Process for producing a porous mineral membrane on a mineral support

The invention relates to a process for preparing mineral and in particular, carbon-containing membranes on a porous mineral support. According to the process, at least one layer of a stable emulsion of mineral particles in a solution containing a thermosetting resin is deposited on a permeable, porous, mineral support. The resin undergoes a polycondensation treatment followed by coking, and the coke formed insures the mechanical connection of the mineral particles both to one another and to the support. The membranes produced according to the invention can be used in separating processes, particularly in microfiltration and ultrafiltration.

TECHNICAL FIELD OF THE INVENTION 
The present invention relates to a process for producing permeable, porous, 
mineral membranes on porous, permeable supports. These membranes are 
intended for use in separating processes and more particularly 
microfiltration and ultrafiltration. The term mineral membranes is 
understood to mean carbon-containing or ceramic material membranes. 
PRIOR ART 
Among separating processes, microfiltration and ultrafiltration or reverse 
osmosis most frequently have recourse to organic membranes, which have to 
be mechanically maintained on a support. In addition, these membranes 
remain relatively sensitive to corrosion and temperature changes. 
In order to solve this problem, homogeneous carbon or asymmeteric ceramic 
supports have appeared on the market. In order that said supports can be 
used in ultrafiltration or microfiltration, it is necessary to deposit a 
membrane based on ceramic powder (zirconia, alumina, titanium, clay, etc). 
The necessary procedures are known. They consist of either using a 
sol-gel, or the engobe coating of a defloculated emulsion. The behaviour 
of the thus deposited membrane is obtained by thermal sintering of the 
ceramic grains. 
Despite the good thermal behaviour of these ceramics, they do not always 
satisfy the criteria necessary for corrosion resistance and/or alimentary 
compatibility. 
It is also known that carbon has an excellent chemical and thermal 
resistance and a recognized alimentary compatibility. However, although it 
is possible to deposit carbon-containing product emulsions on a porous 
support, said product cannot be sintered. 
SUMMARY OF THE INVENTION 
One of the objects of the invention is to surmount this disadvantage in 
order to obtain a mechanically strong carbon-containing membrane and whose 
texture can also be adjusted. 
Another object of the invention is to obtain a membrane made from a 
mechanically strong ceramic material and whose texture can also be 
adjusted by a process which does not involve sintering. 
These objects are achieved according to the invention by a process for 
producing a permeable, porous, mineral membrane, characterized in that on 
a permeable, porous, mineral support is deposited at least one layer of a 
stable emulsion of mineral particles in a solution containing a cokable 
thermosetting resin and that the resin undergoes a polycondensation 
treatment, followed by coking, the coke formed ensuring both the 
mechanical connection of the mineral particles to one another and to the 
support. 
The mineral particles can be of carbon, the word carbon being understood in 
the widest sense and designating coke, carbon black, graphite, etc or of a 
ceramic material, such as silicon carbide, silicon nitride, titanium 
oxide, zirconium, etc. 
The mineral particles can be in the form of small fibres or grains, such as 
trichites. Their dimensions are chosen as a function of the sought texture 
for the membrane. 
For example, the thermosetting resin can be a phenolic resin or any resin 
which, after coking, leaves an adequate coke level to ensure the necessary 
behaviour of the deposit. 
The emulsions used can be of the "engobe" type, i.e. defloculated emulsions 
in an appropriate agent. 
For example, the particles are vigorously mixed in a polyvinyl alcohol to 
which has been added a defloculating agent, such as an ammonium salt. To 
this mixture is added the thermosetting resin used for the agglomeration 
of the particles. Following the deposition of this engobe on the support, 
floculation takes place which, after drying, ensures that the thus 
deposited layer is continuous and has a regular thickness. This complete 
procedure is called "engobe coating". 
It is also possible to use for the emulsion a "master paste" of paint based 
on mineral particles, emulsion taking place in a solute containing the 
thermosetting resin. In this case, the deposit on the support takes place 
by coating. 
In all cases, the resin undergoes a polycondensation treatment, followed by 
a coking treatment at between 400.degree. and 3000.degree. C. 
The permeable, porous support can be of various materials able to withstand 
the resin coking temperatures, i.e. carbon in the widest sense, ceramics, 
etc. The support can advantageously be of carbon, as are the particles. 
For example, it is possible to choose for the support a polygranular 
carbon or a carbon fibre/carbon matrix composite. In this case, an all 
carbon filtering element is obtained, i.e. having all the properties 
inherent in carbon and consequently usable in numerous fields. 
The production process according to the invention makes it possible to 
obtain, as desired, a membrane adhering to its support and having a 
texture adjusted to its intended use. Thus, as a function of the texture 
of the support, it is possible to successively deposit emulsion layers of 
mineral particles of different grain sizes in each layer, starting with 
the largest particles. In this case, there is no need for coking between 
each layer deposit, it merely being necessary to carry out a careful 
polycondensation, coking taking place when all the layers have been 
deposited. Thus, a very permeable, selective, asymmetric composite 
membrane is obtained. 
It should be noted that in order to adjust the texture, it is possible to 
impregnate the support with an appropriate impregnating agent, but such a 
process leads to a homogeneous material, which although selective, is not 
very permeable. Thus, the superiority of the process according to the 
invention is readily apparent. 
In the case where the support has a relatively loose or weak texture, the 
number of layers deposited to obtain the membrane with the desired texture 
can be considerably reduced if the first layer is produced with an 
emulsion of a fibrous material. This fibrous material is advantageously 
constituted by trichites of carbon or ceramic material, such as silicon 
carbide, silicon nitride, etc. Thus, to a loose texture support, it is 
necessary to apply a succession of layers of mineral particles of 
decreasing grain size, whereof the first is relatively large in order to 
prevent "punching" or penetration of the support. A deposit with an 
emulsion containing a fibrous material behaves like a non-woven fabric not 
reducing the permeability of the support and serving as a support for the 
following layers containing the finer particles. 
The actual production process of the invention makes it possible to obtain 
a membrane on variable geometry supports, i.e. plates, tubes, shaped 
members, etc. It is particularly appropriate for the deposition of 
membranes on the inside of tubular supports. 
It should be noted that, if for reasons of texture or cost, it is wished to 
deposit on the said support layers of different natures (e.g. one layer 
containing silicon carbide trichites, followed by one or more layers 
containing carbon powder), this is made possible by the process according 
to the invention.

APPLICATION EXAMPLES 
The following examples given in an indicative and non-limitative manner 
serve to illustrate the invention. 
EXAMPLE 1 
The support is a carbon fibre/carbon matrix composite with an air 
permeability 200,000 cm.sup.3 .multidot.s.sup.-1 .multidot.m.sup.2 
.multidot.bar.sup.-1 and with an average pore diameter of 30 .mu.m. 
Engobe coating takes place on said support, i.e. the deposition of the 
engobe as defined hereinbefore, said engobe being constituted by an 
emulsion of 4 .mu.m coke particles, whilst the thermosetting resin is a 
phenolic resin. Following polycondensation and coking of the resin, a 
membrane is obtained having an air permeability of 120,000 cm.sup.3 
.multidot.s.sup.-1 .multidot.m.sup.2 .multidot.bar.sup.-1 and with an 
average pore diameter of 20 .mu.m. 
EXAMPLE 2 
The process steps used to produce the membrane in this example are set 
forth in the flow diagram in FIG. 1. 
The support is the same as in example 1. Two successive engobe coatings are 
formed on this support using emulsion containing carbon particles of 
different grain sizes, the first containing 4 .mu.m coke particles and the 
second 0.1 .mu.m carbon black particles, the thermosetting resin being a 
phenolic resin. Each engobe coating is followed by polycondensation of the 
resin. Following the final coking, a membrane is obtained having an air 
permeability of 50,000 cm.sup.3 .multidot.s.sup.-1 .multidot.m.sup.-2 
.multidot.bar.sup.-1 and with an average pore diameter of 1 to 5 .mu.m. 
The supported membrane produced in this example is shown in cross-section 
as 10 in FIG. 2. As seen in FIG. 2, a support 12 is covered on its surface 
with a first layer 14 of 4 .mu.m coke particles 16, which do not penetrate 
into support pores 15. The coke particles 16 are bonded to each other into 
support 12 by means of a coke layer 18 formed by coking of the resin. 
On top of layer 14 is an additional layer 20 formed of 0.1 .mu.m carbon 
black particles 22 bonded together and to layer 14 by coke layer 18. 
EXAMPLE 3 
The support is the same as that of example 1. Engobe coating takes place on 
said support using an emulsion containing small carbon fibres (diameter 7 
.mu.m, length 0.2 mm), the thermosetting resin being a phenolic resin. 
Following polycondensation and coking of the resin, a membrane is obtained 
with an air permeability identical to that of the support and with an 
average pore diameter of 12 .mu.m. 
Such a membrane can either be used directly in filtering operations or can 
receive, as in example 2, a deposit of 0.1 .mu.m carbon black particles, 
which reduce the permeability to 50,000 cm.sup.3 .multidot.s.sup.-1 
.multidot.m.sup.-2 .multidot.bar.sup.-1 and the average pore diameter to 5 
.mu.m. 
The membranes produced according to these three examples can be used in 
microfiltering, or supports for receiving a new engobe layer and can 
undergo selectivity adjustment for use in ultrafiltration. 
EXAMPLE 4 
The support is an alumina ceramic support with a permeability of 10,000 
cm.sup.3 .multidot.s.sup.-1 .multidot.m.sup.-2 .multidot.bar.sup.-1 and 
average pore diameter 10 .mu.m. 
Engobe coating is carried on this support using an emulsion containing 
silicon carbide trichites, the thermosetting resin being a phenolic resin. 
Following polycondensation and coking of the resin, an asymmetric, porous 
medium is produced, which has a high permeability and which can be used in 
microfiltration.