INTERNATIONAL CEMENT REVIEW AUGUST 2012

The natural water content of raw materials contributes to the heat consumption of the whole cement

production process (see Figure 1). Hence, the raw material deposit determines in advance the thermal heat consumption within reach. Although dry limestone deposits are largely to be preferred for new projects, in some regions (eg southeast Russia and parts of Ukraine and Belarus) widespread wet chalk deposits prevail. In these cases, using this wet raw material is the only option. Table 1 offers an overview of modern chalk-based clinker production lines.

Most of them (1-6) use the so-called Compact Kiln System1. Only the Mordov-3 project (7) was designed as a conventional four-stage preheater kiln fed by dry raw meal2. To cover the heat requirement for drying the wet raw materials in the vertical raw mill, external hot gas (~700kJ/kg clinker) is being added to the kiln exhaust gas (yellow arrow in Figure 1). The overall specific heat

consumption for this dry-process comes to 3800kJ/kg of clinker. At Mordov Cement, an internal power plant with gas turbines is in operation. The waste heat provides energy for drying ‘free of charge’ (see Figure 1).

But when looking for the process with minimal thermal heat consumption for wet chalk the choice must be semi-wet (see Figure 1).

The process labels in Table 1 have been assigned in accordance to the systematic process definition given in Kapphahn, 20103.

Mechanical dewatering of cement raw slurriesTypical raw material suspensions contain water in the range of 33-42 per cent per mass. Modern wet kiln feed may consist

IThe exception proves the ruleby Michael Kapphahn, Holcim (Deutschland) AG, Germany

While modern cement production technology favours the dry-process, there are occasions were a semi-wet process is the only choice – for instance, if the limestone deposits used as a raw material source only supply wet product.

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Figure 1: heat consumption for modern kilns with PH and PC vs kiln feed moisture

Table 1: overview of modern chalk-based clinker production lines

Location Operational Process Kiln feed Moisture Equipment Capacity Thermal heat Remarks (*)

since (%) (tpd) consumption

(MJ/t clinker)

1. Aalborg, Denmark 1988 Wet slurry + DS 30 Compact kiln system 4500-4600 4430* performance test

with 2-stage PH+PC

2. Sebriakovsk, Russia 1991 Semi-wet filter cake* >21 Compact kiln system 2300 4300 additional slurry

with 3-stage PH+PC injection

3. Lägerdorf, Germany 1995 Semi-wet filter cake 20.5 Compact kiln system 4500-4800 3600* performance test

with 3-stage PH+PC

4. Chelm, Poland 1999 Semi-dry lumpy chalk 23-25 Compact kiln system 5000 ~3700-3800* depending on chalk moisture

with 3-stage PH+PC from quarry

5. Rugby, UK 2000 Wet slurry + DS 30 Compact kiln system 3800-4000 4600

with 2-stage PH+PC

6. Mordov-2, Russia 2007 Semi-wet filter cake 21 Compact kiln system 2300 3600

with 3-stage PH+PC

7. Mordov-3, Russia 2009 Dry dry raw meal 0 Conventional 6000 3100* plus ca 700 kJ/kgclinker

4-stage PH+P waste heat

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AUGUST 2012 INTERNATIONAL CEMENT REVIEW

of water-minimised chalk-silica slurry with 33-36 per cent moisture and dry fly ash. Finally, the overall kiln feed moisture approaches 30 per cent4.

Figure 2 illustrates the energetic advantage of the semi-wet process. Let’s assume that chalk (23 per cent moisture), ground sand and iron waste are being transferred into slurry (40 per cent water content, blue arrow in Figure 2). In the case of the wet-process, a theoretical 1477MJ/t of clinker of thermal energy is consumed for drying from 40 to 21 per cent. In the comparison, the rest of drying energy down to zero per cent makes no more of a difference.

The semi-wet process takes advantage of mechanical dewatering. To produce a 21 per cent wet filter cake 6kWh/t of clinker of electrical power (see upper red arrow in Figure 2) is required. The power consumption is equivalent to a negligible 9MJ/t of clinker. This is only 1.5 per cent of the dewatering energy used in the wet-process!

The second red arrow in Figure 2 relates to the semi-wet process when using high-grade chalk (LSF>400). Such a pure chalk filter cake can be produced by means of membrane filtration. Although the specific power consumption of membrane filtration is more than double, the much drier filter cake (13-15 per cent) marks the minimum of thermal heat consumption possible for chalk (see Figure 1).

The cake filtration is based on the different permeability of the filter medium (textile weave of filter cloth and the growing filter cake layer on it) for water and solids.

At the beginning, the empty plate pack consisting of individual filter plates (see Figure 3) is pressed hydraulically

together to become sealed. Then it is filled with slurry. When a pressure of 2-3bar is reached step two begins. Increasing pressure leads to the formation of a thin filter cake layer on the inner surface of the filter cloths which cover each plate. As the filter cake layer grows, more pressure is required to overcome the increasing resistance.

In case of chamber filtration, the pressure reaches 23-25bar at step three. The formation of filter cake comes to an end when both inner cake layers merge to a compact filter cake.

In reality it is difficult to maintain the maximum filtration pressure for a long time (see Figure 4). However, if the size of the high-pressure diaphragm pump related to the chamber press volume is correctly chosen, a fast control loop can manage it. It is wise to operate two pumps

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Figure 2: thermal and mechanical dewatering (MJ/t of clinker) vs kiln feed moisture

Figure 4: process sequence of a chamber filter press

Figure 3: chamber filter press

courtesy of Andritz/Rittershaus & Blecher Co

Filter press full of slurry

Start of filter cake formation

High throughput

Partial filter cake formation

Filtration resistance increases

Throughput decreases

Complete filter cake formation

High filtration resistance

Minimumthroughput

Core blown out

Discharge of filter cake

= filter cake

= filtrate

= slurry

= filter cloth

= pressure

Sequence of pressure/ throughput curve

Thro

ughp

ut (

Q),

Pre

ssur

e (P

)

Time (t)

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INTERNATIONAL CEMENT REVIEW AUGUST 2012

KILN PROCESS TECHNOLOGY

– a centrifugal pump with high flow rate for feeding (0-4bar) and a high-pressure diaphragm pump with a small volume per stroke.

In step four the high-pressure pump stops, the feeding channel is blown out and the pack is released for emptying plate by plate.

The chamber filter cake with moisture above 20 per cent appears sticky. To prevent cake pieces from sticking on the sealing frame of the plates when falling out of the press, the entire step four has to be entirely operator-controlled.

Usually a full cycle of filtration takes 15-17 min for steps 1-3 and ~10 minutes for step four. Each press (12 chamber presses with 103 plates each, Figure 3) gives two filtration cycles per hour, producing 2 x 11m3 = 42t filter cake per press and hour.

A membrane filter press is slightly different in construction and operation (see Figure 5).

Steps 1-2 coincide with chamber filtration. At 4-5bar the feeding system is switched off (step three). A flexible, mostly polypropylene, membrane introduced between each second plate and filter cloth is being expanded inwards the chamber volume by pressed air or water. As a result, the pressure inside the chamber grows providing energy for further filtration.

At the Lägerdorf plant, Holcim has gained experience with both types of filtration. Chamber filtration was introduced in raw material preparation in 1967. Over the years, the components of raw mix slurries changed. During 1967-1995, a chalk-clay-mixture was used for filter cake production (cake noodles for Lepol grate feeding). In 1995, clay as a raw material component was substituted by fly ash, sand and waste iron ore. Today, Lägerdorf’s raw mix slurry and filter cake consist of chalk with a controlled LSF (target 400) and wet ground quartz sand

(~15 per cent residual on 90µm control sieve).

Between 1999-2002, the raw material preparation for the lime production with Lepol-kiln No 9 had to be changed to the semi-wet process to take advantage of the chloride-wash-out effect of filtration. Unfortunately, trials with chamber filtration failed for high-grade chalk (LSF>600, CaCO3>98 per cent) dewatering. High-pressure gradients in the feeding channel probably led to premature filter cake formation of pure chalk filter cake and blocked the connection between the chambers preventing the further slurry transport.

Additional tests on converted membrane presses were successful. It can be concluded that quick dewatering slurries require a smooth and moderate pressure increase. This is achieved by

the membrane swelling. Pressed air is blown behind the sealed membrane with 12-14bar. The membrane filtration alone achieved cake moistures of 18-19 per cent. To bring down the cake moisture further during step three (Figure 5) pressed air with 7-8bar is simultaneously blown through the filter cake itself. The air stream takes free water and a part of capillary-bound water away from filter cake. The residual moisture of pure high-grade chalk filter cake came down to an average 13.5 per cent for the year 2001.

A vision for Lägerdorf is linked to a transition from chamber to membrane filtration. The kiln feed will consist of pure chalk filter cake (13-15 per cent moisture) and dry input of other raw material components directly to the flash dryer of a four-stage compact kiln system. Thermal heat consumption will come down to ~3200MJ/tclinker.

The specific power consumption for high-grade chalk filtration from 40 per cent to 13.5 per cent will amount to 14kWh/t of clinker (lower red arrow in Figure 2).

However, such low cake moistures can be achieved only with high-grade chalk (LSF>400). Increasing clayish impurities in chalk avoid an effective dewatering. For

Figure 5: process sequence of a membrane filter press

courtesy of Andritz/Rittershaus & Blecher CoTime (t)

Thro

ughp

ut (

Q),

Pre

ssur

e (P

)

Sequence of pressure/ throughput curve

Filter press full of slurry

Membranes at support body

Start of filter cake formation

High throughput

Partial filter cake formation

Filtration resistance increases

Membranes at support body

Throughput decreases

Feeding switched off

Membranes compress the filter cake

Short and effective subsequent dewatering

Core blown out

Discharge of filter cake

Membranes without load

= filter cake

= filtrate

= slurry

= membrane

inflation media

= filter cloth

= membrane

= pressure

The semi-wet process takes advantage of mechanical

dewatering.

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AUGUST 2012 INTERNATIONAL CEMENT REVIEW

chalks with LSF<400 cake moisture below 17-18 per cent can hardly be achieved. Nevertheless, under the same conditions membrane filtration in combination with cake dry blowing enables to get 2-3 per cent less cake moisture be achieved compared to chamber filtration. A membrane press can always be operated in chamber filtration mode. Hence membrane filter presses provide higher process flexibility.

Material handlingChalk is characterised by an unconsolidated rock structure (compressive strength < 5MPa), extremely fine particles (D50 ~5µm) and a high porosity (40 per cent per volume). This soft limestone is easily extracted by any type of excavator5.

The closer the chalk deposit is to natural water table, the more the pores fill with water (23 per cent and more). Even in well-drained chalk quarries high above the water table, rainwater may transform dry chalk (12-18 per cent moisture) into wet chalk. Mechanical stress on chalk lumps presses free water towards the lump surface thereby creating a thin water layer. The lumps become covered with muddy chalk paste and will stick on each wall, transition chute or may form heavy conglomerates. Winter operations under lasting frost conditions is a challenge6.

Reliable chalk transportation from the quarry to the next preparation site in all four seasons by railway, truck or conveyor belt systems requires high operational costs. In the transition period and during frosts raw material transportation will be often face the threat of interruption. The most vulnerable part are conveyor belt systems7.

On the other hand, chalk is easily transformed into a suspension. Slurry treatment has been the most convenient form of chalk handling, in the past, the present and the future.

Considerable innovation has been introduced to reduce the water content in the chalk slurry. In the past, when untreated chalk slurry could not be pumped below some 42 per cent moisture by weight, this figure now has come down to approximately 33-34.5 per cent, only 9-10 per cent higher than quarried with a negligible fall from suspension when left to stand for week8.

Wet chalk must be treated in accordance with its nature: 1. chalk should be transferred into a suspension state as soon as possible. Hydraulic transport has to be chosen. A ‘mobile chalk slurrifier’ has been operating in Lägerdorf (see Figure 6) since 2009 in real quarry conditions through all seasons9

2. slurry pumping is highly cost-effective, reliable and suited for winter operation9 3. the raw material transportation choice excludes dry- or semi-dry processes and a semi-wet process is prescribed.

SummaryDue to local geography, investors in Russia are often bound to wet chalk deposits without any alternative access to dry limestone. Nevertheless, they are still reluctant to decide for the semi-wet process.

However, the well-known advantage of dry-process technology in respect to

thermal heat consumption is valid only in a certain range of natural raw material moisture and in case of raw materials with >20 per cent moisture, will cease to exist and turn into a disadvantage. In this case, the modern semi-wet process appears superior to the dry-process in respect of thermal heat consumption as well as material handling. The exception proves the rule! ______________I

Sources1 GRYDGAARDT, P (1998) “Get out of the wet” in: International Cement Review, No 4, pp77-84. 2 KÜHNE, K (2007) “The new semi-wet process clinker production line for the Russian cement producer Mordowzement” in: Cement International, No 3, pp2-8.3 KAPPHAHN, M (2010) “Modern cement production with chalk” in: ZKG International, No 7/8, pp69-83.4 BORGHOLM, H E (1988) “Ein neues Halbtrocken-Ofen-System fuer 4000 t/d im Aalborg Portlandzementwerk“ in: ZKG International, pp595-600.5 SCHRÖDER, D (1993) “Schaufelradbagger als Alternative zum Sprengbetrieb für semi-hartes Gestein“ in: ZKG International, 46, No 8, pp423-429.6 KAPPHAHN, M (2010) “Winter operations” in: AT International, No10, pp40-47.7 KAPPHAHN, M (2012) “A contribution to the current Wet-Dry-discussion in Russia” 7th International Cement Conference Petrocem 2012, St Petersburg, Russia, 22-25 April. Organised by Journal Cement and its applications, St Petersburg, Russia.8 CLOWES, DJ (2002) “The background, operational experiences and upgraded replacement of the Dunstable to Rugby high pressure pipeline after 35 years continuous operation” in Proceedings of Hydrotransport 15, 15th International Conference on Slurry Handling and Pipeline Transport, Banff, Canada, 3-5 June. Organised by BHR Group, Cranfield, Bedfordshire, UK. 9 ANON (2010) “Aktuelle Forschungen und Trends“ in: AT International, No 3, pp24-32.

The author can be contacted, at:Holcim Group Support LtdThermal Process Technology,Switzerland.

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Figure 6: mobile chalk slurrifier at Lägerdorf

Wet chalk must be treated in accordance with its nature.

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