Production of anhydrous aluminum chloride from hydrated alumina

A process is described for the direct chlorination of hydrated alumina (preferably alumina trihydrate) to aluminum chloride hexahydrate (ACH) by reaction with concentrated hydrochloric acid. Preferably all of the initial hydrated alumina is converted to ACH. The ACH partially calcined to form an amorphous mixture of aluminum oxides and oxychlorides. This mixture is then reductively chlorinated to form anhydrous aluminum chloride which is suitable as a source of electrolytically produced aluminum metal.

TECHNICAL FIELD 
The invention herein relates to the formation of chlorided aluminum 
products. 
BACKGROUND ART 
It has been known for some time that anhydrous aluminum chloride can be 
electrolytically reduced at low temperatures of about 
700.degree.-750.degree. C. to produce aluminum and chlorine with less 
energy consumption than with Hall technology reduction of alumina. Over 
the years a number of processes have been described to produce aluminum 
chloride suitable for electrolytic production of aluminum. Some processes, 
such as those disclosed in U.S. Pat. Nos. 4,039,647 and 4,039,648, involve 
the direct chlorination of aluminum oxide materials derived from the Bayer 
Process under reducing conditions in a molten state. Other chlorination 
processes involve the chlorination of Bayer Process alumina using a solid 
carbon reductant, such as partially calcined coke, as taught and described 
in U.S. Pat. No. 4,284,607. A principal source of alumina in all cases, 
however, has been Bayer Process alumina. In the Bayer Process bauxite is 
mixed with a hot concentrated sodium hydroxide solution to dissolve the 
alumina and separate it from the other major components ("impurities") of 
the bauxite (silica, iron oxide and titania). The dissolved alumina is 
then crystallized as alumina trihydrate, .alpha.-Al.sub.2 O.sub.3.3H.sub. 
2 O. Because the alumina trihydrate is formed in a sodium hydroxide 
environment, it contains a significant amount of soda (usually 0.4 to 
0.6%; all percentages herein are by weight unless otherwise stated). This 
high soda level is also carried over to the product alumina. A high soda 
level in the product alumina is undesirable because of its effect on the 
operation of the fluid bed chlorination process to produce anhydrous 
AlCl.sub.3. Soda consumes valuable chlorine as it is chlorinated, and the 
products of the chlorination of soda (NaCl and NaAlCl.sub.4) accumulate in 
the fluid bed, adversely affecting process operation and requiring 
frequent and costly shut-downs for cleaning of the bed. The high soda 
content alumina also causes difficulties in the separation step of 
entrained unreacted bed material (alumina and/or coke) and AlCl.sub.3 from 
soda chlorination products. 
Other chlorination processes have been described for the chlorination of 
relatively high purity alumina. The alumina is typically produced from the 
calcination of aluminum chloride hexahydrate, (AlCl.sub.3.6H.sub.2 O; 
"ACH"). For instance, in U.S. Pat. Nos. 4,465,566 and 4,465,659 assigned 
to the assignee of present application, ACH derived from acid leaching of 
aluminous material, such as clay, is partially calcined to produce a 
product which has low levels of soda and is active toward calcination. 
Because the ACH is formed in an acid environment, the alumina produced 
from its calcination has low levels of soda, commonly no more than 0.02%. 
Even though this alumina source is quite satisfactory for chlorination and 
production of anhydrous AlCl.sub.3, the acid leaching of the aluminous raw 
materials is energy and capital intensive compared to caustic leaching of 
bauxite. It would therefore be advantageous to have a process for the 
production of anhydrous AlCl.sub.3 that combines the economics of Bayer 
Process and the low soda levels in alumina of the acid leaching processes. 
DISCLOSURE OF INVENTION 
The invention herein is a process for the conversion of hydrated alumina to 
anhydrous aluminum chloride which comprises: 
a. reacting a source of solid hydrated alumina with concentrated 
hydrochloric acid to convert at least a portion of the hydrated alumina to 
solid aluminum chloride hexahydrate; 
b. recovering a solid product from the conversion, the product consisting 
essentially of aluminum chloride hexahydrate or a mixture of aluminum 
chloride hexahydrate and unreacted hydrated alumina; 
c. partially calcining the solid product of step (b) to produce an 
amorphous mixture of aluminum oxides and oxychlorides with a low content 
of water and HCl; and 
d. chlorinating the mixture of step (c) in the presence of a reductant to 
form anhydrous aluminum chloride. 
In a preferred embodiment the source of hydrated alumina is the product 
from the Bayer Process. 
MODES FOR CARRYING OUT THE INVENTION 
In the process herein, the raw material to be converted is hydrated 
alumina. Any form of hydrated alumina is suitable as a raw material, 
including alumina monohydrate and alumina trihydrate ("ATH"). In practice, 
it will be found that the most common raw material is the "Bayer alumina" 
described above. (For brevity in the remainder of this specification, the 
raw material will be considered to be ATH. It will be understood, however, 
that the process of this invention will be equally applicable to all other 
hydrated aluminas.) 
The principal reaction of the first step of the present invention is the 
conversion of ATH to ACH by reaction with concentrated hydrochloric acid, 
according to the following reaction: 
EQU Al.sub.2 O.sub.3.3H.sub.2 O[s]+6HCl[1+g]+6H.sub.2 O[1] 
.fwdarw.2AlCl.sub.3.6H.sub.2 O[s] (1) 
For this reaction the concentration of the hydrochloric acid will be 
preferably in the range of 15-35% acid, preferably 20-30%. No additional 
water beyond that present as the concentrated acid solution is added. 
Additional concentrated acid and or gaseous HCl can be added as the 
reaction progresses or as additional ATH is fed to the process, so that 
the process may be run as either a batch or a continuous operation. If the 
acid concentration is reduced much below about 15%, the reaction rate will 
be significantly reduced and precipitation of the hydrated aluminum 
chloride will be curtailed. 
The conversion reaction is believed to function to form the ACH precursor 
for partial calcination product by reactively dissolving both the hydrated 
alumina and the major portion of its included impurity oxides. The 
dissolved aluminous material then combines with the chloride portion of 
the hydrochloric acid to form ACH which in the defined range of acid 
concentration precipitates spontaneously as a solid. The impurity oxides, 
on the other hand, at this acid concentration remain substantially 
dissolved in the acid, so that the subsequent solid/liquid separation 
process separates high purity ACH from the impure solution. However, if 
the concentration of the acid is raised much beyond approximately 30-35%, 
impurities present in the raw material will also precipitate out with the 
ACH in significant quantities. 
The acid conversion reaction is normally operated in the temperature range 
of 40.degree.-120.degree. C. in a closed vessel. The closed vessel 
prevents escape of any gaseous chloride materials and facilitates the 
recovery and recycle of the HCl and water. The reaction is conducted under 
nominally ambient pressure conditions, but during the course of the 
reaction there will be a small pressure increase within the closed vessel 
due to the vapor pressure of evolved HCl and water. The preferred 
temperature for the conversion is in the range of 70.degree.-80.degree. 
C., but that preferred range can vary depending on the particular 
materials from which the reaction vessel is constructed, due to differing 
degrees of resistance to the corrosive attack of the hot concentrated 
acid. The initial percent solids in the reaction mixture is in the range 
of 5-40%. The preferred range is 10-20%. At the end of the reaction, the 
slurry has a solids content in the range of 12-60% with the preferred 
value being between 25-50%. 
The reaction will normally be run to complete conversion of the ATH and 
ACH. The actual reaction time will depend on the temperature of reaction, 
the amount of material to be reacted and the acid concentration, but will 
normally be in the range of from 15 to 150 minutes. During this conversion 
the ATH dissociates in the acid and the ACH forms as solid particles which 
are insoluble in the acid and precipitate to the bottom of the reaction 
vessel. Impurities which are present in the ATH are also dissolved in the 
acid but do not precipitate in significant amounts if the acid 
concentration is not excessive. 
Following completion of the conversion reaction, the precipitated ACH 
(which may contain some unreacted ATH) is separated from the acid solution 
by conventional solid/liquid techniques. It is thereafter washed at least 
once (preferably several times) with highly concentrated (approximately 
35%) HCl solutions to remove remaining traces of the reaction liquor. 
Washing with water or dilute acid is to be avoided, since the ACH will 
dissolve in such liquids. For that reason it is also desirable to keep the 
concentration of the wash acid as high as possible to minimize any 
redissolution of the ACH particles. 
The separated acid is for economic reasons preferably recycled for reuse in 
the conversion step. Since it will contain some impurities, preferably 
also at least a portion of the recycle stream is treated to remove those 
impurities to prevent impurity build-up in the process stream. 
Following recovery and washing of the ACH (and any unconverted ATH) the 
product is calcined at temperatures in the range of 
200.degree.-1000.degree. C., preferably in the range of 
500.degree.-850.degree. C. and most preferably in the range of 
600.degree.-750.degree. C. for time periods sufficient to partially 
calcine (i.e., substantially dehydrate) the product. The times will be 
from about 30-240 minutes, typically about 120 minutes. During calcination 
a significant portion of the ACH and any unconverted ATH is thermally 
decomposed into active aluminous material and a hydrochloric acid-water 
vapor stream. The liberated HCl is advantageously recovered for reuse in 
the conversion of ATH to ACH. 
ACH decomposes according to the general following reaction: 
EQU Heat+2AlCl.sub.3.6H.sub.2 O[s].fwdarw.Al.sub.2 O.sub.3(1-y) 
Cl.sub.6y.xH.sub.2 O[s]+(9+3y-x)H.sub.2 O[1+g]+6(1-y)HCl[g](2) 
while the unconverted portion of the ATH decomposes according to the 
following reaction: 
EQU Heat+Al.sub.2 O.sub.3.3H.sub.2 O[g].fwdarw.Al.sub.2 O.sub.3.xH.sub.2 
0[s]+(3-x)H.sub.2 O[1+g] (3) 
The ACH decomposition reaction (2) is the only reaction involved when the 
ATH is fully converted, as preferred herein. When the ATH is only 
partially converted, both reactions (2) and (3) occur during calcination. 
The ACH (along with any unconverted ATH is heated for a time and at a 
temperature sufficient to reduce the residual hydrogen content (i.e., the 
x in equations (2) and (3)) while maintaining as high a level as possible 
of residual chloride (i.e., the y in equation (2)) in the solid product 
and to produce a highly active product for the subsequent chlorination. 
The necessity for low residual hydrogen content is to minimize the loss of 
chlorine to hydrogen chloride during the chlorination stage by reaction 
with the residual hydrogen according to the equation: 
EQU H(residual)+1/2Cl.sub.2 .fwdarw.HCl (4) 
It may be calculated from the combining ratios of hydrogen and chlorine 
that for every 1 kg of combined residual hydrogen as much as 35 kg of 
chlorine will be converted to hydrogen chloride. Accordingly, the levels 
of hydrogen in the calcined product are crucial in determining the 
economic viability of the process. High residual chloride levels serve to 
reduce the consumption of valuable chlorine in the subsequent chlorination 
step. The partial calcination of ACH will also remove the major portion of 
water and HCl from the ACH producing what is believed to be a mixture of 
aluminum oxides and oxychlorides. 
The optimum calcination temperature of the fully or partially converted ATH 
is that temperature which results in the highest molar ratio of Cl/H. The 
residual hydrogen level in the calcined product is a function of 
calcination temperature. To produce acceptable feed material for the 
subsequent chlorination, the calcination temperature should, as described 
above, be in the range of 350.degree.-1000.degree. C. 
Additionally, although the level of chloride decreases with increasing 
calcination temperatures, as expected, it has also been found that there 
is only a gradual decrease in residual chlorine in the temperature range 
from 450.degree. C. to 700.degree. C. and that above about 750.degree. C. 
the chlorine levels drop sharply to a lower level in the temperature range 
from 750.degree. C. to 1000.degree. C. Indeed, by calculating the molar 
ratio of residual chlorine to residual hydrogen, it has been found that 
there is an optimum temperature range for the calcination of 500.degree. 
C. to 850.degree. C., preferably between 600.degree. to 750.degree. C., 
where the molar ratio of Cl/H approaches one. These calcination 
temperature ranges thus optimize the product in regard to the chlorine 
utilization in the subsequent chlorination process step. The 350.degree. 
C. minimum reflects the lower limit of practical reaction rates, while 
above the 1000.degree. C. maximum chlorine is rapidly drawn off rather 
than being retained to participate in the subsequent chlorination step. 
The ACH product from ATH conversion may be fed to a multiple-hearth furnace 
for calcination, in which case it passes through the furnace 
countercurrent to the upward gas flow. The particles are fed to the top 
hearths of the furnaces and fall slowly from hearth to hearth, being 
simultaneously dried and decomposed. During operation, the center shaft 
rotates slowly and the material on each hearth is raked to a discharge 
point. The alumina discharged from the furnaces is cooled and transferred 
to the final product storage bins. 
Alternatively, ACH can be heated in a two-stage decomposer/calciner. The 
decomposer is an indirectly heated, fluidized-bed reactor operating at 
200.degree. C. to 400.degree. C. The indirect-fired decomposer off-gases 
are cleaned, cooled, and sent to acid recovery. Approximately 25% of this 
cooled gas is recycled to the decomposer by a blower which maintains 
fluidization of the ACH bed. The calciner is a direct-fired, fluidized-bed 
reactor or multiple-hearth roaster that operates between 450.degree. C. 
and 1000.degree. C. About 90% of the ACH decomposition occurs in the 
decomposer; the remaining decomposition occurs in the calciner. The gas 
streams from both are then sent to the acid-recovery area. 
As another alternative, ACH may be calcined in an indirectly heated flash 
calciner or rotary kiln which permits efficient HCl gas-handling and 
recycling. 
Following the thermal decomposition of the fully converted or partially 
converted ATH according to the present invention, the partially calcined 
product is subjected to a reductive chlorination to produce the desired 
anhydrous aluminum chloride. All the oxide impurities in the product will 
chlorinate during this reductive chlorination step to produce their 
respective chlorides. Thus, Na.sub.2 O will be chlorinated to NaCl, 
NaAlCl.sub.4 and Na.sub.3 AlCl.sub.6. The impurity levels in the partially 
calcined product dictates the total chlorine loss and the minimum 
chlorination temperature. The minimum operating chlorination temperature 
is the temperature at which components of chlorination vapor pressure are 
high enough to remove feed impurities as vapors at the same rate the 
impurities are introduced so that the impurities will not accumulate in 
the reactor. This a function of impurities level, particularly of the soda 
level in the feed product, for a higher soda level requires a higher 
temperature to remove the soda-derived impurities. However, this also 
allows the operator to compensate for different soda impurity levels in 
the ATH raw materials simply by adjusting the subsequent chlorination 
temperature. For fully converted ATH, where the soda level in the 
aluminous product is typically about 0.005 ppm, the minimum operating 
chlorination temperatures is about 440.degree. C., while for unconverted 
ATH (where soda level in the aluminous productis typically 0.4%) the 
temperature is about 610.degree. C. Partially converted ATH will have an 
intermediate minimum operating temperature, e.g., 50% converted ATH will 
typically have about 0.15% soda in the partially calcined product and a 
minimum operation temperature of 537.degree. C. The actual operating 
temperature is dictated by kinetics and heat balance of the chlorination 
reaction. However, it is clear that the lower the soda levels in the feed 
product to the chlorinator the lower the possible operating temperature. 
This has a substantial economical advantage in producing anhydrous 
AlCl.sub.3, because of the decrease in corrosion of reactor as a result of 
lower operating temperature and less of the corrosive NaAlCl.sub.4. In 
addition, less reductant (e.g., carbon) is required in the reaction at low 
temperature. For example, at or below 700.degree. C., consumption of 
carbon reductant is typically 0.4 lb/lb of aluminum whereas at 900.degree. 
C. the carbon consumption approaches 0.67 lb/lb aluminum. 
In general, the fully or partially calcined ACH prepared according to the 
present invention is sufficiently activated to enable chlorination to be 
effected in the presence of virtually any reductant known in the art to be 
useful for reductive chlorination. Reductants useful in the chlorination 
of dehydrated ACH calcined according to the present invention are 
carbonaceous materials and include gaseous reducing agents such as carbon 
monoxide and producer gas (mixtures of carbon monoxide, carbon dioxide, 
and hydrogen) as taught by U.S. Pat. No. 4,264,569; COCl.sub.2, CCl.sub.4 
or mixtures thereof; and/or solid reductants, such as partially calcined 
green coke according to the teaching of U.S. Pat. No. 4,284,607, activated 
carbon derived from coal according to the teaching of U.S. Pat. No. 
4,105,752; activated fully calcined coke; or even fully calcined coke. 
Even though it has heretofore been well known to those skilled in the art 
that using fully calcined coke for reductive chlorination of aluminous 
material results in a poor chlorination rate, particularly at low 
chlorination temperatures, it has been found that it can be used, provided 
it is combined with ACH calcined according to the present invention. Each 
reductant offers advantages and also some disadvantages; thus the 
selection of the specific reductant will depend on the desired overall 
process circumstances. 
Chlorination in the present invention may be carried out at pressures 
ranging from about 0.1 atm to about 15 atm, preferably from about 1-5 atm, 
and at temperatures from about 400.degree.-950.degree. C., preferably 
550.degree.-750.degree. C., depending on the reductant, its impurities and 
the soda content in the material to be calcined. As will be understood by 
those skilled in the art, it is preferable from an energy savings point of 
view to effect chlorination at lower temperatures when possible, the 
temperatur being determined by the level of activation of the material to 
be chlorinated and by the level of soda impurity present. The anhydrous 
AlCl.sub.3 so provided from the chlorination of the product of this 
invention is suitable for use as feed to an aluminum electrolytic 
production cell. 
The examples below will illustrate the process of this invention.

EXAMPLE 1 
Control (ACH) 
100 grams of ACH derived from single stage crystallization of acid leached 
clay were calcined for 2 hours at 700.degree. C. in a rotary kiln using 
air as a carrier gas. The off-gases, water and HCl were scrubbed. The 
Al.sub.2 O.sub.3 product contained the following impurities: 
______________________________________ 
Impurities Amount, ppm 
______________________________________ 
P.sub.2 O.sub.5 
0.01 
MgO 0.01 
K.sub.2 O 0.005 
Fe.sub.2 O.sub.3 
0.005 
SiO.sub.2 0.005 
CaO 0.03 
Na.sub.2 O 0.005 
Cl 6.7 
H.sub.2 0.24 
______________________________________ 
EXAMPLE 2 
Control (alumina) 
100 gm of Bayer Process ATH (Al.sub.2 O.sub.3.3H.sub.2 O) were partially 
calcined as in Example 1. The product of this calcination contained the 
following impurities: 
______________________________________ 
Impurities Amount, ppm 
______________________________________ 
P.sub.2 O.sub.5 
0.003 
MgO 0.006 
K.sub.2 O 0.005 
Fe.sub.2 O.sub.3 
0.04 
SiO.sub.2 0.02 
CaO 0.03 
Na.sub.2 O 0.45 
Cl none 
H.sub.2 0.13 
______________________________________ 
It is clear that the alumina product from acid leaching of aluminuous ores 
(Example 1) has substantially lower levels of soda as well as other 
impurities, as compared to the Bayer Process ATH (Example 2). 
EXAMPLE 3 
Fully converted ATH 
100 gm of Bayer Process ATH were fully converted to ACH which was calcined 
in a rotary kiln at 650.degree. C. for 2 hours. The alumina product from 
this calcination contained the following major impurities: 
______________________________________ 
Impurities Amount, % 
______________________________________ 
P.sub.2 O.sub.5 
0.003 
Fe.sub.2 O.sub.3 
0.003 
SiO.sub.2 0.005 
CaO 0.001 
Na.sub.2 O 0.004 
Cl 5.1 
H.sub.2 0.1 
______________________________________ 
Comparing the data in Example 1 and Example 3, it becomes apparent that the 
present process can produce a similar quality alumina product with regard 
to soda levels and Cl/H molar ratios. The present invention thus avoids 
the high energy and capital requirements of acid leaching while still 
producing the desired product. 
EXAMPLE 4 
Partial conversion of ATH 
100 gm of 54% converted ATH were calcined as in Example 1 and the product 
was found to contain the following major impurities: 
______________________________________ 
Impurities Amount, % 
______________________________________ 
Fe.sub.2 O.sub.3 
0.002 
SiO.sub.2 0.02 
CaO 0.005 
Na.sub.2 O 0.09 
Cl 3.5 
H.sub.2 0.25 
______________________________________ 
Comparing the results of Examples 2, 3 and 4, it is clear that the soda 
level in the product from partially converted ATH is intermediate between 
that of the products of unconverted ATH (Example 2) and fully converted 
ATH (Example 3). The chlorine level is proportionate to the percentage of 
ACH in the partially converted product. In addition, the Cl/H ratio is 
more favorable in terms of chlorine consumption than that of unconverted 
ATH. 
EXAMPLE 5 
Control of chlorination 
25 gm of feed material (80% of aluminous product from Example 1 and 20% of 
activated carbon) was chlorinated in 1" fluidized bed reactor at 
650.degree. C. The results of chlorination measured by the amount of 
aluminum chloride produced per unit time are as follows: 
______________________________________ 
Relative Rate Of AlCl.sub.3 
Time, min Production, gm/min 
______________________________________ 
120 0.133 
255 0.144 
______________________________________ 
The average relative rate of AlCl.sub.3 production is about 0.138 gm/min. 
EXAMPLE 6 
Chlorination of calcined (unconverted) ATH 
25 gm of feed material (80% alumina product from Example 2 and 20% of 
activated carbon) were chlorinated as in Example 5. The average 
chlorination rate was about 0.11 gm AlCl.sub.3 /min. 
EXAMPLE 7 
Chlorination of partially converted ATH 
25 gm of feed material (80% aluminous product from Example 4 and 20% of 
activated carbon) was also chlorinated as per Example 5. The average 
chlorination rate was again slightly higher (0.154 gm AlCl.sub.3 /min) 
than that of the acid leach derived aluminous material. 
EXAMPLE 8 
Chlorination of fully converted ATH 
25 gm of feed material (80% aluminous product from Example 3 and 20% of 
activated carbon) was chlorinated as in Example 5. The results showed that 
the average chlorination rate is about 0.15 gm AlCl.sub.3 /min. This 
indicates that the partially calcined ACH from converting ATH according to 
this invention has a chlorination rate as good as or better than that of 
acid leach derived aluminous material. 
INDUSTRIAL APPLICABILITY 
The process of this invention and the resultant anhydrous aluminum chloride 
product are applicable to the industrial production of aluminum metal. The 
product is a raw material from which aluminum can be produced with less 
energy usage than required for conventional Hall process aluminum 
production. 
It will be immediately evident to those skilled in the art that the 
invention herein encompasses embodiments which, while not specifically 
described above, are clearly within the scope and spirit of the invention. 
Consequently, the scope of the present invention is to be determined 
solely by the appended claims.