JPH08157211A - Method for improving whiteness of aluminum hydroxide - Google Patents

Abstract

PURPOSE: To obtain aluminum hydroxide with excellent whiteness by an easy method and at low cost by bringing a caustic solution of aluminum into contact with a specific adsorbent.

CONSTITUTION: The objective aluminum hydroxide is obtained by (a) bringing a caustic solution of aluminum into contact with a adsorbent comprising a compound represented by formula or calcined, and (b) filtering he solution before and/or after said contact, wherein A represents Mg²⁺, Ni²⁺, Ca²⁺, Zn²⁺; B Al³⁺, Fe³⁺, Cu³⁺; C OH^-, Cl^-, Br^-, NO₃ ^-, CH₃COO^-, C₂O₄ ^2-, CO₃ ^2-, SO^2-, PO₄ ^3-, Fe(CN)₆ ^3-, Fe(CN)₆ ^4-; 1/2≤Z≤2. According to the method for improving whiteness of aluminum hydroxide, divalent, trivalent or higher electronegative or negative ionic colored materials are easily adsorbed. Calcined hydrotalcite, which is an undesirable condition, can also remove monovalent contaminants from a solution.

COPYRIGHT: (C)1996,JPO

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to caustic and high smelt streams, especially aluminum hydroxide (also called alumina trihydrate) and streams related to the production of alumina from bauxite by the Bayer process (sometimes called Bayer stream). Related to the purification of. The present invention is further intermediate in whiteness, ie, a whiteness level of about 85% or higher based on 100% TiO ₂ standard, more preferably about 90 or 92% whiteness, and a target of about 0.20. It relates to means for producing aluminum hydroxide / hydrate having an absorbance level. For the purposes of the present invention, the terms "whiteness" and "whiteness" may be used.

[0002]

BACKGROUND OF THE INVENTION Recovery of aluminum hydroxide from bauxite by the Bayer process is accomplished by digesting a hydroxide-containing ore with a caustic solution. Most of the alumina dissolves in this liquor, but the least preferred ore component (sometimes called "red mud") remains undissolved. The red mud could be removed from this liquor after bauxite was pressure digested with a caustic such as sodium hydroxide. The aluminum hydroxide is then separated from the sodium aluminate supersaturated solution (also known as "green liquor" or "container"), usually by precipitation.

During such precipitation, the supersaturated sodium aluminate is cooled and mixed with a seed material, or a slurry of aluminum hydroxide serving as a seed source, to induce the formation of more aluminum hydroxide. . After settling, the slurry is pumped through a classifier system where the coarse portion of crystallized aluminum hydroxide is separated from the liquor. Crystallized (or precipitated)
The finer parts of aluminum hydroxide are further classified into two finer parts called "secondary species" and "tray species". The latter is the finest part from this classifier system. These second and tray species are often recycled to the settler to serve as seeds for further precipitation of aluminum hydroxide. The resulting spent sodium aluminate may be recycled to the digester for mixing with fresh (or incoming) bauxite.

Sodium aluminate solutions can also be prepared by digesting precipitated aluminum hydroxide (primary species), secondary species, tray species or combinations thereof in a manner known as re-digestion. . Some B
The ayer equipment produces excess species (generally tray species), but they either recycle the species to a bauxite die digester thereafter or reheat through another specially designed die digester. It may be soaked.

A high concentration of impurities is undesirable in the sodium aluminate solution used to make aluminum hydroxide. This is because such impurities reduce the whiteness or color purity of the hydroxide / hydrate precipitated from it. Therefore, it is desirable to minimize the presence of such impurities in the sodium aluminate stock solution prior to crystallization. Typically, such impurities are about 75% based on the 100% TiO ₂ standard.
Or produce aluminum hydroxide having a whiteness level below. Therefore, their use is prohibited in many applications that generally require relatively high whiteness levels (about 80% or higher). The present invention uses the same 100% TiO 2.
Produces aluminum hydroxide with a whiteness level of about 85% based on ₂ standards. For certain embodiments, about 9
Brightness levels of 0 or 92% are consistently achieved.

The presence of organic and inorganic impurities results in relatively low stock solution production rates and reduced alumina purity. Organic impurities can cause difficulties other than the following: That is, low alumina yield, excessively fine hydroxide particles, formation of colored liquid and aluminum hydroxide, low sedimentation rate of red mud, loss of caustic substance due to formation of organic sodium compound, increase of liquid density, and viscosity increase. Increasing, high boiling point, and undesired bubbling of liquid.

A number of methods are known for removing colorants from sodium aluminate solutions. These include Australian Patent No. 12085/83, which teaches that a stock solution is treated with reactive MgO or Mg (OH) ₂ and then calcined above 900 ° C. Schedule
US Pat. No. 4,046,855 to ers et al. also discloses a method of treating an aluminate liquor with a magnesium compound to remove organic colorants therefrom. Japanese Patent No. 5
No. 7-31527 produces high purity aluminum hydroxide by adding one or more alkaline earth metal compounds to such a solution. Typical additives included are magnesium or calcium oxides, hydroxides, carbonates, silicates and nitrates, and barium carbonates, silicates, nitrates or sulfates.

German Patent No. 3,501.350 discloses that calcium oxide / hydroxide and keeserite (Kieselite) (MgSO _4. H ₂ O) are added to the Bayer liquid to reduce the concentration of impurities, especially the content of iron. Of adding a mixture of US Patent No. 4,9 to Goheen et al.
No. 15,930, aluminum hydroxide of improved whiteness is produced by contacting a liquid stream with a mixture of tricalcium aluminate and deactivated hydrotalcite. Finally, Nigro US Pat. No. 5,068,
No. 095, the caustic solution is treated with calcined hydrotalcite to remove colorants, especially iron. About 10g /
A high charge of 1 was especially preferred for producing hydroxide / hydrates with a whiteness of greater than about 80%. The present invention realizes an improvement over the method of Nigro et al.

[0009]

It is desirable to provide a simple means for removing ionic colorants and contaminants from caustic containing sodium hydroxide and sodium aluminate, with reduced product loss. Providing means for improving the whiteness / whiteness level of aluminum hydroxide, providing a low cost, low capital means for removing greater amounts of colored contaminants from sodium aluminate solutions, To process a sodium aluminate liquor to produce a powder having a whiteness level of 85% or more based on 100% TiO ₂ standard, A-30 alumina hydrate sold by Alcoa, Australia. Whiteness higher than the whiteness level of about 65-75% associated with the rate and significantly whiter than that of Alcoa (about 6%) C-31 to produce a hydrate which can be manufactured cheaply than hydrate would like.

It also provides an improved chemical grade hydrate having a low insolubles content for the zeolite, alum and sodium aluminate markets, and overcomes the problems and disadvantages of the above pretreatment methods. It would also be desirable to do.

The majority of the whiteness measurements given here are T
The instrument was taken using an technidyne luminance meter model S-4, but the instrument is a Technical Assoc.
i-ation of Pull & Paper I
The requirements for ndustry (or TAPPI) were maintained annually and corrected by purchased standards.
The hydrate color on absorption was measured with a filtered 13.3% caustic solution of hydrated alumina prepared in a Parr bomb at 140 ± 5 ° C. for 2 hours. The solution was protected from exposure to light. Light absorption is then B
ausch & Lomb Spectronic 200
Measured on a 0 spectrophotometer at 450 nm in a 10 cm cell, corrected for a blank caustic solution.

[0012]

SUMMARY OF THE INVENTION In accordance with the foregoing, a caustic solution, typically re-digested aluminum hydrate, a solution of supersaturated sodium aluminate, or aluminum hydroxide or water removed from the Bayer stream. Methods are provided for improving the whiteness level of a compound. One preferred embodiment of this method is (a) filtering the solution to remove contaminants,
Except for those whose size is typically 50 microns or less, (b) this filtrate is of the formula A _W B _X (OH) _Y C _Z ·
Contact with an adsorbent consisting essentially of a calcined compound with nH ₂ O, where A is Mg ²⁺ , Ni ²⁺ , F
represents a divalent metal cation selected from the group consisting of e ²⁺ , Ca ²⁺ and Zn ²⁺ , and B represents a trivalent metal selected from the group consisting of Al ³⁺ , Fe ³⁺ and Cu ^3+. Represents a metal cation of, and C is OH ⁻ , Cl ⁻ , Br ⁻ , NO ₃ ⁻ , CH.
₃ COO ⁻ , C ₂ O ₄ ²⁻ , CO ₃ ²⁻ , SO ₄ ²⁻ , PO ₄
^Represents a monovalent to tetravalent anion selected from the group consisting of ^3- , Fe (CN) ₆ ^3- and Fe (CN) ₆ ^4- , and w, x, y, z and n satisfy the following conditions: , 0 <z ≦ x ≦ 4
≦ w ≦ 1 / 2y and 12 ≧ n ≧ 1/2 (w−x)), followed by (c) adsorbing additional contaminants onto the calcined compound.

[0013] In one preferred basis, the method further comprises (d) separating or filtering the contacted compounds and additional contaminants from the solution after step (c).
Yet another preferred step is to add high whiteness, preferably about 85% or more based on 100% TiO ₂ standard, of aluminum hydroxide as a seed material to the solution. In some cases, this species may be a finely divided portion of the aluminum hydroxide formed during step (d) above.
As an optional step, (i) recalcining the contacted compound separated from the solution in step (d), and (ii) contacting this recalcined compound with an additional solution, thereby increasing efficiency Recycling the adsorbent as described above. The adsorbent in step (b) preferably consists essentially of calcined or activated hydrotalcite (sometimes abbreviated as "Activ.HTC" in the accompanying figures and tables).
The product is added to the caustic stream preferably in an amount of less than or equal to 1.0 g / l, more preferably less than or equal to about 0.3 g / l, or as little as about 0.1 g / l for a dosage of about 15-30 minutes. In addition to the total processing time below. The present invention further provides 100
% Ti0 ₂ to provide a standard improvement of more than about 85%, based on the whiteness of the aluminum hydroxide.

In describing this preferred embodiment,
With a sufficient amount of calcined or activated hydrotalcite
Aluminum using a filtration step before and / or after contact
There is repeated reference to the treatment of sodium acid. Only
However, the present invention derives from NaOH and certain paper processing technologies.
Suitable for other caustic streams including that solution known to
It should be understood to be used. Activated hydrota
Lucite is the preferred adsorption used in combination with filtration
The present invention is not limited to the formula A_wB_x(OH)_yC_z・ NH
₂Further metal hydroxides belonging to structural groups with O
Sometimes things are used. In the above formula, A is a divalent cation
, B represents a trivalent cation, and C represents a monovalent to tetravalent cation.
Represents an anion, and w, x, y, z and n are
Relationship, o <z ≦ z ≦ x ≦ 4 ≦ w ≦ 1 / 2y and 12 ≦
n ≦ 1/2 (w−x) is satisfied. Especially in this group
The preferred member is often of formula A₆B₂(OH)₁₆C_z・
4H ₂Identified by O. A in the previous formula is Mg²⁺, N
i²⁺, Fe²⁺, Ca²⁺And / or Zn²⁺And B
Is Al³⁺, Fe³⁺, And / or Cr³⁺And that
C is OH^-, Cl^-, Br^-, NO₃ ^-, CH₃C
OO^-, C₂O_Four ^2-, CO ₃ ^2-, SO_Four ^2-, PO_Four ^3-,
Fe (CN)₆ ^3-And Fe (CN)₆ ^Four-A list containing
One or more anions selected from
1/2 ≦ z ≦ 2, depending on the anion charge being applied.
It Several publications have described these compounds as hydrota
Collectively described as Lucite. However, the present invention
For the purpose of, such compounds are their alternating blue
Bivalent and trivalent in site like layers
It was divided into various subgroups according to the cations. example
For example, Pyroaurites (or
Or “sjogrenetes”) is the basic formula Mg₆Fe
₂OH₁₆CO₃・ 4H₂Has O. On the other hand, tacobite
(Takavites) is the expression, Ni₆Al₂OH₁₆CO
₃・ 4H₂Includes compounds similar to O.

Another of the terms "hydrotalcite"
Definition is the formula Mg₆Al₂OH₁₆CO ₃・ 4H₂Satisfied O
It includes all natural or synthetic compounds. This is sometimes
Is 6MgO.Al₂O₃・ CO₂・ 12H₂As O
Can be rewritten. In its ionic form, hydrotal
The site is [Mg₆Al₂(OH)₁₆]²⁺[CO₃]^2-・
4H₂Appears as O. The main structural unit of this compound is
Lucite or magnesium hydroxide
It has a toroidal shape and Mg ions share multiple edges.
Located between the ions (OH). Magnesi in this structure
By replacing any of the um with trivalent aluminum
And still maintain the basic sheet-like structure of brucite.
A bottom layer of magnesium and aluminum is created.
Charge imbalance resulting from such aluminum ion substitution
To compensate for the anion (in the above equation, the "C"
(Indicated by letters) and water molecules are inserted,
Between such brucite-like layers (C_z・ NH
₂The intermediate layer of O) is formed. Greatest parent to combine with water
Anions that have hydrodynamic power and form hydrotalcite are
Carbonate (CO₃ ^2-).

The spatial distribution of carbonate ions in hydrotalcite can vary depending on how freely the Al ³⁺ ions replace the Mg ²⁺ ions therein. The spacing of brucite layers also depends on the amount of aluminum displaced into the basic structure of hydrotalcite. As the aluminum displacement increases, the interlayer spacing generally decreases due to the increased electrostatic attraction between the hydrotalcite positive hydroxide layer and the negative interlayer. The thickness of the intermediate layer may be further varied depending on the size and arrangement of those anions replaced by carbonate ions in the basic structure of hydrotalcite.

Natural deposits of hydrotalcite have been found in the Snarum, Norway and Ural Mountains. Typical occurrences are in the form of serpentine, talc schist, or as spinel pseudomorphs. Like most ores, natural hydrotalcites are virtually impossible to find in pure form. Natural ores typically contain one or more other minerals including magnesia chlorite and muscovite.

Several methods are known for producing synthetic hydrotalcite. A more common way is to use hydrotalcite as a fine powder, -20 mesh granules,
Alternatively, it is manufactured as an extruded product having a 1/8 inch diameter. In U.S. Pat. No. 3,539,306, aluminum hydroxide, aluminum amino acid salts, aluminum alcoholates, water-soluble aluminates, aluminum nitrate and / or aluminum sulfate are compounds containing carbonate ions, and magnesium oxide, hydroxide. It is mixed with a magnesium component selected from magnesium or a water-soluble magnesium salt in an aqueous medium maintained at a pH of 8 or higher. The resulting product is used as a gastric antacid according to the above-mentioned document. Mi
Reissued U.S. Pat. No. 34,164 to sra, the disclosure of which is fully incorporated herein by reference, discloses another method of synthesizing hydrotalcite.
This method consists of heating magnesium carbonate and / or magnesium hydroxide to make active magnesia, then mixing the latter with an aqueous solution of aluminate, carbonate and hydroxyl ions.

Another known method for synthesizing hydrotalcite involves adding dry ice or ammonium carbonate to a mixture of magnesium oxide and alpha-alumina. Yet another method ( The Amer
ican Mineral-ogist , Vol. 5
2, pp. 1036-1047 (1967))
Produces a hydrotalcite-like material by titrating MgCl ₂ and AlCl ₃ with NaOH without carbon dioxide. The suspension thus produced is at 60 ° C.
It is dialyzed for 30 days at room temperature to form a hydrated Mg-Al carbonate hydroxide with the properties of both hydrotalcite and man-siteite.

In a preferred embodiment of the invention, the caustic solution may be treated by contact with a material consisting essentially of calcined or activated hydrotalcite. By using the term "consisting essentially of ..."
It is meant that all contacting adsorbents should contain 85 or 90%, and more preferably about 95 to 98% hydrotalcite.

In its fully dehydrated state, the calcined hydrotalcite has the formula: Mg ₆ Al ₂ O ₈ (OH)
Believed to have _two . When only partially activated or calcined, hydrotalcite contains relatively large amounts of water ions. In an alternative embodiment, granulated calcined hydrotalcite may be used, and said granule form is about 10 to 35 hydrotalcite powder.
% By mixing with one or more binder materials.

The activation or heat treatment of hydrotalcite to form the calcination transformation may be carried out in any conventional or newly developed medium maintained at a temperature between about 400 and 650 ° C. it can. Preferred activation /
The calcination temperature, between about 425-550 or 600 ° C., generally maximizes the surface area and pore volume of this compound.

After heat activation, a porous, skeletal structure is formed from which most, if not all, of the water and carbonate ions have been driven off. This product has an average pore diameter of about 55 Å, about 2.9 g / c.
m ³ skeleton (or solid component) density, and 0.3 cm
^It has a total pore volume of ³ / g. The specific surface area of hydrotalcite is also 20 m ² / g to about 50-200 m ² / g
It is known to increase during such time (as measured by BET nitrogen adsorption) after such thermal activation.

In a first embodiment of the invention, FIG.
Aluminum hydroxide with improved whiteness / whiteness, calcined hydrotalcite, pyroaurite
-Aurite), tachovite,
Alternatively, it is produced by contacting a solution containing hydroxide with a substance consisting essentially of a mixture thereof. The contacted material is then preferably separated from this solution for recalcination and recycling. Such recalcination and recirculation is optional and may not be necessary if the adsorbent input is low. Aluminum hydroxide species with high whiteness, 85% or more, are then typically about 60-85 ° C (1
Used as a seed material for initiating precipitation from solution to generate purer aluminum hydroxide at one or more temperatures between 40-185 ° F). After initiation, the high whiteness level of this product can be maintained by recycling the classified species from the settling stream.

In the second embodiment, referring to FIG.
The color contamination level of the luminate solution is then determined by Bayer
Reduced by pre-washing tray species to remove liquid
To be made. The tray species are then re-digested, resulting in
The resulting sodium aluminate solution has the formula A₆B (OH)₁₆
C-4H₂Contacted with the calcined form of the adsorbent with O
It A in the previous formula is Mg²⁺, Ni²⁺, Fe²⁺, Ca²⁺And
And Zn²⁺Is selected from the group consisting of³⁺, Fe
³⁺And Cr³⁺Is selected from the group consisting of, and C is
OH^-, Cl^-, Br^-, NO₃ ^-, CH₃COO^-,
C₂O_Four ^2-, CO ₃ ^2-, SO_Four ^2-, PO_Four ^3-, Fe (C
N)₆ ^3-And Fe (CN)₆ ^Four-Selected from the group consisting of
And 1/2 ≦ z ≦ 2. Contacted or used
The adsorbent then produced whiter aluminum hydroxide
Separated from solution to precipitate.

In a preferred embodiment, the powdered calcined hydrotalcite is added directly to the caustic solution (or stock solution) to be treated. The amount of adsorbent added can be determined by testing a representative sample to avoid underdosing or, more importantly, uneconomical overdosing. With the additional filtration step of the present invention, significantly lower active hydrotalcite dosages (compared to the prior art) will achieve higher whiteness levels than achieved with known methods. In particularly preferred conditions, doses of 1.0 g / l or less proved sufficient by prefiltration, but smaller doses of about 0.5 g
/ L, 0.2 g / l or even 0.1 g / l also removed a sufficient amount of color / contamination. Although the total contact time can vary from a few minutes to over an hour, current data according to this embodiment of the invention show that a total treatment time of about 15 minutes or less achieves sufficient colorant removal. Show.

Saturated, spent or contacted adsorbents are usually separated from caustic liquids by known or successively developed methods such as filtration, use of primary separators, gravity settling and / or centrifugation. Excluded. In particularly preferred conditions, a pressure filter is used. Even higher hydrate whiteness may be achieved if the contacting and settling of the activated hydrotalcite is followed by another filtration step (according to FIG. 3). Such post-contact filtration is believed to remove from the solution almost all of the inactivated form of hydrotalcite formed during the process, along with any organic matter that is adsorbed or otherwise insolubilized.

Once the calcined hydrotalcite powder has been mixed with one or more binders and then extruded, molded or otherwise formed into large particles, such particles are then separated from the column, It can be packed in a fluidized bed or other container through which the solution can pass. A third alternative contacting method involves exposing the caustic solution to a calcined hydrotalcite semi-solid sludge, or slurry. This processing method is particularly applicable to NaFeO ₂ , FeOO
Good for removing typical contaminants such as H, Fe ₂ O ₃ and even some humic acids.

Immediate filtration after contact of the solution with a material consisting essentially of calcined hydrotalcite, pyroaurite, tacobite, or mixtures thereof is also aluminum hydroxide of improved whiteness / whiteness. Occurs, but it is a less preferable condition. The contacted adsorbent separated from this solution is preferably recalcined and recycled to a continuous or semi-continuous process for cost efficiency.

The method of the present invention is generally susceptible to adsorbing divalent, trivalent or higher charged electron-negative or anionic colorants. Calcined hydrotalcites can also remove monovalent contaminants from solution, although less preferred conditions. Without being limited to any particular theory of action, the preferred embodiment proceeds as follows. Upon calcination (or activation), both carbonate and water are expelled from the basic structure of hydrotalcite according to the formula:

[0031]

## STR1 ## _{Mg 6 Al 2 OH 16 CO 3} · 4H 2 O → Mg
₆ Al ₂ O ₈ (OH) ₂ + CO ₂ + 11H ₂ O

Contact with anionic or electron-negative contaminants then gives rise to the coloration mentioned above, which results in empty anions in the calcined product during contact with the solution and rehydration. Occupy the position of. For some contaminants, it is believed that the tricalcium aluminate intermediate is formed upon re-digestion of hydrate. This intermediate then attracts hydrophobic, high molecular weight, organic coloring molecules so that they are removed together during the post-contact filtration operation.

[0033]

[Example] The following ambient conditions (atmospheric)
For the example, a concentrated synthetic sodium aluminate solution was prepared as follows. 536 g NaOH pellets were added to 1484 g deionized water in a stainless steel beaker with overhead stirring. The temperature of the solution was raised from 18 ° C to 88 ° C using a heating plate to dissolve the NaOH. 724 g of A-30 hydrate (65.0% whiteness) was added to this solution and stirred.
After the mixed solution was evaporated and boiled down, additional deionized water was added. The solution was then filtered through # 40 paper for 45 minutes, measured for specific gravity and refrigerated. The goal is Na ₂ CO
₃ total caustic value (T / C) of 352 g / l, 0.66
Alumina to caustic ratio of 7, 235 g / l of Al ₂ O ₃ and a specific gravity of 1.372.

[0034]

[Table 1]

The following control conditions were then maintained for many of the tests below. Species: 50g sieved to 200/325 mesh
/ L C-30 hydrate (76.8% whiteness), Hydrotalcite: digested hydrate when treated with activated hydrotalcite, lab activated at 550 ° C for 80 minutes. Solution temperature: 95 ° C., synthesis solution: total caustic = 175 g / l (as Na ₂ CO ₃ ); AC ratio = 0.6; specific gravity = 1.191, precipitation conditions: 74 ° C. water bath for 24 hours.

The following series of tests were conducted using the solutions prepared under ambient conditions. Test 1: Control sample-no filtration and no addition of activated hydrotalcite, Test 2: Only one filtration step was performed, Test 3: Activated hydrotalcite treatment only, Test 4: Activated hydrotalcite treatment. And subsequent filtration, Test 5: Perform only one filtration step (as a repeat or reference sample), Test 6: Perform two filtration steps, Test 7: Pre-filter before activated hydrotalcite treatment, And test 8: pre-filtration, activated hydrotalcite treatment and post-filtration. This series of tests yielded the following% whiteness and absorbance measurements.

[0037]

[Table 2]

The second series of tests consisted of 1 in the Parr reactor.
It was carried out in a caustic solution that had been cylinder digested at 43 ° C for 30 minutes, because it redissolved the alumina into the caustic solution quickly and completely, and chemically changed some of the organic colorants present. The purpose was to simulate the conditions of the digester of the factory that can be used. This gave the following absorbance and% whiteness level results, the latter of which is summarized graphically in FIG.

[0039]

[Table 3]

A series of tests were then conducted using various adsorbent exposure times and doses (with pre and post filtration) to obtain the following brightness data results.

[0041]

[Table 4]

A series of tests were also conducted to determine the recyclability of activated hydrotalcite in this method. Each cycle of the four cycle test used a fresh supply of re-digested hydrate solution. The synthetic caustic solution contained 0.33 g / l of adsorbent in the first cycle, and the following cumulative filtered solids, which contained hydrotalcite reactivated in the second to fourth cycles: For 15 minutes. The contacted solids are filtered off, dried overnight and then at 550 ° C. for 8 hours.
It was reactivated for 0 minutes. In cycles 2 through 4, both C-30 and C-31 hydrates were used as seed material for comparison purposes.

[0043]

[Table 5]

From the above data, thermal regeneration and reuse of activated hydrotalcites over 4 cycles resulted in less than 10% reduction in overall color removal efficiency.

Having thus far described particularly preferred embodiments, it is to be understood that the invention can be embodied otherwise within the scope of the appended claims.

[Brief description of drawings]

Furthermore, the features, objects and benefits of the present invention will become more apparent from the detailed description of the preferred embodiments made with reference to the following drawings.

FIG. 1 is a flow chart depicting one embodiment for producing the improved aluminum hydroxide according to the present invention. The above embodiment comprises contacting the sodium aluminate solution with an activated hydrotalcite adsorbent prior to filtration (optional steps of this embodiment are represented in the dashed box. ).

FIG. 2 is a flow chart of the second embodiment, which filters re-digested tray seed liquor after contact with activated hydrotalcite.

FIG. 3 is a flow chart depicting a third embodiment of the present invention, which filters a liquid stream both before and after contact with activated hydrotalcite.

FIG. 4 is a flow chart depicting the quantitative improvement in whiteness achieved when treating a caustic liquor prepared at atmospheric pressure according to the present invention.

FIG. 5 is a flow chart depicting the quantitative improvement in whiteness achieved when such a solution is subjected to simulated pressurized (“bomb”) digestion.

FIG. 6 is a graph comparing average whiteness levels (y-axis) achieved under various filtration conditions (x-axis).

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Nancy M. Fitzgerald Dowling Avenue, Pittsburgh, PA, USA 3901 (72) Inventor Bennett El. McCormick 1215 Linden Street, Cheswick, PA

Claims (8)

[Claims] 1. A solution of caustic or sodium aluminate
The whiteness level of aluminum hydroxide removed from the solution was revised.
To improve, (a) add the solution to the formula A₆B₂(OH)₁₆C_Z・ 4H₂O
 (In the formula, A is Mg²⁺, Ni²⁺, Fe²⁺, Ca²⁺And Zn
²⁺Is selected from the group consisting of³⁺, Fe ³⁺and
Cu³⁺Selected from the group consisting of, C is OH^-, Cl^-,
Br^-, NO₃ ^-, CH₃COO^-, C₂O_Four ^2-, CO₃
^2-, SO_Four ^2-, PO_Four ^3-, Fe (CN)₆ ^3-And F
e (CN)₆ ^Four-Selected from the group consisting of, and 1/2
≦ z ≦ 2) more essential than the calcined compound
Contacting with an adsorbent consisting of, and (b) filtering the solution before and / or after said contacting.
Step, consisting of caustic solution or sodium aluminate
The whiteness level of aluminum hydroxide removed from the solution.
How to improve. 2. The method of claim 1, further comprising the step of: (c) adding to the solution aluminum hydroxide having a whiteness level of about 85% or higher based on 100% TiO ₂ standard as a seed material. The method described. 3. Further comprising: (d) separating the calcined compound from the solution, (e) recalcining the separated compound, and (f) adding the recalcined compound. The method according to claim 1 or 2, comprising the step of recycling into a solution. 4. A process according to claim 1 or 2, wherein step (a) comprises contacting the solution with about 1.0 g / l or less of activated hydrotalcite. 5. The method of claim 4 which comprises contacting the solution with about 0.3 g / l or less of activated hydrotalcite. 6. The method according to claim 1, wherein the solution contains a solute consisting of sodium hydroxide, sodium carbonate or sodium aluminate, or a mixture thereof. 7. The method of claim 1, further comprising recycling the calcined compound into an additional solution. 8. Removed from the caustic solution, and (a) said solution.
Liquid is the formula A₆ B₂ (OH)₁₆C_Z・ 4H₂O (where A is
Mg²⁺, Ni²⁺, Fe²⁺, Ca²⁺And Zn²⁺Consisting of
Selected from the group, B is Al³⁺, Fe ³⁺And Cu³⁺From
Selected from the group consisting of C and OH^-, Cl^-, Br^-, N
O₃ ^-, CH₃COO^-, C₂ O_Four ^2-, CO₃ ^2-, SO_Four
^2-, PO_Four ^3-, Fe (CN)₆ ^3-And Fe (CN)₆
^Four-Selected from the group consisting of, and with 1/2 ≦ z ≦ 2
An adsorption consisting essentially of a calcined compound having
A step of contacting with an agent, and (b) filtering and contacting the solution.
Remove the calcination compounds and contaminants that were touched,
And the above-mentioned filtration is carried out by (ii) before the step (a)
After step (a) or (iii) before step (a)
And subsequent steps, and (c) adding a seed material to the solution.
Then 100% TiO₂About 85% or less
Add aluminum hydroxide with whiteness level above
Is a product decolorized by the method consisting of
100% TiO₂About 85% or more based on standard products
Improved aluminum hydroxide having a whiteness level above
Product.