Process for the production of exhaust gas filters

A large scale method is provided for the production of exhaust and industrial gas filters comprising aluminium oxide produced by the decomposition of alumina hydrate deposited on a substrate. A plurality of substrates are mounted on a stand, immersed in an alkali metal aluminate solution while the stand is subjected to motion through the solution, thereby causing the solution to flow over and through the substrates while alumina hydrate is deposited on them.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the production of exhaust and 
industrial gas filters on a large scale. 
In recent years, governments have become increasingly aware of the dangers 
to public health resulting from exhaust fumes of internal combustion 
engines, particularly in confined spaces. Among the constituents of 
exhaust gases which give rise to concern are carbon monoxide, 
incompletely-burnt hydrocarbons, nitrogen oxides and lead containing 
particulate matter. Regulations have been issued in many countries to 
reduce the amount of such emissions in exhaust gases. Attempts have been 
made to satisfy the requirements of such regulations in several ways: 
(a) Internal combustion engines have been designed so that they operate in 
such a manner as to produce an inherently "cleaner" exhaust. 
(b) Gasoline in which the added lead content is reduced or absent, is being 
sold on a wider scale than previously, and is mandatory in many areas. 
(c) Attempts have been made to filter the exhaust, or to provide a catalyst 
which would lead to a more complete combustion of carbon monoxide and 
unburnt hydrocarbons. 
While progress has been made with the first two of the methods set out 
above, they have the disadvantage that the development of a completely new 
engine for motor vehicles is extremely expensive so that manufacturers 
would wish, as far as possible, to continue to produce engines whose 
development has already largely been carried out. In addition, the 
production of lead-free petrol means that refineries would need to be run 
in a less efficient manner and use more crude oil than is possible when 
the octane rating of lower-octane hydrocarbons can be increased by the 
inclusion of lead compounds. 
2. DESCRIPTION OF THE PRIOR ART 
U.S. Pat. No. 3,231,520 and British Pat. No. 1,058,706 disclose a structure 
comprising a substrate having an adherent film or layer of alumina formed 
thereon which serves as a support for catalytic materials to promote a 
variety of reactions including the oxidation of exhaust gases from 
internal combustion engines. The alumina film is formed on a substrate of 
a metal or non-metal which may have a variety of configurations. The 
adherent alumina film is formed by contacting the substrate with a 
solution of an alkali metal aluminate forming a hydrated film of alumina 
on the substrate which is then dried and calcined to produce a hard 
tenacious film of predominantly gamma alumina. U.S. Pat. No. 3,227, 659 
discloses that the alumina-coated structure of U.S. Pat. No. 3,231,520, in 
addition to being a useful support for catalysts, may be impregnated with 
a phosphorus-containing material such as an alkali metal phosphate which 
is useful for the treatment of exhaust gases containing lead-containing 
particulate matter. U.S. Pat. No. 3,140,651 discloses that this structure 
may be impregnated with a chromium-containing material, such as an alkali 
metal or an alkaline earth metal chromate, for treating exhaust gases 
containing lead-containing particulate matter. U.S. Pat. No. 3,362,783 
discloses a useful configuration of the alumina-coated structure wherein 
the substrate comprising metal wool is encased in a metal casing prior to 
coating with the alumina film so that the alumina coats not only the metal 
fibres, but also coats the casing thereby bonding the metal fibres to the 
inside of the casing. This particular structure was effectively employed 
in the treatment of exhaust gases when it was coated or impregnated with 
an oxidation catalyst. These alumina coated structures have the ability to 
withstand severe abrading and vibration which makes them particularly 
useful as a catalyst support for the treatment of automobile exhaust 
gases. Further use of these structures in this or other services may be 
desirable. 
British Pat. No. 1,271,710 shows that alumina coated substrates may also be 
used on their own for the treatment of exhaust gases. It was found that 
the alumina itself acted as a filter to remove lead-containing particulate 
matter from the exhaust gases which could then be subjected to further 
treatment using a suitable oxidation catalyst, without this oxidation 
catalyst being rendered inactive by the lead. 
In preparing the alumina-coated substrates in accordance with the various 
Patents described above, it was usual to deposit the alumina by immersing 
the substrate, which was generally formed from knitted metal mesh or wire, 
but could also comprise other forms, such as balls, bars, chains, plates 
or tubes. Generally, the alumina was deposited from a solution of an 
alkali metal aluminate. This can easily be generated by dissolving 
aluminum metal in an alkali metal hydroxide, most usually sodium 
hydroxide. The reactions involved in this process are as follows: 
1. Dissolution of aluminum 
EQU 2Al+2H.sub.2 O+2NaOH=2NaAlO.sub.2 +3H.sub.2 
2. Decomposition on the wire 
EQU 2NaAlO.sub.2 +4H.sub.2 O=Al.sub.2 O.sub.3.3H.sub.2 O+2NaOH 
The net overall reaction is: 
EQU 2Al+6H.sub.2 O=Al.sub.2 O.sub.3.3H.sub.2 O+3H.sub.2 
As will be seen from these equations, the aluminum oxide is deposited in 
the form of the trihydrate (Gibbsite). 
Generally, the film of aluminum oxide is at least 1 mil (0.025 mm) in 
thickness, and is preferably not less than about 10 mils (0.25 mm). 
Generally, a film thickness of from 10 to 100 mils (0.25 to 2.5 mm) is 
suitable. 
After drying, the coated substrate is generally calcined at a temperature 
which is usually in the range from 285.degree. to 820.degree. C., more 
preferably from 540.degree. to 820.degree. C. This treatment dries off 
water of crystallization, and converts the aluminum oxide into the 
gamma-phase, which has a high surface area per unit weight and is very 
adsorptive. 
The processes described in the prior art were suitable for the manufacture 
of small numbers of exhaust gas cartridges, but additional problems are 
encountered when attempts are made to manufacture such cartridges on a 
larger scale, which would be necessary if they were to be made a standard 
fitting for motor vehicles. In particular, it has been found that there is 
great difficulty in ensuring that substantially equal amounts of alumina 
are deposited upon the different cartridges in a batch. It is found, that 
when such cartridges are simply immersed in a solution of sodium 
aluminate, there was very considerable variation in the weight of alumina 
deposited upon individual cartridges. In some instances, twice as much 
alumina was deposited as in others. 
Some improvement can be provided by the use of highly pure aluminum (99.99% 
purity) but this is twice as expensive as virgin aluminum (99.5% purity). 
Belgian Pat. No. 849,373 notes certain of the difficulties encountered when 
attempting to carry out the coating process, and proposes a method in 
which the aqueous solution of alkali metal aluminate is formed by 
dissolving aluminum oxide in sodium oxide solution, and the composition of 
the alkali metal aluminate solution is adjusted during the course of 
deposition, by passing it through a regenerating device in which a weak 
solution of alumina, leaving the tank in which coating takes place, is 
concentrated by evaporation, and is regenerated by adding alumina, 
filtered and returned to the coating tank with the addition of a quantity 
of water which is equivalent to that evaporated in the regenerating 
system. 
Such a process is quite complicated, involving evaporation and filtration 
of the solution. Moreover, the alkali metal aluminate is not generated 
directly from aluminum metal, but is instead formed from aluminum oxide 
and sodium oxide. 
OBJECTS OF THE INVENTION 
The object of the present invention is to provide a method by means of 
which coatings of aluminum oxide can be formed in a simple and convenient 
manner, while avoiding the various disadvantages of the prior art 
discussed above. 
A further object of the invention is to provide a method for depositing 
coatings of aluminum oxide on substrates from a solution of alkali metal 
aluminate, such as sodium aluminate, without using complex and expensive 
means for the continuous regeneration and purification of said alkali 
metal aluminate solution. 
Further objects of this invention will become apparent on further reading 
of this Specification. 
SUMMARY OF THE INVENTION 
The above stated objects of the invention can be achieved by a method for 
the production of exhaust gas filters comprising aluminum oxide produced 
by the decomposition of alumina hydrate deposited on a substrate contained 
within a metal casing, by depositing alumina hydrate from an alkali metal 
aluminate solution, which comprises mounting a plurality of said 
substrates upon a stand therefore, immersing said stand in said alkali 
metal aluminate solution, and subjecting said stand to motion through said 
alkali metal aluminate solution whereby said alkali metal aluminate 
solution is caused to flow over and through said substrates while alumina 
hydrate is deposited on said substrates. 
Although it is possible to coat the substrates and then to insert the 
coated substrate within a metal casing, it is preferred to insert the 
substrate, which is preferably made from a metal wire, within the casing, 
before immersing the stand and filled casings in the alkali metal 
aluminate solution. 
According to one embodiment of the invention, the alkali metal aluminate 
solution is obtained by dissolving aluminium metal in an alkali metal 
hydroxide solution. 
According to another embodiment of the invention the alkali metal aluminate 
solution is obtained by dissolving alumina hydrate in an alkali metal 
hydroxide solution. 
According to another embodiment of the invention, the alkali metal 
aluminate solution is obtained by dissolving bauxite or a bauxitic clay in 
an alkali metal hydroxide solution. 
It has been found that the use of such a method for the production of 
exhaust gas filters, makes it possible to use aluminum of commercial 
purity in the generation of the alkali metal aluminate solution without 
the disadvantages previously encountered when using this material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The apparatus comprises a coating tank (1) and a wash tank (2). Each is 
provided with heating means (not shown) such as a means for injecting 
steam. Exhaust gas filters to be coated in tank (1) are supported on a 
stand (3) and a stand carrying filters in the process of being coated in 
tank (1) is identified as (3'). This stand (3') is supported from hoist 
(4) by means of a compressed air cylinder (5), to be discussed more fully 
below, fed with compressed air by means of line (5'). 
A fan (6) is provided to extract gaseous and liquid droplets from the 
vicinity of the coating and wash tanks, and this is vented to the 
atmosphere at (7). A furnace (8), fed with flammable gas (9) is provided 
to calcine the coated gas filters and to convert the aluminum oxide 
trihydrate into the gamma-phase, as described above. 
As shown in greater detail in FIGS. 2 and 3, the stand comprises three 
shelves (10) which are preferably perforated for free circulation of 
liquid in the tank. Baskets (11) are provided around the edges of the 
shelves in order to support blocks of aluminum (12). This would be omitted 
when the alkali metal aluminate was generated in another vessel. Exhaust 
gas filters (13) to be coated with aluminum oxide in accordance with the 
invention are supported on the shelves (10). 
In accordance with the embodiment of the invention described in these 
drawings, the air cylinder (5) is provided in order to enable the stand 
carrying the exhaust gas filters to be reciprocated vertically within the 
alkali metal aluminate solution in tank (1). By feeding compressed air 
along line (5') the cylinder is caused to operate and the stand, carrying 
filters is raised and lowered as the cylinder operates. 
In the specific instance of the plant used in the examples, the tanks (1) 
and (2) are each built of mild steel and are about four foot six inches in 
diameter and about seven feet tall. This is a suitable size to enable the 
coating of batches of fifty exhaust gas filters, each having a capacity of 
51/2 liters. The specific air cylinder employed had a stroke of about six 
inches and operated continuously in a 12 second cycle. A swivel hook 
attachment enabled the stand to rotate slowly in the reacting alkali metal 
aluminate solution. 
The invention is not restricted to such a specific method of bringing about 
the motion of the filters through the alkali metal hydroxide solutions. 
For example, instead of subjecting the stand to vertical reciprocation, 
the stand could instead be reciprocated horizontally in the bath, or 
alternatively it could be rotated in the bath. In such a case, however, 
the filters being coated would have to be oriented in such a way as to 
permit a free flow of the alkali metal aluminate solution through the 
filters. Various other types of apparatus suitable for moving a stand 
through a solution are well known in the art and need no particular 
description. 
It is to be emphasized that the function of the air cylinder (5) is to 
reciprocate the stand within the tank. Apparatus serving only to lower the 
stand into the tank and to remove it after a period of immersion, the 
stand not moving during the immersion, would not provide an acceptable 
coating of the substrates, and the use of such apparatus would not be in 
accordance with the teachings of this invention. 
Although a gas-heated oven (8) is shown in the drawing, other forms of oven 
for calcining the coated filters can be employed, for instance an electric 
furnace. 
In general, the coating conditions used in accordance with the invention 
are the same as those that have already been described in the art, for 
instance in British Pat. No. 1,271,710 referred to above. The substrate is 
preferably of extended dimensions, and is particularly of a length and 
geometric surface area substantially greater than that of discrete 
particles. The substrate employed in the structure of the invention is not 
restricted to any particular configuration nor to any particular material. 
The substrate may be formed of a metal or non-metal, although metal is 
preferred, and may include such materials as steel, stainless steel, alloy 
steel, iron, iron alloys, nickel, chrome-nickel alloys, aluminum, an 
aluminum-coated metal or titanium, including sintered metal materials, or 
refractory or ceramic materials including, for example, high melting 
glass, refractory metal oxides, e.g. magnesia, alumina, zirconia and 
silica, or refractory metal silicates or carbides. The configuration of 
the substrate may include bars, balls, chain, mesh, plates, saddles, 
sheet, tubes, wire or the like. 
Although it is preferred to employ a sodium aluminate solution, it should 
be understood that other alkali metal aluminate solutions, e.g. potassium 
aluminate, should also be used. 
In accordance with the present invention, it is most convenient to form the 
sodium aluminate solution by dissolving aluminum metal in sodium 
hydroxide. The aluminum which is used can be in any convenient form. 
It is a particular advantage of the process of this invention that it is 
possible to employ virgin aluminum having a purity of about 99.5%. Such 
material could not conveniently be used in the heretofore-known processes, 
because the relatively high content of impurities leads to irregularities 
in the reaction. It is also possible, however, to use other forms of 
alumium, such as the more highly purified and expensive S.P. aluminum, 
which has a purity of about 99.99%. In general any aluminum at least about 
99% pure can be used. 
The temperatures that can be used in accordance with this embodiment of the 
process are generally above about 50.degree. C., more preferably in the 
range from about 80.degree.-100.degree. C. Higher temperatures can, 
however, be used if desired. 
The process previously described is one in which alumina hydrate (Gibbsite) 
is deposited on a mesh of stainless steel wire wool from a dilute sodium 
aluminate solution at high temperatures (about 90.degree.-95.degree. C.), 
the alumina in the solution being replenished continuously by the addition 
of aluminum metal. 
An alternative to dissolving aluminum metal, to produce the desired 
solution, would be to produce a solution having a high alumina to soda 
ratio and a high soda concentration by boiling or autoclaving, with 
recirculation if necessary, strong sodium hydroxide solutions with alumina 
hydrate (Gibbsite), filtering is necessary, and decomposing the resulting 
liquor at a convenient temperature in the presence of the stainless steel 
wool. The maximum alumina to soda ratio which can be tolerated to give a 
reasonably stable solution varies, the ratio increasing with increasing 
soda concentration. 
Typical solutions, before decomposition would have free soda strengths (as 
Na.sub.2 O) from about 60 to 300 grams per litre, initial weight ratios of 
Al.sub.2 O.sub.3 to Na.sub.2 O in the solution of about 1.3:1 to about 
1.67:1 and Al.sub.2 O.sub.3 concentrations of about 78 to about 500 grams 
per litre. The optimum strength is approximately 200 grams per litre of 
Na.sub.2 O. To obtain the required deposition of alumina hydrate on the 
surfaces to be coated, the liquors would be decomposed at temperatures 
from about 60 to about 95.degree. C. 
Similar sodium aluminate solutions may be the so-called Bayer liquors, or 
modifications thereof. Bayer liquor is the sodium aluminate solution 
produced by treating bauxite or bauxitic clay with caustic soda, in the 
Bayer Process, according to which aluminum hydroxide or oxide is produced 
for use as raw material for the production of aluminum metal. The present 
coating process can be operated using sodium aluminate coating solutions 
from any or a mixture of above sources. 
Although the coating process described can utilize solutions prepared 
either by dissolving aluminum metal in sodium alkaline hydroxide, or by 
dissolving alumina hydrate in an alkaline hydroxide, or solutions arising 
from the Bayer process or a mixture of these sources, the optimum results 
to data have been obtained by the use of solutions of aluminum metal in an 
alkaline hydroxide as described. 
Although the substrate upon which the aluminum oxide is deposited can vary 
widely, as indicated above, it has been found most convenient to employ 
knitted steel wire. One suitable form is a cut wire having a triangular 
section and variable thickness and strand length. Other forms of knitted 
steel wire can be obtained, for example a smooth drawn wire which is 
knitted into a stocking and then rolled into a cylinder of the required 
thickness and weight. 
In general, the thickness of the film of alumina which is deposited should 
not be less than about 1 mil (about 0.025 mm), and preferably not less 
than about 4 mils (about 0.1 mm). Deposits of alumina of almost any 
thickness are possible, but coatings thicker than about 150 mils (about 
3.75 mm) are generally not advantageous. Most usually, the thickness of 
the film will be from about 4-30 mils (about 0.1-0.75 mm). 
To show the advantage which is achieved in accordance with the present 
invention, the following non-limiting Example is provided: 
EXAMPLE 
About 118 Kg of caustic soda flake was added to about 1135 liters of water. 
When this had dissolved, the temperature was raised to about 60.degree. C. 
by live steam injection and about 71.5 Kg of virgin aluminum added on the 
otherwise empty filter stand. The temperature rose rapidly and 
effervescence increased until the liquor boiled at about 105.degree. C. 
The aluminum was removed, the liquor allowed to cool and the dissolution 
resumed with 20 kilo batches of metal. The temperature remained constant 
at about 95.degree. C. 
The following day the aluminum had all dissolved and a clear aluminate 
liquor had been produced. The stand was loaded with fifty 5.2 litre 
wire-filled preweighted cases on three tiers and about 24.5 Kg of aluminum 
were charged to the bottom shelf. The liquor was heated to about 
70.degree. C. and the stand was inserted. Water was added until the stand 
was fully immersed, and the temperature was brought back to about 
70.degree. C. During the run the aluminum moved on the shelf, resulting in 
most of the hydrogen evolved during the run passing through half the 
cases. Despite the temperature being raised to about 75.degree. C. after 
eight hours, the total coating cycle took about 27 hours. When the 
calcined cases were weighed, the spread of weights was considerable, some 
cases picking up twice as much alumina as others. 
For the second experiment, the liquor was made up fresh, but a thin film of 
alumina hydrate covered the tank walls and the stand. To eliminate the 
possibility of the aluminum moving as before, the ingots were suspended in 
expanded metal baskets around the tank wall about halfway between bottom 
and surface. To reduce the coating time, the reaction temperature was set 
at about 85.degree. to 90.degree. C. 
The reaction was complete in about 24 hours but a considerable variation in 
coating weights still occurred: i.e. about 42% were within .+-.10% of the 
required weight 
about 60% .+-.15% 
about 75% .+-.20% 
The spread of weights appeared random, but in an attempt to reduce 
temperature gradients it was decided to operate the steam valve 
intermittently rather than continuously to eliminate possible temperature 
gradients. Also as a layer of grease could be seen on the top of the 
reaction foam all subsequent runs were carried out with degreased cases. 
Knitted mesh inserts were used in an attempt to eliminate variations in 
wire packing density but coating proved very slow--over 24 hours coating 
time. The weight distribution was again poor, only about 10% falling 
within .+-.10% of the mean. 
The above sets out the results which are achieved when the apparatus in 
accordance with the invention is used without the air cylinder to provide 
vertical reciprocation of the stand. When, however, the experiment is 
carried out in the same manner, but making use of the air cylinder having 
a stroke of about six inches and operating on a 12 second cycle, about 80% 
of the coated filters had within .+-.10% of the desired weight. 
In a further experiment, the size of the filter which was tested was 
altered. In the experiments detailed above, the filters had a diameter of 
about 15.75 cm and a length of about 33 cm, and fifty filters were placed 
on the stand. In the further experiment, eighty filters, each about 45.75 
cm long and having an about 10.16 cm by 7.62 cm oval section were packed 
in two tiers on the stand, and coated as before. About 76% of the coated 
filters obtained in this way fell within the desired range of .+-.10% of 
the desired weight. 
The use of the apparatus in accordance with the invention makes it possible 
to achieve repeated re-use of the alkali metal aluminate solution. No 
processing problems were observed when ten separate batches of filter were 
coated on ten successive days. The efficiency of utilization of the 
aluminum was about 75-80% throughout the series, for virgin aluminum and 
about 90% for S.P. aluminum. No significant decrease in efficiency was 
noticed as the age of the reaction liquor increased. Losses of sodium 
hydroxide over the period were low and topping up with sodium hydroxide 
was required only each week. 
The distribution of coating weight varied, but overall more than about 70% 
of the filters fell within 10% of the desired weight. 
Numerous substitutions, modifications, and changes can be made in the above 
defined process without departing from the inventive concept. The scope of 
the invention can, however, best be understood with reference to the 
following claims.