be considered in the ground to set coal homogeneous of mixed coal to ensure inclusion properties of raw coal in for a period of time (the length of time depends in the volume of mixed coal, usually 2 ~ 3 days) to maintain relatively uniform. This method is used both foreign and domestic coal preparation plant.

[Case] 4, 6 on the second layer of ash and far difference of density of Zhungeer Coal Mine is the main feature of Honi Da coal. recommends coal blending mode of learning from strong points to make up  deficiencies. In view of the downhole failure to implement separate production and transport, the quantity and quality of the two layers of coal mining with random, ground production system also do not take any mixed coal measures, thus the determines the 4, 6 second layer coal mixed coal ratio (1:5) which can not be guaranteed, blending into the wash nor from the implementation. In fact, the 4 6 on the two floor of coal gets into a random mixture of selection. What is difficult to control the separation process will eventually cause serious consequences after the election, unstable product quality.

3.    To confirm the problems regarding coal preparation technology

The separation process of scheme comparison for qualitative analysis of the technology, not fully consider the various factors related to the separation process, lack of quantitative data, the argument is not sufficient, so conclusion is difficult to convince the people.

The newly issued clause 5.1.7 (boldface mandatory provisions) clearly stipulates that" the coal preparation method should be according to the related factors of coal properties (such as particle size composition, density, optional, floating, sulfur content and occurrence characteristics, gangue lithology), product requirements, separation efficiency, sales revenue, cost of production and infrastructure investment determines by comparison of technology and economy ". The above provisions has been a qualitative change comparing with the old "standard", this is the main difference and progress of new standard and old standard the main . The core requirements should be based on the representative  optional data to consider a variety of factors associated with the separation process, through comparison of technical and economic and comprehensive analysis to determine the separation process. Definition of coal preparation method, besides should reflect the principle of maximum production rate, but also embodies the principle of maximizing economic benefits.

[Case] Northeast Hegang Mining Area, a foreign company to design and construction of a coking coal preparation plant (1.50Mt/a) adopts desliming prior to two pressure feeding two product dense medium cyclone + coarse slime separation process in spiral separator. Finally, comparing with the mine plant which adopts Before the election the undeslimed non pressure feeding three product heavy medium cyclone+ the slime heavy medium cyclone separation technology ,the production of slime coal has reduced about 5%. An annual revenue loss is tens of millions dollars . This is a typical case for unsuitable choosing of fine coal (including coarse slime ) separation technology.

[Case]Shaanxi Yulin and Hengshan mine coal preparation plant (4.00Mt/a), long flame coal puts into the coal washing. The biggest characteristic of this quality of coal is is most coal seam roof and floor of tonstein intercalations in rock with mudstone that is easy to disintegration in water and easy to clay. Coal preparation process uses coal completely into the washing process, namely desliming prior.50 ~ 1.5mm adopts owing pressure two products dense medium cyclone separation material, 1.5 ~ 0.15mm coarse slime adopts spiral separator, -0.15mm slime adopts filter press direct recovery.

Since the commissioning, the main washing coal is 4-2 coal seam of this mine, the coal layer contains an average thickness of 200mm tonstein. The main problems are even under the situation of volume of coal washing is small (Real average coal washing capacity is less than 1/5 of design rated capacity), wash water is soon to get black and thickening, a serious shortage of coal slurry system capacity, slime can't deal with it resulting of circulating water tank liquid level rising and overflow, only a few hours can’t drive to production. The mainly two reasons are as following:

（1）The design adopts feeding dense medium cyclone separation with pressure, produce a lot of secondary slime . Feeding is hit by centrifugal pump blade, high-speed stirring, shale gangue contained in raw coal, large numbers to be mud. According to field test, -0.15mm fine mud getting into thickener is the 18% of coal, compared with the original design, is higher 3.5 folds of predicted value 5.23%. So causes a serious shortage of capacity of coal slurry system.

（2）Shale gangue cement after the case is not easy sedimentation, bad filtration efficiency. According to the conventional selection of thickener and filter area are obviously not enough, fine mud is not timely recovery, a large number of accumulation in the wash water results  circulating water turning black and thicken soon .Circulating water tank liquid level rising and overflow.

Later there is a indispensable solution adding a filter press shop, based on the original design of three sets filter press machine (300m2/set),to add 6 filter press(400m2/ set).The filter area has increased two times than before, increased investment nearly 15 million yuan.

These are the opposite cases, the main reason is because of the improper selection of coal preparation method. In addition, the case also shows that the gangue Shiyan has great influence on coal selection method.

The coal contains a large number of easy mud mudstone gangue, "pre discharge" is widely used and mature technology application. China is mainly composed of two kinds of pre-discharge process, dense medium vessel bath and mobile jigger.

Note: Here do not discuss process adaptability, each kind of raw coal has different adaptation process. No best coal preparation process, only the most suitable coal preparation process!

Pre row gangue technologies can exclude large shale gangue as soon as possible from the system, so the following main election process and coal slurry system is very favorable. Effective size separation of dense medium vessel bath shallow slot in the technical specification in general is written in 13mm-200mm, there are some manufacturers in the propaganda even said that the minimum limit of separation is 6mm, in fact, these are misleading. From the practical experience, only in the particle size more than or equal to 25mm, heavy medium shallows slot good separation effect.

In Australia and South Africa, the mainstream process of pre dumping is "selective crushing machine".

Mention of selective crushing machine, the majority of China's coal preparation colleagues will scoff, they believe this technology out of the one decade, how could be used in Australia's mining technology such developed countries.

The selective crusher used in Australia and South Africa , the principle is same as the China eliminated 10 years technology, but the utilization efficiency and adaptation is absolutely different. Selective crusher in the process of Australia and South Africa coal preparation pre discharging, reaches highly successful and equipment failure rate is low, maintenance costs less than the movable jigger and dense medium vessel bath, the operation is simple.

The main reason is the difference between Australian coal and China's coal quality , in current situation, domestic only Jincheng Anthracite Coal Group mine and part of Ningxia coal mine for selective crusher as a pre discharging process, other areas are not suitable. In addition, just because of the manufacturing details of processing equipment. When asking why equipment price difference such obviously, I often do the simple analogy is, Xiali and Mercedes Benz can be on the road, you say what is the difference? Do not discuss the details here.

4.    The problems when adopts mobile jigger to do preparation technology

The moving sieve jigging machine adopting to pre row gangue separation process is more, but the reasonable conditions of the process is often lack in-depth analysis. For example, ignore the movable sieve jig in gangue accounts for the proportion, under the condition of the gangue content is little to use moving sieve jigging machine pre dumping blindly; in addition, many design institutes add buffer storage in the front of moving sieve jigging machine, resulting in material limit rate increases, makes the jigger can’t work normally.

[Case] Shaanxi Shenfu mining area (South) of a mine coal preparation plant (6.00Mt/a), adopts moving jigger to pre discharge. While the mine coal gangue content is less, less content of gangue block, +50mm block in coal gangue content accounted for only less than 9%. The movable sieve jigger to pre discharge is not reasonable. Because little content coal gangue adopts the movable sieve jig discharge, is not conducive to give full play to the jig processing ability of moving screen. Will lead to a corresponding quantities increasing of moving sieve jigging machine selection.

[Case] Shanxi Luan mining area of a coal preparation plant adopts moving sieve jigging machine ,but adding buffer storage in front of moving sieve jigging, increasing the limit ratio, makes the jig is not working properly.

5. The problems appears when the case of above two medium system design

If heavy dense medium coal preparation plant layouts more than two independent production systems, some design workshop space is tension, so will combine the production process between some links(like thin dielectric barrel, magnetic separator)  of two medium system into one set machine. It seems that reduce equipment units,but  is not reasonable. In order to improve the density of the medium, the precision of the flow control, keep each production system to be independent is indispensable.

6. The problem of high medium consumption of dense medium

The medium consumption is the important technical index of dense medium sorting and it is also the factor to impact cost. Reducing medium consumption is one of the key design of the dense medium separation technology. Mandatory provision 5.3.9 in depicts " separation of coal per ton of magnetite powder technology consumption shall comply with the following provisions :lump coal <0.8kg; mixed coal and powder coal <2.0kg. " But a lot of design in the production is often ineffective, medium consumption is too large, the majority more than 2kg, even 3 ~ 5kg, in the mud a serious level of Guizhou even more than 8kg medium consumption.

The influence factors of medium consumption index are various, must fully consider and adopt comprehensive technical measures in order to get good results. These measures include:

 (1) A reasonable selection of screen area and the sieve gap (qualified segment 0.5mm, thin dielectric section and an arc-shaped sieve 0.75 ~ 1.0mm, if the screen material and ultimately to flotation is not greater than 0.5 mm), eliminate the sculping screen run medium ;

(2) Except prevent arc screen from sculping, sculping screen must be qualified medium, try to reduce the number of medium to dilute media segment, to reduce the loss caused by the cleaning medium;

(3)Try to avoid using the double sculping screen. A lot of design in the HMS system, using double sculping screen, will merge product sculping and grade two homework into a double screen. Although the link is simplified and convenient layout and other advantages, but also has a lower sieve separating efficiency is not high, will increase the drawbacks of medium consumption;

(4)Before the material gets into the sculping screen ,adding average material facilities (such as sieve, trough) to make the material uniformly on sculping screen surface, in order to improve the efficiency;

(5) Increase the pressure head of sculping screen and guarantee the enough water volume (small  water and high pressure), so as to reduce the quality of dielectric material on the sieve;

(6) Choosing the high gradient magnetic separator for recovering medium selection, the separation efficiency is high up to 99.9%, which can effectively reduce medium consumption;

(7) Strictly control the feeding of magnetic separator (100 m3/h.m), ensure the reasonable feeding material(15%~25%, the best value is 20%), ensure that the separator material into the magnetic material content does not exceed the allowable value (18.5t/h - M). In order to make the magnetic separator condition and magnetic separation efficiency to maintain the best condition, it is very important to reduce medium consumption. Ore pulp (i.e. dilute medium) concentration depends not only on the sculping screen size of the spray water, but also has a direct relationship with whether setting desliming process before the election. Therefore the development and calculation of process should combine with selection of magnetic separator;

(8) The reasonable selection of additional media. In order to ensure timely additional medium, the processing ability of the gangue in the separator should be enough left or dedicated a separator as a purge adding new medium.

7.Common problems in equipment selection process

Process and equipment failure, adverse consequences can only be put into operation in the coal preparation plant after gradually exposed. Because once the project completes, the plant space and the area has been fixed, then to make up the correct selection of equipment's failure is very difficult. Regardless of economic losses, only the implementation of technical equipment change will be very difficult. If you accept a fait accompli, will results in a difficult to overcome the passive and trouble to the production and management of coal preparation plant, or even leave a permanent regret. No matter how good the design process, how advanced and perfect technology; ultimately depends on the long-term stability of the equipment, and the normal operation can be reflected. Heavy medium coal preparation plant design production in recent years, there are not so many problems about the process, most of the problems exposed are due to selection of equipment failure.

So far, equipment selection problem exists in almost every domestic design of coal preparation plant.

This kind of equipment selection error, if in the international project, keeping same  attitude, the result will be very miserable, enough to make the design units and engineering general contract firms bankruptcy.

[Case] South Africa famous mining engineering technology provider, BATEMAN. Because of the failure of a project, not only lose tens of millions of dollars EPCM costs (engineering design and project management contract), and by the owners of the claim. The direct result in delisting from the stock market, finally purchased by its competitor.

Therefore, the design engineer must attach great importance to the importance of equipment selection process in the design of coal preparation plant.

8.The selection of equipment failure, mainly in the following two aspects

（1）There is not a clear view regarding performance and feature of device. Just to pursue coherence of model to cause the bigger or smaller model. Comparing with the index provided by ,there exists some distance.

[Case] Shandong Juye coal mine coal preparation plant washing system adopts general contracting. De medium screening uses the linear vibration sieve, although feasible, but because horizontal linear vibrating screen is for de medium, the unit area processing capacity is relatively small (3 ~ 4t/m2), so general sieve area selection specifications is larger, and large linear vibration sieve structure rigidity is poor, easily broken cross beam. For the -50mm mixed coal products sculping, the processing capacity of unit area is relatively large (5 ~ 6t/m2), banana strong structure rigidity is more suitable for the screen.

Slime coal de medium screen area is 4 x 22 m², the unit area processing capacity has reached 6t/m², more than the index which is nearly doubled, illustrates the selection area is less nearly doubled. And waste removal mesoporous sieve in order to maintain and clean coal de medium screen specifications consistent, selection area is too large, the unit area processing capacity is less than 2t/m², significantly lower than the ’s index. In gangue de medium screen area is too large, but also bring excessive dewater defects, leading to the separator inlet concentration is low, affecting the efficiency of magnetic separation. The selection of equipment do not have to deliberately pursuing unified specifications, resulting in unnecessary waste of investment, for the general project is even more so.

[Case] Some of the design adopting banana sieve for sculping screen type of lump coal dense medium separation products is not reasonable.

[Case] Dense medium cyclone feeding pump selection mistakes of Shaanxi Yuheng coal preparation plant cause much economic loss which took a good lesson for us.

[Case] A lot of process design, adopting thickness of the slime dewatering respectively recycling principle is reasonable. But on the particle size is less than 0.15mm of fine coal slime with pressure filter dewatering and recovery, there are large water filter cake, dewatering efficiency is low, processing capacity exponentially decreases with the problem, the design should be treated with caution.

（2）The one-sided pursuit of imported equipment, should give priority to domestic technical clearance of equipment.

[Case] It’s not a advertising for Jin Jing group, now press filter selection if you continue to use foreign brands, you must be tyrant .

9.There are problems in the process layout

1) Many design institutes on the occasion of surface production system layout, don’t set up storage of coal fields and coal buffer bin, main shaft wellhead lifting equipment is directly connected to the production system of coal preparation plant. This first of all does not meet the clause 3.2.1 and 3.2.3 in new issued < Specification for design of coal washing project>,simultaneously there are some deficiencies:

(1) Connecting directly to make the production system of coal preparation plant equipment hour capacity must be same as hoisting equipment, and in order to ensure the hoist safety and production system of coal preparation plant to equipment system, which will cause investment enormous waste.

(2) Production system of coal preparation plant with any equipment failure, due to coal flow system locking relationship will lead to hoisting equipment (tape machine) parking. If the main mine equipment is hoister winch will bring great harm to mine production.

2) The design of coal preparation plant is often called as the modular layout. But in fact ,most of main building design is not really a modular structure, not even a modular process.