WO2003095364A1

WO2003095364A1 - Method for clarifying bayer process liquors

Info

Publication number: WO2003095364A1
Application number: PCT/AU2003/000554
Authority: WO
Inventors: Gerald Dunstan Roach; John Bernard Cornell; Philip Scott Heckley; Geoffrey William Riley; Denis Nicoli
Original assignee: Alcoa Of Australia Limited
Priority date: 2002-05-10
Filing date: 2003-05-09
Publication date: 2003-11-20
Also published as: AUPS224802A0

Abstract

A method for reducing the solid content of a feed Bayer Process liquor containing residual solids, the method comprising the steps of : introducing an inert particulate material into a clarifier vessel having an upper portion and a lower portion; introducing a first flocculating agent into the clarifier vessel; allowing at least a portion of the inert particulate material to form a first layer of suspended inert particulate material in the lower portion of the clarifier vessel; introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of inert particulate material; removing an overflow from the upper portion of the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor.

Description

"Method for Clarifying Bayer Process Liquors"

Field of the Invention

The present invention relates to a method for clarifying Bayer Process liquors. The method of the present invention is particularly suited to the clarification of primary settler overflow liquor.

Background Art

The background to the present invention will now be discussed in the context of the clarification of Bayer Process primary settler overflow liquor. However, the scope of the present invention should not be considered to be limited to such, with the invention having utility in the clarification of a range of Bayer Process Liquors.

The Bayer process is widely used for the production of alumina from alumina containing ores, such as bauxite. The process involves contacting alumina- containing ores with recycled caustic aluminate solutions, at elevated temperatures, in a process commonly referred to as digestion.

The sodium aluminate solution so produced also contains insoluble residues from the bauxite ore, and the solids are separated from the solution in a thickener or clarifier. The solids, known as 'red mud', are taken as underflow from the thickeners, then typically washed to recover caustic values and render such suitable for disposal. The overflow, however, typically still contains finely divided red mud particulates, comprising iron oxides, iron hydroxides, silica and the like.

The presence of these compounds in the final alumina product is highly undesirable and the settler overflow is often passed through one or more filters to remove such, before being seeded with aluminium hydroxide to induce the precipitation of further aluminium hydroxide therefrom. The precipitated aluminium hydroxide is separated from the caustic aluminate solution, with a portion of the aluminium hydroxide being recycled to be used as seed and the remainder recovered as product. The remaining caustic aluminate solution is recycled for further digestion of alumina containing ore.

However, the filters remain operable for relatively short periods of time prior to blinding. Periods of operation of approximately 8 hours are typically achieved before maintenance is required. This typically involves opening the filter assembly, removing the solids, and back flushing the filter medium with clean spent liquor. This causes considerable disruption in an otherwise largely continuous process, and demands considerable labour.

In an effort to avoid the use of such filters, or at least extend the period for which they remain operable, it has been suggested that the overflow from the settler be treated with one or more flocculants before being passed through another settling step. Further, it is known to introduce an inert, insoluble solid as a filter aid to increase the operational longevity of the filters.

Connelly et al. United States Patent number 5,387,405 dated Feb. 7, 1995 disclose a secondary 'polishing' process for Bayer Process liquors in which bio- carbohydrates such as dextran are used in conjunction with an inert insoluble solid as a filter aid to treat a primary settler overflow before passing such to a secondary settling stage, and the contents of the Connelly patent are herein incorporated by reference.

The Connelly patent states (at column 2, lines 58-60) that 'a Bayer Process operation may accomplish suspended solids ranging from essentially zero to no more than 5 mg/L'. The patent includes experimental data in which 500 mL samples of primary settler clarifier overflow were combined with a filter aid before adding varying concentrations of bio-carbohydrate flocculant in a magnetically stirred 600 mL beaker with acceptable results. However, attempts to apply the method disclosed by Connelly in test apparatus approximating a conventional clarification vessel have met with limited success.

Emmett, R.C. et al. in a paper titled 'Recent Developments in Solid/Liquid Separation Technology in the Alumina Industry', (Light Metals 1992) describes recent improvements in various separation steps used in Bayer refineries, including some experimental use of a solids contact clarifier having a top-fed combination feed well and flocculant compartment. Incoming feed is mixed with recirculated settled solids, recirculation being achieved using an underflow pump. Emmet et al. report that trials using the solids contact clarifier with tricalcium aluminate or causticiser precipitate showed promise, but were unable to produce consistent overflow solids less than 10mg/L.

The preceding discussion of the background to the invention is intended to facilitate an understanding of the present invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was part of the common general knowledge in Australia as at the priority date of the application.

Throughout the specification, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or group of integers but not the exclusion of any other integer or group of integers.

Throughout this specification, unless the context requires otherwise, the term "inert" is used in relation to a substance to indicate that the substance is not substantially chemically acted upon by the Bayer Process Liquor, or substantially dissolved, therein under the typical conditions of the Bayer Process.

Disclosure of the Invention

In accordance with the present invention, there is provided a method for reducing the solid content of a feed Bayer Process liquor containing residual solids, the method comprising the steps of:

introducing an inert particulate material into a clarifier vessel having an upper portion and a lower portion;

introducing a first flocculating agent into the clarifier vessel; allowing at least a portion of the inert particulate material to form a first layer of suspended inert particulate material in the lower portion of the clarifier vessel;

introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of inert particulate material;

removing an overflow from the upper portion of the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor.

Preferably, the method also comprises the step of:

removing an underflow from the clarifier vessel, said underflow including solids from the feed Bayer Process liquor, and inert particulate material.

Preferably, the steps of introducing an inert particulate material into a clarifier vessel and introducing a first flocculating agent into the clarifier vessel more specifically comprise the steps of:

mixing the inert particulate material with the first flocculating agent; then

introducing the inert particulate material-first flocculating agent mixture into the clarifier vessel.

The first flocculating agent and the inert particulate material may pass through an in-line mixing device before being introduced to the clarifier vessel.

Mixing the inert particulate material with the first flocculating agent prior to introducing such into the clarifier vessel enables the inert particulate material and first flocculating agent to be mixed thoroughly without unduly disturbing the first layer of suspended inert particulate material. Flocculating agents may be generally divided into two types, being those that predominantly clarify a suspension (a clarifying flocculating agent), and those that predominantly cause settling rates to increase (a settling flocculating agent). Preferably, the first flocculating agent is a clarifying flocculating agent.

The method of the present invention may include the step of:

introducing a second flocculating agent to the clarifier vessel.

The second flocculating agent may comprise the same agent as the first flocculating agent. The second flocculating agent may be introduced directly to the clarifier vessel, or be mixed with the inert particulate material and or the Bayer Process liquor prior to being introduced into the clarifier vessel. The point at which the second flocculating agent is introduced will be determined, at least in part, by the nature of the flocculating agent and the nature of the inert particulate material.

In one form of the invention, the steps of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material comprises:

introducing the feed Bayer Process liquor with at least a portion of the inert particulate material into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material.

In a specific form of the invention, the feed Bayer Process liquor and the at least a portion of the inert particulate material pass through an in-line mixing device before being introduced to the clarifier vessel.

Where the method comprises the step of removing an underflow from the clarifier vessel, and the step of mixing the feed Bayer Process liquor with the inert particulate material, the method may comprise the further step of: mixing a portion of the underflow with the feed Bayer Process liquor to provide at least a portion of the inert particulate material added to the clarifier vessel; and

disposing of a portion of the underflow.

In one form of the invention, after the steps of introducing an inert particulate material and a first flocculating agent into a clarifier vessel having an upper portion and a lower portion, and substantially concurrently with the step of allowing at least a portion of the particulate material so introduced to form a first layer of particulate material in the lower portion of the clarifier vessel, the method of the present invention may comprise the step of:

allowing a portion of the inert particulate material so introduced to settle to form a second layer of particulate material beneath the first layer of particulate material, the second layer of particulate material having a higher concentration of particulate material than the first layer of particulate material.

In one form of the invention, the step of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material more specifically comprises the step of:

introducing the feed Bayer Process liquor into the first layer of particulate material.

In a further form of the invention, the step of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material more specifically comprises the step of:

introducing the feed Bayer Process liquor into the second layer of particulate material. The method of the present invention may comprise the step of:

agitating the second layer of particulate material to maintain a substantially constant solids concentration throughout the second layer of particulate material.

Preferably, the method of the present invention comprises the step of:

controlling the extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor enters the first layer of inert particulate material and the density of the first layer of particulate material.

In one form of the invention, the step of controlling the extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor is introduced into the first layer of inert particulate material and the density of the first layer of particulate material comprises:

altering the point at which the feed Bayer Process liquor is introduced into the clarifier vessel.

In one form of the invention, the step of controlling the extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor is introduced into the first layer of inert particulate material and the density of the first layer of particulate material further or alternately comprises:

altering the thickness of the first layer of particulate material.

Further or alternately, the step of altering the thickness and density of the first layer of particulate material may comprise:

varying the rate at which underflow is removed from the clarifier vessel, and thus varying the thickness and density of the second layer of inert material. Further or alternately, the step of altering the thickness and density of the first layer of particulate material may comprise:

varying the rate at which inert particulate material is introduced into the clarifier vessel

varying the rate at which Bayer Process liquor is introduced into the clarifier vessel.

It has been found that a means by which the extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor enters the first layer of inert particulate material may be controlled by adjusting the solids content of the mixture introduced into the clarified vessel, in addition to the point at which such is introduced.

It has been found that when solids are maintained in the clarifier vessel for extended periods, gibbsite precipitation may occur, thereby affecting the clarity of the overflow, and rendering control of the depth of the first and second layers of inert particulate material more difficult.

Where the method includes the step of mixing a portion of the underflow with the feed Bayer Process liquor to provide at least a portion of the inert particulate material added to the clarifier vessel, the method may further include the step of:

washing the underflow prior to mixing the underflow with the feed Bayer Process liquor in order to dissolve the gibbsite or control gibbsite precipitation rate.

Other methods for the control of gibbsite concentrations may include systems primarily involving solid-liquid separation (by gravity, enhanced gravity, filtration, classification or other), followed by dissolution of the gibbsite using fresh caustic or other Bayer Process Liquors to an appropriate level for gibbsite nucleation control.

Preferably, the step of removing an overflow from an upper portion of the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor, more specifically comprises the step of:

removing an overflow from an upper portion of the clarifier vessel having a concentration of solids lower than that of the first layer of particulate material.

Preferably, the concentration of solids between the first layer of inert particulate material and the point at which the overflow is removed is below about 0.080 g/L.

In a highly preferred form of the invention, the invention comprises the steps of introducing a feed Bayer Process liquor with a flocculating agent and an inert particulate material into the lower portion of the clarifier vessel such that the feed Bayer Process liquor-flocculating agent-particulate material mixture passes through at least a portion of a layer of suspended inert particulate material in the lower portion of the clarifier vessel, removing an overflow from the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor, removing an underflow from the clarifier vessel, said underflow including solids from the feed Bayer Process liquor, and inert particulate material, mixing a portion of the underflow with further feed Bayer Process liquor; and disposing of a portion of the underflow wherein the steps are performed substantially simultaneously, in a continuous process.

The step of introducing the feed Bayer Process liquor agent into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material may be performed using any feed arrangement, including sparging or piping into feed distribution equipment. These arrangements might include a tangential feed entry into the bottom or side of the clarifier vessel, or a bottom, central feed arrangement. However, in a preferred form of the invention, the feed Bayer Process liquor is distributed radially from a central discharge sparge.

In one form of the present invention, a rotating rake assembly is provided in the second layer of particulate material, the rotating rake assembly being adapted to agitate the second layer of particulate material providing a relatively consistent underflow density.

Preferably, the Bayer Process liquor has an initial alkali concentration of up to 400g/L TA, where TA represents total alkali concentration expressed as g/L sodium carbonate. Preferably, the Bayer Process liquor has a temperature below its boiling point. Preferably, the Bayer Process liquor has an initial solids concentration of up to 1000mg/L. The Bayer Process liquor may be a green liquor, supersaturated in alumina. In one form of the invention, the Bayer Process liquor is provided in the form of a primary settler overflow liquor.

The inert particulate material is preferably selected from the group calcium oxide, limestones, calcium hydroxides, slaked limes, calcium carbonate, calcite, calcium aluminate, cellulose, crushed carbon, graphite, cellulose based filter aids, alumina, alumina species and other materials such as plastics, or mixtures thereof.

In a particular form of the invention, the inert particulate material is selected from the group: CaO, limestone, Ca(OH)₂, slaked lime, calcite, calcium aluminate, Ca(AIO₂)₂. In a specific form of the invention the inert particulate material is provided in the form of tricalcium aluminate.

Preferably, the inert particulate material comprises particles in the size range 0.5 to 200μm. Preferably, the inert material is introduced at a concentration of approximately 1 to 10g/L. However, higher concentrations, such as approximately 20 g/L may be used.

Appropriate flocculating agents may include low molecular weight anionic polymers, particularly bio-carbohydrates such as dextran. In a particular form of the invention, a dextran flocculant is used. . In a highly specific form of the invention, the dextran flocculant is provided in the form of H152, as supplied by Ondeo-Nalco (product code 85711).

In a further particular form of the invention, an acrylate, acrylamide, hydroxamate terpolymer flocculant, such as HXPAM200, from Cytec is used.

The appropriate density and thickness of the first layer of inert particulate material will be a function of factors including but not limited to: the particle size distribution of the inert particulate material, the density of the inert particulate material, the particle size distribution and density of flocculated particles (entering either as Bayer Process liquor, inert particulate material or a combination of both), the rise rate of Bayer process liquor to the overflow, local fluid velocity as impacted on by vessel geometry and internals and the properties of Bayer Process liquor reporting to the overflow (density, viscosity, presence of other phases such as air, etc).

However, where the inert particulate material is provided in the form of tricalcium aluminate, with a particle size distribution in the range 0.5 to 200 micron, being mixed with H152 dextran flocculant, treating a Bayer Process liquor in the form of primary settler overflow with density of approximately 1.27g/cm³ and viscosity of approximately 1.9cP with clarifier feed being introduced into a 1.8m tall clarifier by a radial sparge to achieve an effective up flow in the range of 2 to 4m/hr in the first layer of inert particulate material, it is preferable that the concentration of solids in the first layer of inert particulate material is between 40 and 120g/L.

For the same operating conditions and treating the same Bayer Process Liquor as above, it is preferable that the first layer of inert particulate material extends at between about 200-900mm above the point at which the feed Bayer Process liquor is introduced into the first layer of inert particulate material.

Again it is recognised that for different conditions, materials and different tank geometries (especially height), the depth of the first layer of inert particulate material will vary, however a certain density of first layer of inert particulate material will be required to remove solid particulates (from both Bayer process liquor and inert particulate material) introduced into the clarifier vessel. This density will be a function of the factors mentioned previously.

For the same operating conditions and treating the same Bayer Process Liquor as above, it is preferable that the density of the second layer of inert particulate material is between about 220g/L to 450 g/L.

This is to enable a consistent underflow to be obtained in order to be able to effectively control total clarifier feed solids when at least part of the underflow is recycled for re-use. The density of the second layer of inert particulate material will be highly dependent on the type of inert particulate material chosen and the height available between the point at which the Bayer Process liquor is introduced into the first layer of inert particulate material and the point at which the underflow is removed from the vessel. These heights will impact on the density and compaction achievable with a specific type of inert particulate material and those skilled in the art will be able to determine the density and height of second inert particulate material to best suit the conditions for a particular application.

The point at which the overflow is removed from the clarifier vessel relative to the first layer of particulate material is also dependent on at least some of the factors described above. However, for the same operating conditions and treating the same Bayer Process Liquor as above it is preferred that the overflow is removed from between about 600 and 1400 mm above the first layer of particulate material.

Best Mode for Carrying Out the Invention

The best method for performing the invention presently known to the applicant will now be described, by way of example only. The following description of the best method should not in any way be seen as limiting the scope of the preceding description of the invention. A block diagram/flow-sheet illustrating some features of the best method comprises Figure 1. A Bayer Process liquor in the form of a primary settler overflow liquor containing residual solids is combined with a flocculant in the form of Nalco H152 at a dosage of 10-20ppm or lower, the primary settler overflow-flocculant combination then being combined with an inert particulate material in the form of tricalcium aluminate of a size range 0.5 to 200μm at a concentration of 10g/L, the combination then being introduced into a clarifier vessel. The clarifier vessel has an upper portion, having a peripheral overflow weir and launder and a lower portion, having a conical floor sloping towards a central underflow outlet. The conical floor section of the clarifier also has a rake assembly as can best be seen in Figure 2.

Feed to the clarifier is introduced by way of a radial sparge arrangement that can be raised or lowered. Feed is introduced from the top via the sparge and the sparge exit is located in the bottom part of the clarifier vessel. Feed is evenly distributed in a radial fashion into the body of surrounding fluid/slurry.

A relatively dense layer of tricalcium aluminate with a density of approximately 300 g/L and a thickness of 300 mm below the feed point is allowed to form. Above the relatively dense layer, a loose bed of tricalcium aluminate, with a solids concentration of approximately 40-120g/L and a thickness of about 500-900 mm is allowed to form. Above the loose bed of tricalcium aluminate, immediately below the overflow , is a region of solids concentration between about 0.005 and 0.080 g/L, and a thickness of about 500-1000 mm is allowed to form. The various layers are illustrated in Figure 3. The relatively dense layer is continuously agitated by way of the rake assembly to ensure relatively constant solids concentration, and thus underflow density, and to assist in consistent withdrawal of solids from the vessel.

Introduced primary settler overflow liquor-H152 flocculant-tricalcium aluminate mixture passes through at least the loose bed of tricalcium aluminate, substantially reducing the residual solids content thereof. An underflow is removed from the clarifier vessel, the underflow including a substantial portion of the residual solids from the primary settler overflow liquor, with the dense tricalcium aluminate. The thickness of the respective layers, and most importantly the loose bed of tricalcium aluminate, may be maintained in the desired range by varying the rate at which the underflow is taken, the rate at which Bayer Process liquor is introduced and/or the point at which the sparge exit is positioned in the clarifier vessel with respect to the dense and loose layers of tricalcium aliuminate.

A portion of the underflow is mixed with the feed primary settler overflow liquor to provide at least a portion of the tricalcium aluminate fed therewith, and a portion is disposed of. Prior to mixing with the feed primary settler overflow liquor, the portion of the underflow is subjected to washing to assist in controlling gibbsite precipitation by dissolution of solid gibbsite into solution. The overflow from the clarifier vessel has a substantially lower solids content relative to the feed liquor. Once the bed has been established, the method may be performed as a continuous operation whilst controlling the height of the loose bed and the distance between the loose bed and the overflow by the methods mentioned. .

Examples

The efficacy of the method of the present invention will now be demonstrated by a series of examples.. However, the examples should not be seen as limiting the scope of the invention as discussed in the preceding description. In the discussion of the examples, the term 'suspended zone' may be used to describe the first layer of inert particulate material, and the term 'underflow zone' may be used to describe the second layer of inert particulate material.

1. Preliminary (laboratory) Tests

1a. Inert Particulate Material (Filter Aid)

Laboratory tests compared the performance of tricalcium aluminate filter aid from two different refinery sources, under the same flocculation conditions. Using refinery source B, a filter aid concentration of 4 g/L gave a similar degree of flocculation to a filter aid concentration of 7 g/L using refinery source A. Therefore the tricalcium aluminate filter aid from refinery source B was used for all subsequent pilot plant trials. 1b. Flocculating Agents

A range of readily available flocculants were tested in the laboratory and on the pilot rig to assess the impact of dose rate, flocculant type and liquor surface(laboratory) or overflow solids (pilot rig)quantity and type. From these tests, it became apparent that flocculant choice would have some impact on the type and quality of solids in the overflow/liquor surface. All flocculants tested enabled some improvement to overflow solids/liquor surface solids, but the extent of that improvement varied. An example of testing comparing Cytec's HXPAM 200 flocculant to Nalco's 6800 flocculant is summarised in Tables 1 and 2, below.

Table 1

Table 2

HXPAM 200 6800

Si0₂ Fe₂0₃ CaO Si0₂ Fe₂0₃ CaO

Thick O/F 287 12.7 1.2 28.7 12.7 1.2

FA - 1 g/L 8.5 1.7 32.9 23.6 7.8 136.2

FA - 5 g/L 7.3 0.2 55.4 13.2 0.9 156.8

FA - 20 g/L 7.9 0.0 117.5 15.2 0.3 192.7 g/kL solids

2. Pilot Tests

2a. Apparatus

A pilot clarifier vessel was constructed, 600mm in diameter and 1.8m high, with feed lines and pumps to allow the addition of tricalcium aluminate and flocculant to the Bayer Process Liquor. An underflow system was installed to allow recycling of settled solids back to an agitated feed tank. Inert particulate solid was able to be added into the agitated feed tank at up to 10g/L Bayer Process liquor flow. The slurry so formed could be pumped via a line to the clarifier vessel at controlled flow using a flow control valve. Flocculant was added into the line. The flocculated slurry could then be passed through an in-line spiral mixer before being introduced into the clarifier vessel. The clarifier vessel was equipped with two feed systems, being (i) a sparge with outlet located 100mm from the bottom of the conical base of the clarification vessel, such that the feed Bayer Process liquor was distributed radially, and (ii) a feed well that was located off-centre to the cone (not ideal) with the outlet at the top of the conical base. Some experimentation has been undertaken in relation to the sparge design, and is specifically discussed subsequently.

Figure 4 is a top view of the clarifier vessel, showing both the sparge and feed well. Figure 5 is a schematic flow diagram for the test system. The flow to the clarifier was set at 1 kL/hr for the majority of the testwork. This relates to an upflow velocity of 3.5m/hr in the main body of the vessel when using the sparge and an up flow of 3.7m/hr in the main body of the vessel when using the feed well.

2b. Feed Well Tests

The feed well was fed via bifurcate arms to give tangential flow to the feedwell. The feedwell extended down to the top of the cone. Standard tests were carried out using 20 ppm Nalco H152, 1000 L/hr flow and 10 g/L filter aid fed to the feed tank. Results were disappointing after 6 hours of operation with overflow clarities >100 g/kL. It was hoped that the feedwell system would run without the need for a bed, but the results indicate that the existence of a bed is critical. Further feed well tests are described below.

A further test was carried out on the assumption that the bed is critical and if less flocculant is used then the bed will be less dense and therefore have the potential to capture more solids. A test was run with similar conditions described previously but with 5ppm H152. Poor overflow solids similar to the earlier 20ppm H152 feed well test runs were obtained, i.e. >100g/kL.

2c. Sparge Feed Tests

Early tests with the clarifier vessel were run with source A filter aid dosages between 1-4 g/L and 5 ppm of an acrylate, acrylamide, hydroxamate terpolymer flocculant (HXPAM200, from Cytec), with the liquor fed through the sparge. Overflow solids from those tests were often worse than the feed liquor solids.

The filter aid was changed to source B, at an increased dose, and the flocculant was changed to Nalco H152 (a dextran bio-polycarbohydrate) at 20ppm. The overflow solids concentration (measured as NTU) decreased significantly after a period of time as seen in Figure 6 for tests at 5g/L and 10 g/L filter aid. During these tests no underflow solids were removed so the bed height increased over time. From these experiments it became apparent that a certain bed height was required for the liquor to pass through to effectively remove most of the fine mud solids. At the end of the 10 g/L test, the bed height was measured to be ~800mm above the feed position. Bed height was measured using a conductivity probe but also by withdrawing a sample at different levels using a thin tube and syringe. From Figure 6, it can be seen that good overflow clarities were achieved after the bed height reached ~600mm above the feed position.

At 10 g/L filter aid in the feed, an appropriate bed height was established faster and the reduction in overflow solids was reached sooner. Overflow solids of 7g/kL were achieved (measured gravimetrically).

Two further tests were undertaken to determine the concentration of filter aid required to maintain low overflow solids once the bed was built. The bed was built for 4 hours at 10g/L filter aid and then the filter aid concentration was dropped to either 5 g/L or 0.5 g/L. The overflow clarity for the 5 g/L filter aid could be maintained at -10 g/kL, but at 0.5g/L filter aid, overflow solids of 15-20 g/kL were obtained.

2d. Extended Pilot Plant Testing

An extended pilot plant run was conducted for 16 hours to determine (i) if appropriately low overflow solids could be achieved with recycled filter aid/mud from the underflow, and (ii) whether gibbsite precipitated in the bed.

Once a bed was established, fresh filter aid addition was reduced to a rate of 0.5 g/L and a proportion of the underflow was directed back to the feed tank to maintain a total filter aid concentration of 10 g/L in the feed. A small bleed stream was also drained from the underflow to waste to maintain close to steady state solids concentration in the system.

Maintaining a consistent 10 g/L filter aid concentration in the feed tank proved to be difficult because the solids concentration in the underflow was highly variable (i.e. 50-250 g/L). Low overflow solids were obtained when the feed tank solids concentration was > 6 g/L. At concentrations below this, overflow solids rose steadily to ~ 30 g/kL.

X-ray diffraction phase analysis of the bed solids indicated that gibbsite was not present. This was further supported by titration analysis that showed no alumina/total caustic ratio drop between the feed liquor and underflow liquor. However, as will be described subsequently, gibbsite precipitation is a factor in experiments of longer duration. Furthermore, soluble calcia was not observed to increase with levels of 31 ppm and 29 ppm in the feed liquor and overflow liquor respectively.

The robustness of the clarifier with respect to feed liquor solids concentration and overflow clarity was excellent; Figure 7 shows the turbidity measurements (in NTU) for the 16 hour run. Excursions in the clarity of the feed liquor did not have a significant affect on the clarifier overflow.

A further pilot run was carried out using Cytec HX200 flocculant at 20ppm. Similar conditions were used as in the Nalco H152 run described above. The overflow solids did not decrease as seen with H152 after the expected period to build the bed. Turbidity was measured at 120-160 in the overflow, gravimetric analysis indicated 0.39 g/L. The bed height was measured after 5 hours at 100mm above the cone (~300mm above the feed point), and very distinct. It was also noted that the bed density was in the order of 500g/L. "Sighter" tests during the run comparing H152 and HX200 suggested that the HX200 formed smaller floes and settled into a denser bed - this supports the pilot run data.

2e. Further Extended Pilot Plant Testing

Further tests involved a new sparge/feed well design, comprising a four-armed rake with holes located along each rake arm and chains fitted to aid underflow removal. The new sparge design performed with mixed results. It achieved its designed objective in providing a stable underflow density, which is imperative in maintaining a consistent solids concentration in the feed. The underflow solids concentration was approximately 220 g/L, with the feed tank solids concentration maintained between 9-10 g/L during the 15-hour trial.

The bed height was very high (1100 mm above the cone) for most of this test and this caused the overflow solids to be very high also (20-30 g/kL). A three-fold increase in flocculent dosage to 30 ppm did not lower the bed height.

A decrease in flow from 1000 L/hr to 500 L/hr reduced the bed height to 600 mm above the cone within half an hour. There was a corresponding increase in the bed solids concentration from 50 g/L to 65 g/L. With the reduction in bed height, overflow clarities in the order of 5 g/kL were observed. An increase in flow back to 1000 L/hr increased the bed height and poor clarities returned.

It was established that there were three settling zones within the clarifier vessel. The 'Underflow Zone' (corresponding to the second layer in the description of the invention) had a solids concentration of -220 g/L and extended from the discharge point to the height of the cone. Above the underflow was a 'Suspended Zone' (corresponding to the first layer in the description of the invention), which consisted of a flocculated 'bed' of tricalcium aluminate and mud solids at a concentration of 50-60 g/L. The height of this bed was found to be critical to the efficiency of the clarifier. A suspended bed of -600-900 mm in height above the cone was found to give excellent overflow clarities.

The 'Clear Zone' was the upper zone immediately above the bed. Samples taken from this zone indicated that the solids concentration was slightly higher than the overflow concentration, suggesting that there may have been some turbulent action (or further settling) between the bed interface and the overflow. It was important that the clear zone was high enough to prevent the unsettled solids from entering the overflow.

A test was performed over an extended period of three days to ascertain when or if the precipitation of gibbsite became problematic. As described above, earlier experiments running for 16 hours did not evidence gibbsite precipitation. Clarifier feed consisted of 10 g/L tricalcium aluminate sourced from source B, with 95% of the underflow solids flow recycled back to the feed tank and the remaining flow sent to waste. Fresh filter aid was added to maintain the mass balance. 10 ppm of Nalco H152 was added as the flocculent. Flow in the clarifier was set at 750 L/hr in an attempt to achieve reasonable overflow clarities. The initial bed height was 750 mm above the cone of the clarifier vessel.

As the experiment progressed it became clear that bed height was an important parameter for controlling overflow clarities. During the first 12 hours under steady state conditions, the bed height increased by 350 mm. Flow rate was the only lever employed at the time that lowered the bed height and increased the clear zone. Over the length of the experiment, flow was reduced from 750 L/hr to 600 L/hr to 400 L/hr in an attempt to manage the rising bed height.

Overflow clarities ranged between 7 and 40 g/kL and averaged 22 g/kL, much higher than the target overflow clarity of < 10 g/kL. The major contributing factor for this was believed to be the higher than target bed height of 700-900 mm. Figure 8 shows the relationship between overflow clarities and the bed height.

Although bed height control was not ideal during this test, it was believed that the sparge design might have played a part in the higher than expected overflow solids. Holes in the rake arms were all facing in the same direction causing a whirlpool-like effect in the clarifier. Thus the hydrodynamics were different to the sparge having a standard radial feed and may account for the poorer overflow results in this test and the previous 15-hour run.

Laboratory data showed that gibbsite precipitation occurred when the clarifier solids were held for extended times. Underflow samples were taken from a previous clarifier run and held in a rotating water bath at 95°C for 24 and 48 hours. The results showed that the tricalcium aluminate slurry was stable for 24 hours but after 48 hours a 55 point A/TC ratio drop occurred due to gibbsite precipitation. The three day continuous run confirmed the laboratory data, with gibbsite precipitation observed after 35 hours of operation. It is not known whether the precipitation was initiated in the feed tank or the clarifier. As the gibbsite precipitated, the solids in the system increased and bed control of the clarifier was lost. Gibbsite precipitation may be reduced if the recycled underflow is introduced directly into the feed line, rather than the feed tank where it is exposed to green liquor for approximately one hour before being fed to the clarifier.

A difference in the calcia concentration of 8-10 ppm was noted between the feed and overflow during the first 35 hours. After this time, calcia in the overflow rapidly decreased as it became incorporated with the precipitated hydrate. A similar trend was also observed with iron. Calcia and -A/TC values for the run are given in Figure 9.

A further experiment was performed with a variation on the sparge design. The sparge used had a radial outlet with arms supporting a chain rake, without distribution holes in the arms, as illustrated in Figure 2.

The aim of this experiment was to obtain the best overflow clarity for as long as possible using recycled solids. The sparge used in the initial extended pilot plant experiments, having a simple radial feed, was modified by adding a four-armed rake to enable consistent underflow densities for recycling. Also the underflow solids were injected into the suction side of the pump rather than the feed tank. By doing so, the tricalcium aluminate solids were not immersed in green liquor for long periods in the feed tank. (This method of addition would be preferable to delay gibbsite precipitation in the bed).

Flow in the clarifier was set at 650 L/hr and 10ppm H152 flocculent to achieve reasonable overflow clarities. With a bed height of 600 mm above the cone, overflow clarities were in the order of 3 g/kL. After 2 hours under steady state conditions, the bed height increased a further 400mm and appeared to stabilise at 1100mm above the cone, the overflow clarities during this time remained at about 5 g/kL. Data from the entire run is presented in Figure 10. Bed density measurements supported the 'three zone' configuration discussed above with underflow densities of -450 g/L and suspended zone bed densities of -85 g/L.

The flocculent and recycle was turned off to the clarifier to assess the effect on the bed height. After 90 minutes there was little change suggesting the bed height is due to upflow velocity and not the solids coming in from recycle. It was also interesting to note that the overflow solids did not increase for some time, further supporting the theory that the bed is responsible for mud solids collection.

Figure 11 shows the solids concentration of the feed and clarifier overflow during the 15 hour run with the modified sparge. Higher feed solids from the thickener overflow did not affect the clarity of the clarifier overflow, indicating the robustness of the clarifier. Clarifier overflow solids of less than 5 g/kL were maintained during periods where thickener performance was very low.

Although the design of the pilot clarifier vessel may not be ideal, it has shown that overflow solids of less than 10 g/kL can be achieved using the method of the present invention. Modifications and variation such as would be apparent to the skilled addressee are considered to fall within the scope of the present invention.

Claims

The Claims Defining the Invention are as Follows:

1. A method for reducing the solid content of a feed Bayer Process liquor containing residual solids, the method being characterised by the steps of:

introducing a first flocculating agent into the clarifier vessel;

allowing at least a portion of the inert particulate material to form a first layer of suspended inert particulate material in the lower portion of the clarifier vessel;

2. A method according to claim 1 further comprising the step of:

3. A method according to claim 1 or 2 characterised in that the steps of introducing an inert particulate material into a clarifier vessel and introducing a first flocculating agent into the clarifier vessel more specifically comprise the steps of:

mixing the inert particulate material with the first flocculating agent; then introducing the inert particulate material-first flocculating agent mixture into the clarifier vessel.

4. A method according to claim 3 characterised in that the first flocculating agent and the inert particulate material pass through an in-line mixing device before being introduced to the clarifier vessel.

5. A method according to any one of the preceding claims characterised in that the first flocculating agent is a clarifying flocculating agent.

6. A method according to any one of the preceding claims characterised in that the step of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material comprises:

7. A method according to claim 6 characterised in that the feed Bayer Process liquor and the at least a portion of the inert particulate material pass through an in-line mixing device before being introduced to the clarifier vessel.

8. A method according to claim 6 or 7 characterised in that the method comprises the steps of:

mixing a portion of the underflow with the feed Bayer Process liquor to provide at least a portion of the inert particulate material added to the clarifier vessel; and

disposing of a portion of the underflow.

9. A method according to any one of the preceding claims characterised in that after the steps of introducing an inert particulate material and a first flocculating agent into a clarifier vessel having an upper portion and a lower portion, and substantially concurrently with the step of allowing at least a portion of the particulate material so introduced to form a first layer of particulate material in the lower portion of the clarifier vessel, the method of the present invention may comprise the step of:

10. A method according to any one of the preceding claims characterised in that the step of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of inert particulate material more specifically comprises the step of:

introducing the feed Bayer Process liquor into the first layer of inert particulate material.

11. A method according to any one of claims 1 to 9 characterised in that the step of introducing the feed Bayer Process liquor into the lower portion of the clarifier vessel such that the feed Bayer Process liquor passes through at least a portion of the first layer of particulate material more specifically comprises the step of:

introducing the feed Bayer Process liquor into the second layer of particulate material.

12. A method according to any one of claims 9 to 11 characterised in that the method includes the step of: agitating the second layer of particulate material to maintain a substantially constant solids concentration throughout the second layer of particulate material.

13. A method according to any one of the preceding claims characterised in that the method comprises the step of:

controlling extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor enters the first layer of inert particulate material and the density of the first layer of particulate material.

14. A method according to claim 13 characterised in that the step of controlling extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor is introduced into the first layer of inert particulate material and the density of the first layer of particulate materialcomprises:

15. A method according to claim 13 or 14 characterised in that the step of controlling extent to which the first layer of particulate material extends above the point at which the feed Bayer Process liquor is introduced into the first layer of inert particulate material and the density of the first layer of particulate material further or alternately comprises:

altering the thickness and density of the first layer of particulate material.

16. A method according to claim 15 characterised in that the step of altering the thickness and density of the first layer of particulate material comprises

varying the rate at which underflow is removed from the clarifier vessel, and thus varying the thickness and density of the second layer of inert material.

17. A method according to claim 15 or 16 characterised in that the step of altering the thickness and density of the first layer of particulate material comprises

18. A method according to any one of claims 15 to 17 characterised in that the step of altering the thickness and density of the first layer of particulate material comprises:

19. A method according to any one of claims 15 to 17 characterised in that the step of altering the thickness of the first layer of particulate material comprises:

varying the density of a mixture of Bayer Process liquor and inert particulate material is introduced into the clarifier vessel.

20. A method according to any one of claims 8 to 19 characterised in that the method comprises the step of:

21. A method according to any one of the preceding claims characterised in that the step of removing an overflow from an upper portion of the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor, more specifically comprises the step of:

22. A method according to any one of the preceding claims characterised in that the concentration of solids between the first layer of inert particulate material and the point at which the overflow is removed is below about 0.080 g/L.

23. A method for reducing the solid content of a feed Bayer Process liquor containing residual solids characterised by the steps of introducing a feed

Bayer Process liquor with a flocculating agent and an inert particulate material into the lower portion of the clarifier vessel such that the feed Bayer Process liquor-flocculating agent-particulate material mixture passes through at least a portion of a layer of suspended inert particulate material in the lower portion of the clarifier vessel, removing an overflow from the clarifier vessel, said overflow comprising a liquor with reduced solids content relative to the feed Bayer Process liquor, removing an underflow from the clarifier vessel, said underflow including solids from the feed Bayer Process liquor, and inert particulate material, mixing a portion of the underflow with further feed Bayer Process liquor; and disposing of a portion of the underflow wherein the steps are performed substantially simultaneously, in a continuous process.

24. A method according to any one of the preceding claims characterised in that the feed Bayer Process liquor is distributed radially from a central discharge sparge.

25. A method according to any one of claims 9 to 24 characterised in that the second layer of particulate material is agitated by way of a rotating rake assembly provided therein, thereby providing a relatively consistent underflow density.

26. A method according to any one of the preceding claims characterised in that the Bayer Process liquor is provided in the form of a primary settler overflow liquor.

27. A method according to any one of the preceding claims characterised in that inert particulate material is selected from the group calcium oxide, limestones, calcium hydroxides, slaked limes, calcium carbonate, calcite, calcium aluminate, cellulose, crushed carbon, graphite, cellulose based filter aids, alumina, alumina species and other materials such as plastics, or mixtures thereof.

28. A method according to any one of the preceding claims characterised in that the inert particulate material is selected from the group: CaO, limestone,

Ca(OH)₂, slaked lime, calcite, calcium aluminate, Ca(AIO₂)2.

29. A method according to any one of the preceding claims characterised in that the inert particulate material is provided in the form of tricalcium aluminate.

30. A method according to any one of the preceding claims characterised in that the inert particulate material comprises particles in the size range 0.5 to

200μm.

31. A method according to any one of the preceding claims characterised in that the first and/or second flocculating agent is provided in the form of a low molecular weight anionic polymer.

32. A method according to claim 32 characterised in that the first is provided in the form of a bio-carbohydrate such as dextran.

33. A method according to claim 32 characterised in that the first is provided in the form of H152 dextran, as supplied by Ondeo-Nalco under product code 85711.

34. A method, substantially as described herein, with reference to any one of the examples.