Method for improving the brightness of aluminum hydroxide

There is disclosed a method for improving the brightness level of aluminum hydroxide removed from a caustic solution. The method comprises:(a) pre-filtering the solution to remove contaminants therefrom; and (b) contacting the filtered solution with an adsorbent consisting essentially of a calcined compound having the formula: A.sub.w B.sub.x (OH).sub.y C.sub.z.nH.sub.2 O wherein A represents a divalent metal cation selected from the group consisting of: Mg.sup.2+, Ni.sup.2+, Fe.sup.2+, Ca.sub.2+ and Zn.sup.2+ ; B represents a trivalent metal cation selected from the group consisting of: Al.sup.3+, Fe.sup.3+ and Cu.sup.3+ ; C represents a mono- to tetravalent anion selected from the group consisting of: OH.sup.-, Cl.sup.-, Br.sup.-, NO.sub.3.sup.- CH.sub.3 COO.sup.-, C.sub.2 O.sub.4.sup.2-, CO.sub.3.sup.2-, SO.sub.4.sup.2-, PO.sub.4.sup.3-, Fe(CN).sub.6.sup.3- and Fe(CN).sub.6.sup.4- ; and w, x, y, z and n satisfy the following: 0<z.ltoreq.x.ltoreq.4.ltoreq.w.ltoreq. 1/2 y and 12.gtoreq.n.gtoreq. 1/2 (w-x); and (c) adsorbing additional contaminants onto the calcined compound. On a preferred basis, this method also includes: (d) filtering or sepiating contacted compound and additional contaminants from the solution. Still further preferred method steps include: adding to the filtered solution as seed material an aluminum hydroxide of high brightness, preferably with a whiteness level of about 85% or higher based on a 100% TiO.sub.2 reference standard. Preferably, the method further includes: (i) separating calcined compound from the solution; (ii) recalcining the separated compound; and (iii) contacting recalcined compound with additional solution. A preferred adsorbent for step (b) consists essentially of calcined hydrotalcite. There is also provided an improved aluminum hydroxide product having a whiteness level of 85% or higher based on a 100% TiO.sub.2 reference standard, said compound having been removed from a caustic solution and decolorized by the foregoing method.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the purification of caustic liquors and high 
caustic streams, especially those relating to the production of aluminum 
hydroxides (also called alumina trihydrates) and aluminas from bauxite 
according to the Bayer process (sometimes referred to as Bayer liquor 
streams). The invention further relates to means for making an 
intermediate grade aluminum hydroxide/hydrate product in terms of 
brightness, i.e., having a whiteness level of about 85% or higher, more 
preferably about 90 or 92% whiteness, based on a 100% TiO.sub.2 standard 
and a target adsorbence level of about 0.20. For purposes of this 
invention, the terms "brightness" and "whiteness" are used interchangably. 
2. Technology Review 
The recovery of aluminum hydroxide from bauxite according to the Bayer 
process is achieved by digesting hydroxide-containing ores with a caustic 
liquor. A major portion of alumina dissolves in this liquor while most 
unwanted ore constituents, sometimes called "red mud", remain undissolved. 
After bauxite is pressure digested with a caustic such as sodium 
hydroxide, red mud may be removed from this liquor. Aluminum hydroxide is 
then separated from a liquor of supersaturated sodium aluminate, also 
known as "green" or "pregnant" liquor, typically by precipitation. During 
such precipitation, the supersaturated sodium aluminate is cooled and 
mixed with a slurry of aluminum hydroxide particles acting as seed 
material, or seed stock, to induce the formation of more aluminum 
hydroxide. After precipitation, the slurry is pumped through a classifier 
system where a coarse fraction of crystallized aluminum hydroxide is 
separated from the liquor. The fine fractions of crystallized (or 
precipitated) aluminum hydroxide are further classified into two finer 
fractions called "secondary seed" and "tray seed", the latter being the 
finest fraction from this classifier system. These secondary and tray 
seeds are often recycled to a precipitator to act as seed for further 
aluminum hydroxide precipitation. The resulting spent sodium aluminate may 
be recycled to a digester for mixing with new (or incoming) bauxite. 
Sodium aluminate liquors can also be made by digesting precipitated 
aluminum hydroxide (primary), secondary seed, tray seed or combinations 
thereof in a method known as redigestion. Some Bayer plants produce excess 
seed (generally tray seed) which they may thereafter redigest by recycling 
to bauxite digesters or through a separate, specially designed digester. 
High levels of impurities are undesirable in the sodium aluminate liquor 
used to make aluminum hydroxide because such impurities decrease the 
whiteness or color purity of the hydroxide/hydrate precipitated therefrom. 
It is desirable, therefore, to minimize the presence of such impurities in 
green sodium aluminate liquors before crystallization takes place. Such 
impurities typically cause the aluminum hydroxide produced to have a 
whiteness level of about 75% or less based on a 100% TiO.sub.2 standard 
thus prohibiting their use in many applications where generally higher 
brightness levels (about 80% or above) are required. The present invention 
produces aluminum hydroxide having whiteness levels of about 85% or more 
based on the same 100% TiO.sub.2 reference standard. For some embodiments, 
whiteness levels of about 90 or 92% are achieved consistently. 
When organic and inorganic impurities are present, lower liquor 
productivity and reduced alumina purity result. Organic impurities may 
cause such other complications as: lower alumina yields; excessively fine 
hydroxide particles; the production of colored liquors and aluminum 
hydroxide; lower red mud settling rates; caustic losses due to sodium 
organic formation; an increase in liquor density; increased viscosities; 
higher boiling points; and unwanted liquor foaming. 
Numerous methods are known for removing colorants from a sodium aluminate 
liquor. These include Australian Patent No. 12085/83 which teaches 
treating liquors with reactive MgO or Mg(OH).sub.2 before calcining at 
900.degree. C. or more. Schepers et al. U.S. Pat. No. 4,046,855 also 
discloses treating aluminate liquors with a magnesium compound to remove 
organic colorants therefrom. Japanese Patent No. 57-31527 produces an 
aluminum hydroxide of high purity by adding one or more alkaline earth 
metal compounds to such solutions. Representative additives include 
oxides, hydroxides, carbonates, silicates and nitrates of magnesium or 
calcium; and barium carbonates, silicates, nitrates or sulfates. 
German Patent No. 3,501,350 discloses adding a mixture of calcium 
oxide/hydroxide and kierserite (MgSO.sub.4.H.sub.2 O) to Bayer liquors for 
lowering impurity levels, especially iron contents. In Goheen et al. U.S. 
Pat. No. 4,915,930, an aluminum hydroxide of improved whiteness is 
produced by contacting liquor streams with a mixture of tricalcium 
aluminate and unactivated hydrotalcite. Finally, in Nigro U.S. Pat. No. 
5,068,095, caustic solutions, are treated with calcined hydrotalcite to 
remove colorants, especially iron. High dosages of about 10 g/l were 
preferred to make hydroxides/hydrates with about 80% whiteness levels or 
higher. The present invention represents an improvement over the 
aforementioned Nigro et al. method. 
SUMMARY OF THE INVENTION 
It is a principal objective of this invention to provide uncomplicated 
means for removing ionic colorants and contaminants from caustics 
including sodium hydroxide and sodium aluminate. It is another objective 
to provide means for improving the whiteness/brightness levels of aluminum 
hydroxide with reduced product loss. It is still another objective to 
provide a low cost, low capital means for removing greater amounts of 
color contaminants from sodium aluminate liquors. Another objective is to 
provide means for treating sodium aluminate liquors to produce a powder 
with whiteness levels consistently over 85% based on a 100% TiO.sub.2 
standard. Yet another objective is to produce hydrates which are higher in 
brightness than the roughly 65-75% whiteness levels associated with A-30 
alumina hydrate as sold by Alcoa of Australia, while being less expensive 
to produce than Alcoa's significantly whiter (about 96%) C-31 hydrate. 
The majority of brighiness measurements described herein were taken using a 
Technidyne Bfightimeter Model S-4 maintained annually per Technical 
Association of Pulp & Paper Industry (or TAPPI) requirements and 
calibrated with purchased standards. Hydrate color by absorbance was 
measured on a filtered 13.3% caustic solution of hydrated alumina prepared 
in a Parr bomb at 140.degree..+-.5.degree. C. for 2 hours. The solution 
was protected from exposure to light. Light absorbance values were then 
measured on a Bausch & Lomb Spectronic 2000 Spectrophotometer at 450 nm in 
a 10 cm cell as corrected against a blank caustic solution. 
It is another principle objective to provide improved chemical grade 
hydrates with lower insoluble contents for the zeolite, alum and sodium 
aluminate markets. It is yet another objective to overcome the problems 
and disadvantages of the prior treatment methods described above. 
In accordance with the foregoing objectives, there is provided a method for 
improving the brightness level of aluminum hydroxide or hydrate removed 
from a caustic solution, typically of redigested aluminum hydrate, 
supersaturated sodium aluminate or a Bayer liquor stream. One preferred 
embodiment of this method comprises: (a) filtering the solution to remove 
contaminants, typically those which are 50 microns or less in size; (b) 
contacting this filtered solution with an adsorbent consisting essentially 
of a calcined compound having the formula A.sub.w B.sub.x (OH).sub.y 
C.sub.z.nH.sub.2 O wherein A represents a divalent metal cation selected 
from the group consisting of: Mg.sup.2+, Ni.sup.2+, Fe.sup.2+, Ca.sup.2+ 
and Zn.sup.2+ ; B represents a trivalent metal cation selected from the 
group consisting of: Al.sup.3+, Fe.sup.3+ and Cu.sup.3+ ; C represents a 
mono- to tetravalent anion selected from the group consisting of: 
OH.sup.-, Cl.sup.-, Br.sup.-, NO.sub.3.sup.- CH.sub.3 COO.sup.-, C.sub.2 
O.sub.4.sup.2-, CO.sub.3.sup.2-, SO.sub.4.sup.2-, PO.sub.4.sup.3-, 
Fe(CN).sub.6.sup.3- and Fe(CN).sub.6.sup.4- ; and w, x, y, z and n satisfy 
the following: 0&lt;z.ltoreq.x.ltoreq.4.ltoreq.w.ltoreq. 1/2 y and 
12.gtoreq.n.gtoreq. 1/2 (w-x) followed by (c) adsorbing additional 
contaminants onto this calcined compound. On a preferred basis, the 
foregoing method further includes: (d) separating or filtering contacted 
compound and additional contaminants from the solution after step (c). Yet 
another preferred step adds to the solution as seed material an aluminum 
hydroxide of high brightness, preferably about 85% or higher based on a 
100% TiO.sub.2 reference standard. In some instances, this seed may be a 
fine classified fraction of the aluminum hydroxide produced during above 
step (d). Optional method steps include: (i) recalcining contacted 
compound separated from the solution in step (d); and (ii) contacting this 
recalcined compound with additional solution thereby recycling said 
adsorbent for enhanced efficiencies. The adsorbent in step (b) preferably 
consists essentially of calcined or activated hydrotalcite (sometimes 
abbreviated as "Activ. HTC" in the accompanying FIGURES and TABLES). This 
product is delivered to a caustic stream in preferred dosages of 1.0 g/l 
or less, more preferably about 0.3 g/l or less, even as low as about 0.1 
g/l or less, for total treatment times of about 15-30 minutes or less. The 
invention further provides aluminum hydroxide of improved whiteness, about 
85% or higher based on a 100% TiO.sub.2 reference standard.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
In this description of preferred embodiments, repeated reference is made to 
the treatment of sodium aluminate liquors using filtration steps before 
and/or after contact with sufficient amounts of calcined or activated 
hydrotalcite. The invention should be understood to apply to still other 
caustic streams, however, including NaOH and the liquor solutions known to 
result from certain paper processing techniques. While activated 
hydrotalcite is the preferred adsorbent used in combination with 
filtering, the invention may employ still other metal hydroxides belonging 
to the structural family having the formula: A.sub.w B.sub.x (OH).sub.y 
C.sub.z.nH.sub.2 O, wherein A represents a divalent metal cation, B a 
trivalent metal cation, C a mono-to tetravelent anion, and w, x, y, z and 
n satisfy the following: 0&lt;z.ltoreq.x.ltoreq.4.ltoreq.w.ltoreq. 1/2 y and 
12.ltoreq.n.ltoreq. 1/2 (w-x). Preferred members of this family have often 
been identified by the formula: A.sub.6 B.sub.2 (OH).sub.16 
C.sub.z.4H.sub.2 O, wherein A is: Mg.sup.2+, Ni.sup.2+, Fe.sup.2+, 
Ca.sup.2+ and/or Zn.sup.2+ ; B is: Al.sup.3+, Fe.sup.3+ and/or Cr.sup.3+ ; 
and C is one or more anions selected from the list which includes: 
OH.sup.-, Cl.sup.-, Br.sup.-, NO.sub.3.sup.-, CH.sub.3 COO.sup.-, C.sub.2 
O.sub.4.sup.2-, CO.sub.3.sup.2-, SO.sub.42.sup.2-, PO.sub.4.sup.3-, 
Fe(CN).sub.6.sup.3-, and Fe(CN).sub.6.sup.4- with 1/2.ltoreq.z.ltoreq.2 
depending on the anionic charge being substituted. Some publications 
collectively describe many of these foregoing compounds as hydrotalcites. 
For purposes of this invention, though, such compounds have been divided 
into various subgroups depending on the divalent and trivalent cations 
within their alternating brucite-like layers. For example, pyroaurites (or 
"sjogrenites") have the basic formula: Mg.sub.6 Fe.sub.2 OH.sub.16 
CO.sub.3.4H.sub.2 O. Takovites, on the other hand, include compounds 
resembling: Ni.sub.6 Al.sub.2 OH.sub.16 CO.sub.3.4H.sub.2 O. 
Another definition for the term "hydrotalcite" includes any natural or 
synthetic compound satisfying the formula: Mg.sub.6 Al.sub.2 OH.sub.16 
CO.sub.3.4H.sub.2 O. This is sometimes rewritten as 6MgO.Al.sub.2 
O.sub.3.CO.sub.2.12H.sub.2 O. In its ionic form, hydrotalcite appears as: 
[Mg.sub.6 Al.sub.2 (OH).sub.16 ].sup.2+ [CO.sub.3 ].sup.2-.4H.sub.2 O. The 
main structural unit for this compound is brucite, or magnesium hydroxide, 
in an octagonal sheet-like form, with Mg ions positioned between multiple 
(OH) ions which share a common edge. By substituting trivalent aluminum 
for some of the magnesium in this structure, sublayers of magnesium and 
aluminum are created while still maintaining brucite's basic sheet-like 
structure. To compensate for the charge imbalance from such aluminum ion 
substitutions, anions (indicated by letter "C" in the foregoing formulae) 
and water molecules are intercalated to form interlayers of 
(C.sub.z.nH.sub.2 O) between such brucite-like layers. The anion with the 
greatest affinity to combine with water and form hydrotalcite is carbonate 
(CO.sub.3.sup.2-). 
The spacial distribution of carbonate ions within hydrotalcite can vary 
depending on how freely Al.sup.3.sup.2+ ions substitute from the Mg.sup.2+ 
ions therein. Brucite layer spacing also depends on the amount of aluminum 
substituted into hydrotalcite's basic structure. As aluminum substitution 
increases, interlayer spacing generally decreases due to an increase in 
the electrostatic attraction between the positive hydroxide layers and 
negative interlayers of hydrotalcite. Interlayer thicknesses may vary 
still further with the size and orientation of those anions substituted 
for carbonate in the basic structure of hydrotalcite. 
Natural deposits of hydrotalcites have been found in Snarum, Norway and the 
Ural Mountains. Typical occurrences are in the form of serpentines, talc 
schists, or as a spinel pseudomorph. Like most ores, natural hydrotalcite 
is virtually impossible to find in a pure state. Natural deposits 
typically contain one or more other minerals including penninite and 
muscovite. 
Several methods for making synthetic hydrotalcite are known. The more 
common approaches produce hydrotalcite as a fine powder, in -20 mesh 
granules, or as 1/8 inch diameter extrudates. In U.S. Pat. No. 3,539,306, 
an aluminum hydroxide, aluminum-amino acid salt, aluminum alcoholate, 
water soluble aluminate, aluminum nitrate and/or aluminum sulfate are 
mixed together with a carbonate ion-containing compound and magnesium 
component selected from magnesium oxide, magnesium hydroxide or 
water-soluble magnesium salt in an aqueous medium maintained at a pH of 8 
or more. The resulting product is used as a stomach antacid according to 
that reference. In Misra U.S. Pat. No. Re. 34,164, the disclosure of which 
is fully incorporated by reference herein, yet another method for 
synthesizing hydrotalcite is disclosed. The method comprises heating 
magnesium carbonate and/or magnesium hydroxide to form activated magnesia, 
then combining the latter with an aqueous solution of aluminate, carbonate 
and hydroxyl ions. 
Another known method for synthesizing hydrotalcite adds dry ice or ammonium 
carbonate to a magnesium oxide and alpha alumina mixture. Yet another 
process, described in The American Minerologist, Vol. 52, pp. 1036-1047 
(1967), produces hydrotalcite-like materials by titrating MgCl.sub.2 and 
AlCl.sub.3 with a carbon dioxide-free NaOH system. The resulting 
suspension is dialyzed for 30 days at 60.degree. C. to form a hydrated 
Mg--Al carbonate hydroxide with both hydrotalcite and manasseite 
properties. 
In preferred embodiments of this invention, caustic solutions may be 
treated by contact with a substance consisting essentially of calcined or 
activated hydrotalcite. By use of the term "consisting essentially", it is 
meant that any contacting adsorbent should contain greater than 85 or 90% 
activated hydrotalcite, and more preferably about 95 to 98%. 
In its fully dehydrated state, calcined hydrotalcite is believed to have 
the formula: Mg.sub.6 Al.sub.2 O.sub.8 (OH).sub.2. When only partially 
activated or calcined, hydrotalcite contains more water ions. In 
alternative embodiments, a granular calcined hydrotalcite may be used, 
said granular form being made by combining hydrotalcite powders with about 
10 to 35% of one or more binder materials. 
The activation or heat treatment of hydrotalcite to form calcined variants 
may be carried out in any conventional or newly-developed medium 
maintained at temperatures between about 400.degree.-650.degree. C. 
Preferred activation/calcination temperatures, between about 
425.degree.-550.degree.or 600.degree. C., generally maximize surface area 
and pore volumes for this compound. 
After thermal activation, a substance having a porous, skeletal structure 
is produced from which most if not all water and carbonate ions have been 
expelled. This product has: an average pore diameter of about 55 
angstroms; a skeletal (or solid component) density of about 2.9 g/cm.sup.3 
; and total pore volume of about 0.3 cm.sup.3 /g. The specific surface, 
areas of hydrotalcite are also known to increase from about 20 m.sup.2 /g 
to between about 50-200 m.sup.2 /g (as determined by BET nitrogen 
adsorption) after such thermal activation. 
In a first embodiment of this invention. FIG. 1a, aluminum hydroxide of 
improved whiteness/brightness is made by contacting a hydroxide-containing 
solution with a substance consisting essentially of: calcined 
hydrotalcite, pyroaurite, takovite, or mixtures thereof. Contacted 
substance is then separated from this solution for preferably recalcining 
and recycle. Such recalcining and recycle are optional and may not be 
necessary if adsorbent dosages are low. An aluminum hydroxide seed of high 
whiteness, 85% or higher, is then used as seed stock for causing a purer 
aluminum hydroxide to start precipitating from the solution, typically at 
one or more temperatures between about 60.degree.-85.degree. C. 
(140.degree.-185.degree. F.). After start-up, the high brightness level of 
this product can be maintained by recycling classified seed from the 
precipitation stream. 
In a second embodiment, per FIG. 1b, color contaminant levels of an 
aluminate solution are lowered by pre-washing tray seed to remove Bayer 
liquor, therefrom. The tray seed is then redigested and the resultant 
sodium aluminate liquor contacted with the calcined form of an adsorbent 
having the formula A.sub.6 B.sub.2 (OH).sub.16 C.4H.sub.2 O, wherein A is 
selected from the group consisting of: Mg.sup.2+, Ni.sup.2+, Fe.sup.2+, 
Ca.sup.2+ and Zn.sup.2+ ; B from: Al.sup.3+, Fe.sup.3+ and Cr.sup.3+ ; and 
C from: OH.sup.-, Cl.sup.-, Br.sup.-, NO.sub.3.sup.-, CH.sub.3 COO.sup.-, 
C.sub.2 O.sub.4.sup.2-, CO.sub.3.sup.2-, SO.sub.4.sup.2-, PO.sub.4.sup.3-, 
Fe(CN).sub.6.sup.3- and Fe(CN).sub.6.sup.4- with 1/2.ltoreq.z.ltoreq.2. 
Contacted or spent adsorbent is then separated from the solution to 
precipitate a whiter aluminum hydroxide product. 
In preferred embodiments, powdered forms of calcined hydrotalcite are added 
directly to the caustic solution (or liquor) being treated. The amount of 
adsorbent to be added may be determined by testing representative samples 
so as to avoid underdosing or, more importantly, wasteful overdosing. With 
the added filtration step, or steps, of this invention, significantly 
lower dosages of activated hydrotalcite (as compared with the prior art 
will achieve higher whiteness levels than realized with prior known 
method. On a preferred basis, dosages of about 0.85 or 1.0 g/l or less 
have proven satisfactory with prefiltefing though lesser amounts of about 
0.5 g/l, 0.2 g/l or even 0.1 g/l have also removed sufficient amounts of 
colorants/contaminants. While total contact times may vary from several 
minutes to one hour or more, current data shows that total treatment times 
of about 15 minutes or less accomplish sufficient colorant removal 
according to this embodiment of the present invention. 
Saturated, spent, or contacted adsorbent is typically removed from caustic 
liquors by known or subsequently-developed techniques including 
filtration, using a primary separator, vacuum filter gravity settling 
and/or centrifugation. On a preferred basis, a pressure filter is used, 
most preferably, a continuous pressure filter apparatus. When activated 
hydrotalcite contact and settling is followed by another filtration step 
(per FIG. 1c), even greater levels of hydrate brightness may be achieved. 
Such post-contact filtering is believed to remove from solution most 
unactivated forms of hydrotalcite which may have formed during treatment 
together with any organics that are adsorbed or don't otherwise 
resolubilize. 
When calcined hydrotalcite powders are combined with one or more binders 
before being extruded, foraged or otherwise shaped into large particle 
sizes, such particles can then be loaded into columns, fluidized beds or 
other containment means through which solution may be passed. A third 
contacting alternative exposes caustic solutions to a semi-solid sludge, 
or slurry, of calcined hydrotalcite. This treatment form is especially 
good at removing such representative contaminants as NaFeO.sub.2, FeOOH, 
Fe.sub.2 O.sub.3 and even certain humic acids. 
Filtration only after solution contact with a substance consisting 
essentially of: calcined hydrotalcite, pyroaurite, takovite, or mixtures 
thereof will also result in an aluminum hydroxide of improved 
whiteness/brightness, though on a less preferred basis. The contacted 
adsorbent separated from this solution is preferably recalcined and 
recycled into a continuous or semi-continuous process for enhanced cost 
efficiencies. 
The method of this invention is generally receptive to adsorbing 
electronegative or anionic colorants which are divalent, trivalent or 
higher in charge. Calcined hydrotalcite may also remove monovalent 
contaminants from solution though on a less preferred basis. Without being 
limited as to any particular theory of operation, it is believed that 
preferred embodiments proceed as follows. Upon calcination (or 
activation), both carbonate and water are expelled from hydrotalcite's 
basic structure according to the formula: 
EQU M.sub.g6 Al.sub.2 OH.sub.16 CO.sub.3.4H.sub.2 O.fwdarw.M.sub.g6 Al.sub.2 
O.sub.8 (OH).sub.2 +CO.sub.2 +11H.sub.2 O 
Contact with an anionic or electronegative contaminant then causes said 
colorants to occupy vacated anion positions in the calcined product during 
solution contact and rehydration. For some contaminants, it is believed 
that a tricalcium aluminate intermediate forms upon hydrate redigestion. 
This intermediate then attracts hydrophobic, high molecular weight, 
organic colored molecules which are removed together through post-contact 
filtering practices. 
EXAMPLES 
For the atmospheric examples described below, a concentrated synthetic 
sodium aluminate liquor was prepared as follows: 536 grams of NaOH pellets 
were added with overhead stirring to 1484 grams of deionized water in a 
stainless steel beaker. Using a hot plate, the solution temperature was 
raised from 18.degree. C. to 88.degree. C. and the NaOH was allowed to 
dissolve. 724 grams of A-30 hydrate (65.0% brightness) was added to this 
solution and stirred. The combined solution was allowed to evaporate down 
before additional deionized water was added. This solution was then 
filtered for 45 minutes with #40 paper, measured for specific gravity and 
refrigerated. The target was 352 g/l Total Caustic (T/C) as Na.sub.2 
CO.sub.3, an alumina to caustic ratio of 0.667, 235 g/l of Al.sub.2 
O.sub.3 and a specific gravity of 1.372. 
TABLE 1 
______________________________________ 
INITIAL WHITENESS LEVELS 
% Brightness 
% Brightness Standard 
Readings Average Dev. 
______________________________________ 
Sample hydrate, as-received 
65.1 65.0 0.208 
64.8 
65.2 
C-30 seed material, 
76.5 76.8 0.300 
as-received 76.8 
77.1 
Sample hydrate, 83.9 83.5 0.566 
once filtered 83.1 
Sample hydrate, treated w/4 
91.4 90.8 0.557 
g/l of Active HTC for 
90.7 
min and filtered 
90.3 
C-31 seed material, 
96.7 96.6 0.058 
as received 96.6 
96.7 
______________________________________ 
The following control conditions were then maintained for many of the tests 
described below: 
Seed: 50 g/l of C-30 hydrate (76.8% whiteness) screened to 200/325 mesh 
Hydrotalcite: Laboratory activated at 550.degree. C. for 80 minutes 
Digested hydrate liquor temperature when treated with Activ. HTC: 
95.degree. C. 
Synthetic Liquor: Total Caustic=175 g/l as Na.sub.2 CO.sub.3 ; AC 
ratio=0.6; Spec. gravity=1.191 
Precipitation conditions: 74.degree. C. water bath for 24 hours 
Using a liquor prepared under atmospheric conditions, the following matrix 
of tests was performed: 
Test 1: Control Sample--No filtering and no Activ. HTC addition 
Test 2: Only 1 Filtration Step performed 
Test 3: Only treatment with Activ. HTC 
Test 4: Treatment with Activ. HTC and Filtering thereafter 
Test 5: Only 1 Filtration Step performed (as a repeat or check sample) 
Test 6: Two Filtration Steps performed 
Test 7: Prefiltering before Treatment with Activ. HTC; and 
Test 8: Prefiltering, Treatment with Activ. HTC and Post-filtering 
This matrix of tests resulted in the following % brightness and absorbance 
measurements: 
TABLE 2 
______________________________________ 
ATMOSPHERIC TESTS 
0.2 g/l of Hydrate 
Test Pre- Activ. HTC Post- Color by 
No. filtered 
for 15 min filtered 
Absorbance 
% Brightness 
______________________________________ 
1 No No No 0.101 82.7 
2 No No Yes 0.129 91.8 
3 No Yes No 0.072 84.6 
4 No Yes Yes 0.175 93.5 
5 Yes No No 0.123 92.3 
6 Yes No Yes 0.109 92.8 
7 Yes Yes No 0.161 91.4 
8 Yes Yes Yes 0.128 94.0 
______________________________________ 
These % brightness values are summarized graphically at FIG. 2a. 
A second matrix of tests was performed on a caustic solution that was bomb 
digested in a Parr reactor at 143.degree. C. for 30 minutes in order to 
rapidly and completely redissolve alumina in caustic and simulate plant 
digester conditions which may chemically change some of the organic 
colorants present. This resulted in the following absorbance and % 
brightness levels, the latter of which are graphically summarized at FIG. 
2b. 
TABLE 3 
______________________________________ 
BOMB DIGESTED 
0.2 g/l of Hydrate 
Test Pre- Activ. HTC Post- Color by 
No. filtered 
for 15 min filtered 
Absorbance 
% Brightness 
______________________________________ 
1 No No No 0.062 84.1 
2 No No Yes 0.130 93.3 
3 No Yes No 0.054 83.4 
4 No Yes Yes 0.061 94.2 
5 Yes No No 0.120 92.8 
6 Yes No Yes 0.068 94.2 
7 Yes Yes No 0.047 93.0 
8 Yes Yes Yes 0.039 94.4 
______________________________________ 
The overall effect on % whiteness improvement by combining filtering steps 
with activated hydrotalcite contact is shown at FIG. 3. 
A series of tests were then performed using various adsorbent exposure 
times and dosages (with both pre- and -post filtering) resulting in the 
following brightness data: 
TABLE 4 
______________________________________ 
DOSAGES & TREATMENT TIMES 
Dose of % 
Run # Activ. HTC (g/l) 
Treatment Time (min.) 
Brightness 
______________________________________ 
1 0 0 85.0 
2 0.1 7.5 88.9 
3 0.1 15 89.2 
4 0.2 7.5 88.3 
5 0.2 15 87.0 
6 0.2 30 88.5 
7 0.33 15 88.1 
8(a) 0.33 30 90.8 
8(b) 0.33 30 90.2 
9 0.33 60 88.1 
10 1.0 30 89.4 
11 1.0 60 89.5 
12 2.5 30 90.2 
13 2.5 60 89.3 
14 5.0 30 92.7 
15 5.0 60 89.2 
______________________________________ 
A series of tests was also run to determine the recyclability of activated 
hydrotalcite in this method. Each cycle of a four cycle test used a fresh 
supply of redigested hydrate liquor. Synthetic caustic liquor was exposed 
for 15 minutes to 0.33 g/l of adsorbent for the first cycle and to the 
cumulative filtered solids described below (which included reactivated 
hydrotalcite in the second through fourth cycles). Contacted solids were 
filtered off, dried overnight and reactivated at 550.degree. C. for 80 
minutes. In the second through fourth cycles, both C-30 and C-31 hydrates 
were used as seed material for comparative purposes. 
TABLE 5 
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RECYCLING ACTIV. HTC 
Added Charge of Activ. 
C-30 C-31 HTC & other additives 
Seed* Seed* (g/l) 
______________________________________ 
Starting Hydrate 
0.466 -- -- 
Reprecipitated 
0.171 -- -- 
Cycle 1 0.135 -- 0.33 
Cycle 2 0.144 0.024 0.47 
Cycle 3 0.141 0.026 0.75 
Cycle 4 0.147 0.040 0.79 
______________________________________ 
*The absorbance and brightness of these starting seed samples were 0.346 
and 76.8% for C30 and 0.007 and 96.6% for C31. 
From the foregoing data, the thermal regeneration and reuse of activated 
hydrotalcite over 4 cycles resulted in less than a 10% decline in overall 
color removal effectiveness. 
Having described the presently preferred embodiments, it is to be 
understood that the invention may be otherwise embodied within the scope 
of the appended claims.