NECESSARY FUNDAMENTALS FOR SUCCESSFUL ALUMINA SEED FILTRATION

Dr. Reinhard Bott, Managing Director of BOKELA

Dr. Thomas Langeloh, Managing Director of BOKELA

Jürgen Hahn, Project Engineer

BOKELA Ingenieurgesellschaft für Mechanische Verfahrenstechnik mbH

Gottesauer Straße 28

D-76131 Karlsruhe, Germany

ABSTRACT

For seed filtration, vacuum disc filters are commonly used. In the last few years however, High Performance Disc Filters have been installed in modern alumina refineries replacing standard disc filters due to the tremendous hydraulic throughput rates required. This filter type impresses not only by its dimensions but also by its operational data.

The new design of the High Performance Disc Filter is the result of a consequent innovation. For more than twelve years BOKELA has been upgrading Al-Hydrate filters of many manufacturers. The know how and the experience gained by these projects is reflected in the new design of the High Performance Disc Filter.

The plant design as well as the efficient operation of this filter type requires a detailed understanding of the theoretical filtration fundamentals. Only if the influence of the process and the process and the apparatus parameters on the filter operation on the operational result is understood then feasible operation of the seed filters can be ensured. The standard equation for cake formation describes how the specific solids throughput (ms) depends on the decisive and influenceable parameters of the filtration process. By discussing each of these parameters it can be highlighted to which extent the filter capacity can vary during operation.

KEY WORDS:

seed filtration, disc filter, high performance disc filter, filtration parameters

NECESSARY FUNDAMENTALS FOR SUCCESSFUL ALUMINA

SEED FILTRATION

Dr. Reinhard Bott, Dr. Thomas Langeloh, Jürgen Hahn

1.0 INTRODUCTION

As a hydrometallurgical process the Bayer process is based on the fact that the aluminium minerals of bauxite dissolve in NaOH forming Al-hydrate Al(OH)3. The recovering of this Al-hydrate from the pregnant liquor by precipitation is a key part of the process since it decisively influences the production costs, the product quality and the plant capacity. There are two main objectives of the precipitation: to obtain a high yield of Al(OH)3 from the solution and to produce an Al(OH)3 quality which meets the requirements of the calcination. Therefore, the precipitation line often is in the centre of alumina refinery upgrading projects which mainly aim at increasing of the production rate, improving the product quality and at decreasing the production costs. The filtration of the recirculated seed streams (see figure 1) plays an essential part in this context.

Most of the alumina refineries are producing a sandy-coarse alumina quality which is best suited to meet the requirements of modern reduction plants. The production of this material is based on a two stage precipitation process including a separation of two fractions of hydrate particles. The fine seed fraction is recirculated to the "agglomeration" precipitators, whereas a part of the coarse fraction is recirculated to the "growth" precipitators.

Image243b.GIF (8957 bytes)

Figure 1

Principle flow sheet of Al(OH)3 precipitation in alumina refineries

This process is shown schematically in figure 1. The last precipitator feeds the primary classifier or first stage hydrocyclone. The underflow, which is the aluminium hydrate product, is pumped to the product filters where the solids are filtered and washed, mainly on pan filters and then transported to the calcination. The overflow of the primary classifier or hydrocyclone feeds the secondary classifier or hydrocyclone where it is separated into two fractions. The coarse fraction is returned to the precipitators as seed crystals. The fine fraction is usually pumped to the tertiary thickener where the final clarification of the spent liquor takes place. The underflow of the tertiary thickener contains the so called fine seed, which in some plants is recirculated to the "agglomeration" precipitators. The overflow is led to the evaporation where the causticity is increased before it is recirculated to the digestion.

2.0 INFLUENCE OF SEED FILTRATION ON THE PRECIPITATION

A very important factor with basic influence on the Al(OH)3 precipitation process is the supersaturation of the liquor. A high supersaturation of the pregnant liquor improves the liquor productivity and it increases the yield of product, the particle sizes and the product quality. A high yield of Al(OH)3 from the solution also reduces the energy input needed for pumping and heating of the solution.

Therefore, increasing the liquor supersaturation in the precipitators is an essential measure of refinery upgrading projects. As shown in Figure 1, only about 20 % of the Al-hydrate is withdrawn from the precipitators as product stream whereas about 80 % is recirculated as seed. In plants of older design the recirculated coarse and fine seed fractions enter the precipitators unfiltered with a high amount of spent liquor, which reduces the concentration and the productivity of the liquor as well as it affects the potential for the formation of agglomerates.

Consistently, the implementation of filter stations for separating the coarse or fine seed fraction from the spent liquor is an established measure to increase the supersaturation in the precipitators. Then, only particles with very little spent liquor (12-20 % cake moisture) are recycled to the precipitators.

By this way, with seed filtration a 10 % increase of the plant capacity or more can be achieved.

2.1 Filter Options for Seed Filtration

Seed filtration in alumina refineries is mainly characterised by the tremendous flow rates which have to be processed on the filters (see Table 1). So vacuum drum and standard disc filters which are the typical filter equipment for such mass products have been established for this duty. But the most effective and economic filter type which gained acceptance for seed filtration in the recent years is the High Performance Disc Filter, which differs in essential features from standard disc filters.

Comparing the filter design both of drum and disc filters, there are some characteristics of the design which are obviously in favour of the disc filters if no cake washing is required. For drum filters both the specific space demand and the specific investment costs are nearly twice as high as for disc filters. In Table 2 typical values of specific space demand and specific investment costs are shown for standard disc filters and for High Performance Disc Filters.

   

Coarse

Seed

Fine

Seed
   

Conventional Vacuum Disc Filters

High Performance Disc Filters

Conventional Vacuum Disc Filters

High Performance Disc Filters

x50

[µm]

80 -

100

40 -

60

cSL

[g/L]

350 -

750

200 -

600

VSL

[m³/m²h]

3,4 - 4,8

8 - 14

2,5

3,5

mS

[t/m²h]

1,7 - 2,4

6 - 8

0,5 - 1,5

0,7 – 2

VL

[m³/m²h]

2,5 - 3,5

5 - 9

1,8 - 2,2

2,7 -3,2

mC

[%]

< 20

< 18

14

12

x50 [µm] = mean particle diameter cSL [g/L] = slurry concentration

VSL [m³/m²h] = specific slurry flow mS [t/m²h] = specific solids throughput

VL [m³/m²h] = specific filtrate flow mC [%] = cake moisture content

Table 1

Typical Slurry Data and Operational Results of Al-Hydrate Disc Filters

 

Standard Disc Filter

High Performance Disc Filter

Space Demand per Filter Area

Including Receivers [m²/m²]

0,8 - 0,9

0,7

Investment Costs per Filter Area

Including Receivers [DM/m²]

ca. 8500

ca. 8000

Required Filter Area per 1000 t/h Solids Throughput [m²/(1000 t/h)]

310

160

Investment Costs per 1000 t/h Solids Throughput

100 %

ca. 50%

Table 2

Typical values of specific space demand and specific investment costs for standard disc filters and High Performance Disc Filters

Due to plant upgrading projects the requirements on seed filtration have grown distinctively in the last years. Both the filtration targets and the process conditions for the filtration became more demanding. Improved upstream processes as for instance improved classifier technology, have led to high concentrated feed streams. In coarse seed filtration for instance, feed slurries with a high solids concentration cSL of up to 750 g/l have to be filtered and even concentrations of 900 g/l can occur. Such slurries demand high standards on the filter design. Therefore, conventional vacuum disc filters do not comply with these increased requirements as listed below:

- high performance capacity for processing of large filtrate streams

- high operational flexibility in order to filter different seed materials in a wide range of slurry data and throughput values which should be processed on the same filter apparatus

- low cake moisture

- low content of filtrate solids

- no addition of water to the process

- low maintenance and running costs

- operational characteristics suitable for plant control systems

3.0 THE HIGH PERFORMANCE DISC FILTER TYPE "BOOZER"

High Performance Disc Filters are the most advanced filter technology for Al-hydrate filtration. In order to comply with the increased process demands on Al-hydrate filtration, BOKELA has consequently improved each detail of the vacuum disc filter design and developed the High Performance Disc Filter Type "Boozer". The outstanding hydraulic design of this modern disc filter technology is the precondition for the extraordinary high performance capacity, high operational safety and reliability as well as the low maintenance and operation costs.

Number of discs:

1 / 2 / 3

Filter area:

40 m² / 80 m² / 120 m²

Diameter of disc:

5,6 m

Segments per disc:

30

Segment fixing:

lever or bayonet fixing

Clearance volume:

7,5 l/m²

Weight Standard/ light:

28 kg / 18 kg

Filtrate pipes:

large sized with trapezoidal shape (f = 2,1°/oo)*

Trough design:

common trough with agitator or single trough design

Submergence of disc:

50 %

Filter speed:

up to 5 rpm

Cake discharge:

by air blow back (exactly timed by snap blow valves)

*) characteristic filter value f = cross section area of filtrate pipe per connected filter area

Table 3

Typical technical data of the High Performance Disc Filter Type "Boozer"

High Performance Disc Filters are usually manufactured as single, double or triple disc filters. Each disc has a diameter of 5.6 m and consists of 30 segments forming a filter area of 40 m² per disc. According to individual plant demands the design of this filter can be carried out with different variations (see Table 3).

The most important performance data and features characterising the High Performance Disc Filter can be listed as follows:

  • up to 100 % more specific throughput performance compared to conventional disc filters
  • up to 100 % more pressure difference at the filter cloth by minimised pressure loss due to an optimised hydraulic design of all concerned apparatus details
  • high filter speed of up to 5 rpm (conventional disc filters: max. 3 rpm)
  • 100 % discharge of the cake from the filter cloth even at a high filter speed

(conventional disc filters 75 % of the cake is discharged)

  • 2 - 4 %-points reduced cake moisture content than conventional disc filters
  • high operational reliability and flexibility

The design and the outstanding hydraulic capacity of the High Performance Disc Filter is the basis of the improved filter operation and of a new "Operation Philosophy": Both with common and single trough design the High Performance Disc Filter Type "Boozer" runs without continuous slurry overflow.

3.1 Common Trough Design

With the common trough design the filter is equipped with an agitator for slurry homogenization. The overflow is designed to take the total feed flow even if no quick bleeding drain valve is open. A level measurement system which is suitable for foam on the slurry surface and mist/vapor in the air controls and adjusts the filter speed. The high hydraulic capacity, which enables a high filter speed of up to 5 rpm, offers a great reserve to adapt the filter speed to the slurry flow. Therefore, the common filter trough design allows perfect level control of the slurry. The filter can react on various feed flow rates and if the feed flow and the trough level increase, the control system reacts with increasing filter speed. That way, no slurry overflow occurs, which means all slurry is filtered and both the capacity and removal of spent liquor are maximum.

3.2 Single Trough Design

At higher slurry concentrations with reduced danger of a particle classification, the slurry can be homogenized by the disc rotating in a single trough. Then, no agitator, no sealings, no power for the agitator drive and no maintenance measures are required. But with standard single trough design, often serious problems occur during operation if thick cakes are formed due to low filter speed or high solids concentration in the slurry. The thick filter cake can block the narrow gap between disc and trough wall and the slurry can not flow into the overflow located at the sides of the single trough. Then, the slurry spills over and floods the filter floor. Therefore, the single trough design of the "Boozer" filter is optimized for avoiding this operation failures. The single troughs are connected by a common overflow which ensures a free exchange of slurry between the single troughs. This design ensures that all discs run at the same slurry level. Looking from above, the single troughs look like a common trough since there is only one common slurry level. The slurry feed is located and designed in a way that the stirring effect of the rotating disc is supported by the flow patterns of the incoming slurry.

Figure 2

Single and Common Trough Design of the High Performance Disc Filter (Triple Disc)

4.0 BASIC PARAMETERS FOR THE FILTRATION PROCESS

The design and the efficient operation of the High Performance Disc Filter requires a detailed understanding of the theoretical filtration fundamentals. The standard equation for cake formation (1) describes how the specific solids throughput ms depends on the decisive and influenceable parameters of the filtration process:

Image244b.GIF (3576 bytes)

The specific solids throughput ms depends linearly on the square root or on the reciprocal square root of these parameters. By discussing of these parameters it can be highlighted to which extent the filter capacity can vary during operation. Thereby, the cake formation angle is regarded to be a constant parameter. Furthermore, in the following some other influences on the filter operation are discussed which can not be expressed in terms of equation (1) such as the cake discharge, the filter cloth life time the influence of even filter cakes.

4.1 Pressure Difference

The pressure difference Dp or vacuum respectively is the driving force for the filtration and the specific solids throughput ms depends linearly on the square root of this parameter. On the other side, the generation of the vacuum by vacuum pumps is the most important cost factor of the filtration process. Therefore, the vacuum produced by the pump should be available for the filtration process to a high extent which is only possible if the pressure loss during the filtration is minimal. But on standard disc filters, the greatest part of the generated vacuum is necessary for overcoming of the pressure loss in the filter due to poor hydraulic characteristics. So, the vacuum which is available in the filter segment behind the filter cloth is considerably smaller than the value which is indicated at the vacuum pump. Table 4 shows some typical values of pressure loss in an Al-hydrate disc filter of standard design which we measured in many filters and of the High Performance Disc Filter type "Boozer".

pressure difference at

pressure difference at:

vacuum pump

Dp = 70 kPa

Receiver

control valve

behind
filter cloth

standard disc filter

Dp = 50 – 60 kPa

Dp = 30 - 50 kPa

Dp = 10 - 30 kPa

High Performance Disc Filter type "Boozer"

Dp = 60- 65 kPa

Dp = 55 - 60 kPa

Dp = 35 - 55 kPa

Table 4

Typical values of pressure difference in Al-hydrate disc filters

In the High Performance Disc Filter type "Boozer" the pressure loss is considerably reduced due to the excellent hydraulic design of this filter type. Thus, a pressure difference of 35 to 55 kPa is available behind the filter cloth for cake forming and cake dewatering which is twice the value of standard disc filters.

In terms of specific mass throughput (ms) this means that a doubling of the active pressure difference increases the throughput rate by factor of . Furthermore, a higher available pressure difference provides more security due to a considerably higher operational flexibility.

The filtration pressure difference generated by the pump is overlapped by the hydrostatic pressure in the filter trough. The hydrostatic pressure increases linearly with the slurry level which means that this additional effect is particularly effective at filters with big disc diameters and a high submergence of the filter disc. At coarse seed filtration with the High Performance Disc Filter, we have observed that a filter cake is formed some 10 mm in height at the inner disc radius and about 20 mm in height at the outer disc radius even if the vacuum is cut off.

4.2 Filter Speed

The filter speed (n) is the most important control parameter for filter operation. According to equation (1), the specific mass throughput depends linearly on the square root of this adjustable parameter. The higher the filter speed is, the shorter is the cake formation time during one disc rotation and a thinner filter cake is formed with a better permeability than a thick filter cake. Therefore, the filtrate flow rate increases if the cake thickness decreases and the specific mass throughput rises with the square root of the filter speed.

With standard disc filters the maximum possible filter speed is limited to 2,5 rpm. The High Performance Disc Filter, however, can run with a maximum filter speed of n = 5 rpm which is twice the value of standard filters. According to equation (1) this means a -higher throughput rate. Furthermore, the operational security is decisively improved due to a considerably higher filter speed potential which is available for filter control.

The most important reason limiting the filter speed of standard disc filters is the cake discharge because discharging of thin filter cakes demands a very effective air-blow-back. The High Performance Disc Filter, however, ensures a complete cake discharge even of thin cakes and at high filter speed.

4.3 Slurry Concentration

The concentration parameter (k) is a product parameter which influences the specific mass throughput with the square root of its value according to equation (1). It is defined as follows /1/:

k = cv / (1 - e- cv) with cv = (rSL - rL)/(rS - rL) (3)

Herein, e is the porosity of the filter cake, r S is the solids density, r L means the liquid density and r Sl the slurry density.

In Table 5 the influence of the concentration parameter k on the specific mass throughput is shown for typical values of the slurry density r Sl, a solids density of r S = 2420 kg/m3 and a liquor density of r L = 1220 kg/m3. It can be seen from Table 5 which variations of the mass throughput rate a filter has to manage if the slurry density changes during the filter operation. Referring to a 100 % value at a slurry density of 1540 kg/m³ the filter performance can decrease of some 24 % or increase of about 32 %. In cases of peaks in the slurry density these variations can even be stronger.

Slurry density r Sl

[kg/m3]

k

(e = 0.5)

solids throughput

[%]

1460

0.66

76

1500

0.87

87

1540

1.14

100

1580

1.50

115

1620

2.00

132

Table 5

Specific Solids throughput Ms as Function of the Concentration Parameter k and Slurry Density r Sl, Respectively, for some Typical Density Values of a Coarse Seed Slurry

This phenomenon which often occurs at Al-hydrate filtration praxis requires filters with a high throughput capacity or the possibilty of a filter operation with high filter speed, respectively. With increasing slurry density the cake thickness also increases and the specific filtrate flow reduces which means that the filter must run with a higher filter speed in order to maintain the slurry throughput. Standard disc filters, however, often run with maximum speed during normal operation and increasing of the filter speed is not possible. Then, the feed flow has to be reduced or the filter runs with a continuous overflow.

To avoid previous problems with single troughs mentioned previously in Section 3.2 if the slurry density or the cake thickness increase the "Boozer" filter has all single troughs connected by a large sized common overflow at the emerging disc side and the distance between the disc and the trough side wall is large enough to ensure a secure slurry flow to the overflow side.

4.4 Cake Resistance

The cake resistance (rc) is a product parameter which influences the specific mass throughput according to equation (1) with the reciprocal value of the square root. It reflects the influence of the filter cake characteristics on the filtration and it mainly depends on the particle size and the cake porosity. If the particle size and the cake porosity decrease, the cake resistance increases since the permeability of the cake reduces. The cake resistance is determined indirectly by evaluating of laboratory filtration tests to which we will give no attention here and refer to /1/, /2/. For attaining reliable and comparable values by this method, may have to be analyzed.

Another approach to estimate the influence of the cake resistance is to compare the cake formation time (t1) which is required for a filter cake of a certain thickness when the particle size (x50) changes whereas all other parameters like the pressure difference, the slurry temperature, the causticity and the filter cloth are constant. For a coarse seed, the particle size of which varies during operation in the range of x50 = 90 - 100 µm, we found a cake formation time t1,90µm which is some 15 - 20 % higher than the cake formation time t1,100µm. The specific mass throughput then increases with the square root of the quotient (t1,90µm / t1,100µm) if the particle size increases from 90 µm to 100 µm which means a some 10 % higher throughput rate. This may give an insight to the extent a change of the particle sizes or the cake resistance influences the filter performance during operation.

4.5 Even Filter Cake

For the cake dewatering and for cake washing it is important to produce a filter cake with an even thickness all over the segment, especially for permeable cakes like Al-hydrate filter cakes. Uneven filter cakes are formed for instance if the filter disc consists of less segments with a large segment angle (e. g. 16 - 20 segments per disc) and if the disc submerges only to 35 % or 40 % in the slurry. Then, the cake at the leading edge of the segments is thinner than at the tailing edge. In this case up to 80 % of the gas throughput during the cake dewatering passes through the small cake area at the leading segment edges which is only 10 % of the filter area /3/. The rest of the filter cake keeps poorly dewatered. Also classification of the particles in the trough due to a poor slurry homogenization will lead to uneven filter cakes. Then, the coarse particles form a permeable cake at the outer disc radius whereas at the inner disc radius the filter cake is formed of the finer particles which leads to the same effect as described above.

Furthermore, an uneven and poorly dewatered filter cake is much more difficult to discharge than an even and well dewatered cake.

The design of the High Performance Disc Filter ensures filter cakes with even cake thickness all over a segment due to a large number of segments per disc (30 segments/disc), the large disc submergence of 50 % and the good slurry homogenization in the trough.

4.6 Cake Discharge

The filter cake is discharged from the filter cloth by an air blow back. This process step is of great importance for the filter operation and it should be performed with 100 % cake removal even at thin filter cakes, without blowing back filtrate into the cake and without damaging the filter cloth.

Filter cake which is not removed from the filter cloth due to an incomplete cake discharge reduces the specific mass throughput and leads to an uneven cake thickness on the segment at the next disc rotation. As mentioned previously, this leads to a poor cake dewatering and high gas throughput. Furthermore, an incomplete cake discharge leads to an increased blending of the filter cloth which decreases the throughput rate as well as the cloth life time.

Figure 3

Cake Discharge from a Triple Disc High Performance Disc Filter

Because thin filter cakes are more difficult to discharge from the cloth than thick and heavy filter cakes, an effective cake discharge is also the precondition for running the filter with high filter speed.

The High Performance Disc Filter type "Boozer" ensures a 100 % cake discharge at all discs over the total range of filter speed and is nearly independent from the cake moisture. The basis for an effective cake discharge are mainly the hydraulically optimized iping system with trapezoidal pipe cross sections, the small segment volume and the exact timing of the blow back air by using snap blow valves.

The cake discharge is started by a snap-blow valve opening the blow back air exactly at the time when the segment's filtrate pipe is lined up with the opening in the control plate. In contrast to conventional design where blow back air is steadily blowing with low pressure, the snap-blow valve leads to high shock pressures exactly at the right time for discharge. With fully open snap-blow valve, the trapezoidal shape of the filtrate pipes openings in the control valve and the slide-like opening of the input pipe, enable a quick opening of the cross section for the blow back air. In combination with the short pipes and the small segment volume, this leads to a shock like cake discharge. Due to the well emptied filtrate pipes (as a result of the hydraulically optimized piping) almost no filtrate is blown back re-wetting the filter cake.

4.7 Filter Cloth Life Time

The life time of the filter cloth influences the mass throughput rate and the operation costs. A longer life time of the cloth means reduced filter downtime and higher production rate, as well as reduced operational costs. The most important factors having influence on the cloth life time are the cake discharge, the slurry homogenization in the trough, the confectioning of the filter bags and the segment design (avoiding of sharp edges). If the cake discharge is carried out with inexact timing or if the scraper is wrongly adjusted, the cloth rubs at the scraper which leads to cloth damage. An ineffective cake discharge as well as a particle classification in the filter trough leads to blinding of the cloth reducing the cloth life time and an early and rapid decrease of the filter performance.

 

Standard Disc Filter

High Performance Disc Filter

High Performance Disc Filter

(Advanced Life Time)

Cloth Life Time [h]

800 - 1000

1100 – 1200

1600 - 2000

Solids Throughput per Filter Cloth [t]

2500 - 3200

6500 – 7500

9500 - 12000

Table 6

Typical Values of Cloth Life Time and Solids Throughput per Filter Cloth for Standard Disc Filters and High Performance Disc Filters

In Table 6 typical values for the cloth life time are shown for standard disc and for the High Performance Disc Filters. Due to the improved segment design and improved filter operation with a high effective cake discharge and a good slurry homogenisation, the high performance disc filter cloth life time is enhanced at this filter type. A very interesting figure in table 6 is the solids throughput per filter cloth which for the High Performance Disc Filter is the twice or threefold as for standard disc filters. Two reasons are responsible for this advanced life time: a periodical on-line washing of the cloth by means of spray bars underneath the scraper and a special segment design and cloth fixing which avoids filter cloth damage at the segment foot which often is observed on many disc filters.

5.0 OPERATIONAL RESULTS OF THE HIGH PERFORMANCE DISC FILTER

In Table 7 some operational results of the High Performance Disc Filter are shown. The performance capacity of the BOKELA High Performance Disc Filter with slurry throughput rates of nearly 14 m³/m²h is marking a great leap in vacuum disc filter design. This tremendous performance capacity is so impressive that operators call this filter just "Boozer". Furthermore, the reliable and secure operation as well as its high flexibility has been an important aspect for our clients to decide themselves on the High Performance Disc Filter.

Table 7

Operational Results of the Bokela High Performance Disc Filter Type "Boozer"

ACKNOWLEDGMENTS

The authors would like to thank the clients for trusting in our engineering services.

REFERENCES

Stahl, W. "Fest-Flüssig-Trennung" Papers of the training course "Fest-Flüssig-Trennung" at the Institut für Mechanische Verfahrenstechnik und Mechanik, Universität Karlsruhe.

Merker, K "Vergleichende Untersuchungen an Handfiltern und großtechnischen Vakuumscheibenfiltern zur Bestimmung der Filtrationswiderstände und der Filtrationsleistung" Dissertation, Universität München, 1982.

Schweigler, N. "Kuchenbildung und Entfeuchtung auf dem Scheibenfilter" VDI-Verlag, Reihe 3, Heft 252, 1991.