Latest Technology of Mill Diaphragms

Latest Technology on Mill Diaphragms

presented by:

Christian Pfeiffer Maschinenfabrik GmbH Sudhoferweg 110 - 112

59269 Beckum Germany


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TABLE OF CONTENT: - Introduction - Requirements for a modern intermediate diaphragm - Design features of modern diaphragms

- Support structure with “floating fixation” - Material flow control system - Diaphragm plates

- Mill ventilation system - Conclusion


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INTRODUCTION Grinding of raw materials to produce clinker and the finish grinding of clinker with additives to produce cement is still one of the main energy consuming steps in the production process of the cement plants. About 65 % of the total energy used by a cement plant is consumed by the different grinding facilities. Hence, the optimisation of grinding plants related to their performance and specific energy consumption is very important for the end-user. On this occasion it is mentioned that about 5 % of the world energy consumption is utilised for driving thousands of ball mills whereby an essential part of this percentage is related to the grinding process for producing cement. With the optimisation of ball mills, the valuation and regulation of the material transport through the mill is of great importance. On the one hand, the material-to-be-ground shall remain long enough in the grinding compartment to achieve the specified product fineness at the mill discharge and on the other hand, over-grinding is not wanted, i.e. increased retention time. For this purpose, the intermediate diaphragm of the ball mill has to fulfil a central task: - generally, the diaphragm

separates the mill tube into two compartments and therefore allows an optimal tuning of the coarse and fine ball charge

- the diaphragm controls the

material flow through the mill and consequently influences the grinding efficiency


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For more than 75 years Christian Pfeiffer is now producing diaphragms and the company has gained comprehensive experiences with their production and utilisation. Totally more than 5,000 diaphragms have been sold mainly for applications in the cement industry. A continued development of the technology as well as a constant optimisation of the manufacturing process does allow the company to offer tailor-made solutions at best price-performance ratio to all cement producers. The present paper gives an overview about the most modern innovations for intermediate diaphragms.


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REQUIREMENTS FOR A MODERN INTERMEDIATE DIAPHRAGM Diaphragms have to fulfil a series of wear and process technological requirements. Nowadays, they are tailor-made for each application to match the plant specific requirements like: - width of diaphragm - slot width and slot shape - adaptation to specific filling degree - adaptation of open surface area for optimising mill ventilation - flow control for optimised material transport Furthermore, the support structure of a diaphragm must be stable and maintenance-free and must enable the tube mill to operate for many years. The structure keeping the rolled steel plates must be constructed in such a way that it can accept any ovality of the mill tube as well as the differences between the ambient temperature and the operating temperature of 100 to 140 °C without any problems arising. Because of the different types of feed to ball mills easy changes in the grinding media charges must be possible for each particular application. This aspect gives rise to a further requirement for flexible adaptability of the diaphragm plates in the main wear zone. Due to the continuously increasing cost of labour it is necessary to pay particular attention to time-saving maintenance and servicing when selecting new diaphragms technologies:


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Any replacement parts have to be readily accessible, and it must be possible to replace them by simple methods even after long periods of mill operation. Obviously, a diaphragm of this type should be adjustable after many years of operation. Finally, the diaphragm plates have to be absolutely operationally reliable and must not cause any plant stoppages. The slotted plates on the side facing the 1st grinding compartment must therefore be wear resistant and breakage-proof, and the slots should remain free from clogging by material and grinding media even after long mill operating periods. Diaphragm plates have to be replaceable as easy as possible and without using any great resources. The weight of an individual wear plate must be low enough for the plates to be changed rapidly and with few personnel.


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DESIGN FEATURES OF MODERN DIAPHRAGMS CPB diaphragms are of latest-state-of-the-art and fulfil the above mentioned requirements of dosing and process. Some of the main aspects are described in detail below. Front view of the CPB diaphragm support structure and slotted plates. Design of the diaphragm support structure The diaphragm support structure on the side facing 1st grinding compartment consists of the plate cross. Seated inside the diaphragm is equipped with curved lifters inclined in the direction of material transport. The backside of the diaphragm support structure is completely closed and welded up. It only has a few circular openings as erection aids for the diaphragm backside plates and, in the region of the central openings, trapezoidal openings for drawing off the air. These basic components of the diaphragm support structure are fully welded to one another to produce a stable and inherently rigid support structure. In the centre of the diaphragm support structure there is a central opening with the integrated material flow control system.


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The outer fastening ring which forms the contact with the mill tube takes the thermal stresses and those caused by ovality and movement of the mill tube

as well as the forces exerted by the grinding media charge. This fastening ring is turned to produce a smooth seating surface against the mill tube. At the bolted connections to the mill tube the ring has rectangular openings which take the holding plates which make up the actual connection to the mill tube, the so-called “floating fixation”.

The holding plates are bolted to the mill tube and have a certain amount of clearance in relation to the corresponding openings in the retaining ring. This means that the intermediate diaphragm can move easily in relation to the mill tube, which compensates the coming-up forces. In contrast to diaphragms which are fixed rigidly to the mill tube by bolted connections this type of fixation allows a reliable movement between the diaphragm and the mill tube itself. Flexing as well as thermal and mechanical stress which have their source in the mill tube, are not transferred to the diaphragm. Thus, fixation of the diaphragm plates on the support structure is risk less, bolt and plate breakage does not occur. The trapezoidal openings on the inside of the closed steel plate permit a controlled passage of the ventilation air through the diaphragm support structure. The diaphragm backside plates are therefore also provided with slots at these points.


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At the centre opening of the diaphragm, a larger or smaller angle ring is used depending on the filling ratio of the corresponding grinding compartment. In this way it is possible, in combination with the inner wear plates, to adapt the intermediate diaphragm to suit a wide range of filling ratios for different grinding applications. The maximum central opening can be set in each case. Material flow control system The flow of material is set by a “material flow control system” which is a component part of the intermediate diaphragm and is built into the centre. The material passes through individual openings in the discharge ring into the following compartment. Classical material flow control systems collect the ground cement from the first compartment through the slots of the diaphragm plates and discharge them by different technical solutions into the centre of the diaphragm. The disadvantage of all solutions known so far is the fact that the material is directly affected by the air flow through the centre screen. As a result, the material is usually “blown” too far into the 2nd grinding compartment so that a significant part of the effective grinding length is lost.


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A first improvement was made by introducing slotted plates on the diaphragm side facing 2nd compartment. As standard, the outer ring of the conventionally closed backside plates are replaced by slotted plates. By this, a certain part of material is transferred into the 2nd compartment through this way.

The new solution developed by Christian Pfeiffer (4th generation of intermediate diaphragms) ensures a discharge of the material outside of the centre air flow and an optimal utilisation of the grinding path is ensured. Even after long mill operating periods it is possible to alter the free outlet openings, and hence the flow of material, because there are no adjusting elements on the inside of the diaphragm which jam or weld up after long periods operation. The flow of material is adjusted while the mill is stopped without dismantling any diaphragm elements.

Besides this new generation diaphragm, Christian Pfeiffer still provides the well-known material flow control system of 3rd generation diaphragms.


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Diaphragm plates The plates over an intermediate diaphragm are subdivided into different plate rings which are dimensioned to suit the filling ratio and the position of the main wear zone. The diaphragm support structure has additional holes for the diaphragm plates so that the plate distribution can be altered at any time if there is a change in operating conditions. In this way the main wear zone can be limited to only one, or at maximum two, plate rings. The sizes of the individual

diaphragm backside plates and slotted plates are selected so that they are still easy to handle during installation and dismantling. The slots first taper conically to the narrowest cross section and then widen to the backside of the plate. The slots only enlarge to a negligible extent during the life of the plate so there are no detrimental effects on the grinding process.

The shape of the bolt swages in the diaphragm plates for taking the tear-off bolts is critically important for the service lifetime of the plates. The full wear thickness of a plate can only be utilised if suitable bolts and bolt swages provide a firm seat for the plate even after many years of operation.


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It is clear that the retaining system using a conical bolt swage offers great advantages. With cylindrical bolt swages the plates must be changed at a residual thickness of 15 mm, but with a retaining system using conical bolt swages the plate can be used down to a residual thickness of 8 mm or less.

The conical inlet part of a slot is brought in by CNC – machines together with the bolt swage for a proper fixation of the plates.


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In a second step the slots itself are flame-cut by a CNC controlled robot. Slot width, length and conical enlargement towards the backside can be tailor-made for each particular application.

The CPB diaphragm plates are made of highly wear and breakage resistant rolled steel material which is solely manufactured by well-known steel producers acc. to the specification given by Christian Pfeiffer. Each plate is hardened by a special heat treatment executed in a new heat treatment plant. The plant itself consists of a hardening kiln, a quenching area and a tempering kiln. Temperatures and retention times in the kilns and quenching basin can be djusted for each individual requirement and are controlled and monitored by means of computer.


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MILL VENTILATION A major technological aspect to control the grinding process is the mill ventilation. Several design criteria of a diaphragm have a significant influence on this important topic. In cement grinding the mill ventilation system fulfils two important functions: - Cooling the cement and removing the heat generated during the

grinding process. - Carrying part of the finished product out of the grinding area. It is well known that the energy used during the grinding process in ball mills is almost entirely converted into heat. An adequate mill ventilation system is therefore needed for cooling the product. A good mill ventilation system also ensures rapid removal of the fines which are produced even during the coarse comminution in the first grinding compartment. This can make a decisive improvement in the comminution results. Adequate mill ventilation occurs at air velocities between 1.2 and 1.5 m/s in the free cross-section of the mill. Typical ventilation data for the intermediate diaphragms are given in the schematic sketch.


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Ventilation characteristics of two typical diaphragm applications

Designation Units Numerical Geometric data for the intermediate

diaphragm – mill diameter

Total available slot length

Slot width

Total slot area

Relative free slot area

Relative area of the central grid

m

m

mm

m²

%

%

4.4

107.87

6

0.65

4.58

80

Version A

Filling ratio, grinding compartment 1

Volumetric air flow

Air velocity above the grinding ball charge

Total free area of the diaphragm

(less the area blocked by the grinding

media charge)

air velocity in the diaphragm

%

m³/h

m/s

m²

m/s

25

57 460

1.5

2.128

7.5

Version B

Filling ratio, grinding compartment 1

Volumetric air flow

Air velocity above the grinding ball charge

Total free area of the diaphragm

(less the area blocked by the grinding

media charge)

air velocity in the diaphragm

%

m³/h

m/s

m²

m/s

28

44 980

1.23

1.404

8.9


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It is a part of every optimisation work done by CPB to carry out a detailed study of the entire mill tube prior to the installation of a new diaphragm and other mill internals. The main purpose of such a mill tube inspection is the elimination of obvious bottle necks which may hinder a proper performance of the mill. - mill inlet and outlet have to be optimised according to the requirements

of the mill ventilation - an existing discharge diaphragm has to be checked according to the

free slot and centre screen area - mill main fan and filter have to be checked and, if necessary, adapted

to the new conditions


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CONCLUSION Modern grinding technology with tube mills requires new and tailor-made diaphragms. According to the specific process requirements of the grinding plant each diaphragm is especially designed for the particular tube mill: - the width is adapted to the existing bore hole situation - slot length and width are tailor-made according to product requirement - mill ventilation is optimised by a maximum free area of the diaphragm - centre opening is tailor-made to the ball charge filling level - material flow through the diaphragm is controllable and ensures best

utilisation of the 2nd compartment grinding path With its flexible design and production technologies, Christian Pfeiffer provides their customers best concepts and equipment for their entire grinding facilities.

Latest Technology of Mill Diaphragms

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