A suitable member formed from sintered, powdered, stainless steel is contacted with a nitrate solution of a soluble alkali metal nitrate and a metal such as zirconium in a pH range and for a time sufficient to effect the formation of a membrane of zirconium oxide preferably including an organic polymeric material such as polyacrylic acid

Brandon, Craig A.

Gaddis, Joseph L.

English

NASA Technical Reports Server

United States Patent [191 [I 11 Patent. Number: 4,762,619 
Gaddis et al. [45] Date of Patent: Aug. 9, 1988 
METHOD OF FORMING DYNAMIC 
MEMBRANE ON STAINLESS STEEL 
SUPPORT 
Inventors: 
Assignee: 
Appl. No.: 
Filed: 
Joseph L. Gaddis; Craig A. Brandon, 
both of Clemson, S.C. 
The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
898,449 
Aug. 19, 1986 
Related U.S. Application Data 
Continuation of Ser. No. 755,960, Jul. 17, 1985, aban- 
doned, which is a continuation of Ser. No. 119,334, 
Feb. 7, 1980, abandoned. 
Int. (3.4 .............................................. BOlD 13/04 
U.S. Cl. .................................... 210/639; 210/653; 
210/500.25; 210/500.35; 427/245; 427/247 
Field of Search ............... 210/653, 652, 502, 506, 
210/500.25, 639, 500.35; 55/158; 427/244, 245, 
246, 247; 264/41, 45.8; 755/960 
tS61 References Cited 
U.S. PATENT DOCUMENTS 
3,022,187 2/1962 Eyraud et al. ................. 210/500 M 
3,331,772 7/1967 Brownscombe et al. ...... 210/500 M 
3,344,928 10/1967 Kraus et al. .................... 210/500 M 
3,449,245 6/1969 Johnson et al. ..................... 210/502 
3,577,339 5/1971 Baird et al. ......................... 210/653 
3,926,799 12/1975 Thomas et al. ................ 210/500 M 
OTHER PUBLICATIONS 
Osmosis and Synthetic Membranes, S. Sourirajan, Editor; 
Nat. Res. Con. of Canada, 1977, pp. 295-312. 
Primary Examiner-Peter Hruskoci 
Attorney, Agent, or Firm-Hardie R. Barr; John R. 
Manning; Edward K. Fein 
A suitable member formed from sintered, powdered, 
stainless steel is contacted with a nitrate solution of a 
soluble alkali metal nitrate and a metal such as zirco- 
nium in a pH range and for a time sufficient to effect the 
formation of a membrane of zirconium oxide on the 
stainless steel support, the membrane preferably includ- 
ing an organic polymeric material such as polyacrylic 
acid. 
Thomas, D. G., “Dynamic Membranes . . .  ”, in Reverse 
1571 ABSTRACT 
9 Claims, No Drawings 
https://ntrs.nasa.gov/search.jsp?R=19880019668 2020-03-23T20:06:22+00:00Z
4,762,6 19 
1 
METHOD OF FORMING DYNAMIC MEMBRANE 
ON STAINLESS STEEL SUPPORT 
ORIGIN OF THE INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract and is sub- 
ject to the provisions of Section 305 of the National 
Aeronautics and Space Act of 1958, Public Law 85-568 
(72 Stat. 435; 42 U.S.C. 2457). 
This application is a continuation of application Ser. 
No. 755,960, filed July 17,1985, now abandoned, which 
is a continuation of Ser. No. 119,334, filed Feb. 7, 1980, 
now abandoned. 
DESCRIPTION 
1. Technical Field 
The present invention relates to a method of forming 
a hyperfiltration member employing a membrane dy- 
namically formed on a stainless steel support. 
“Dynamic” membranes of various solute-rejecting 
substances deposited on the surface of porous supports 
or substrates can be used in hyperfiltration methods to 
remove contaminating chemicals from aqueous solu- 
tions by reverse osmosis. Such reverse osmosis mem- 
branes may be used in desalination and other water 
purification equipment, and have potential application 
in the textile industries for separating dyes from aqueous 
solvents. 
2. Background Art 
Prior art methods for forming dynamic membranes 
on porous supports such as for example, ceramic sup 
ports, have utilized what may be described as chloride 
chemistry in that the hydrous metal oxide membrane 
was deposited on the support utilizing chloride salts of 
the metal. Alternately, the hydrous metal oxides, in a 
slurry or in colloidal suspension can be deposited on the 
supports. 
U.S. Pat. No. 3,577,339 to Baird et al, teaches the 
formation of zirconium oxide membranes on a porous 
stainless steel substrate utilizing a solution containing 
hydrous zirconium oxide to form a solute-rejecting 
membrane. U.S. Pat. No. 3,413,219 to Kraus et al dis- 
closes the formation of dynamic membranes of zirco- 
nium oxides and other metals capable of forming hy- 
drous oxides, the membranes being formed by contact- 
ing a porous support with an aqueous solution contain- 
ing colloidal particles of a polyvalent metal capable of 
forming the hydrous metal oxides. 
Other methods of forming dynamic membranes of 
hydrous metal oxides on stainless steel members such as 
tubes, pipes and the like are disclosed in U.S. Pat. No. 
3,331,772 to Brownscombe et al, U.S. Pat. No. 3,344,928 
to Kraus et al, U.S. Pat. No. 3,449,245 to Johnson et a1 
and US. Pat. No. 3,926,799 to Thomas et al. 
As the prior art discussed above shows, methods for 
depositing dynamic membranes such as, for example, 
zirconium oxide-polyacrylic acid membranes on porous 
supports are well established. However, until now it has 
not been possible to form such membranes on stainless 
steel supports without severely corroding the stainless 
steel in the process. Nonetheless, supports made from 
stainless steel are attractive, and indeed necessary in 
certain environments utilizing hyperfiltration technol- 
ogy because of the inherently stronger structural fea- 
tures of such stainless steel supports. 
DISCLOSURE OF INVENTION 
It is therefore an object of the present invention to 
provide a method of forming a dynamic membrane on a 
5 stainless steel support without unduly corroding the 
stainless steel support. 
Another object of the present invention is to provide 
a method of forming a member, useful in hyperfiltration 
applications, which possesses excellent structural integ- 
10 rity by avoiding the corrosion products attending the 
presence of chlorine in the process. 
The above and other objects of the present invention 
will become apparent from the description given herein 
and the appended claims. 
In accordance with the method of the present inven- 
tion, a support, in suitable form, of sintered, powdered 
stainless steel is contacted with an aqueous solution of 
the nitrate salt of a polyvalent metal and a soluble alkali 
metal nitrate in a pH range of from about 3 to about 5, 
20 for a time sufficient to form a hydrous metal oxide mem- 
brane on the support. 
Especially.desirab1e membranes are formed by the 
addition of an organic polymer such as polyacrylic acid. 
BEST MODE FOR CARRYING OUT THE 
INVENTION 
The porous support used in the method of the present 
invention is comprised of powdered stainless steel 
formed in a suitable configuration and sintered into a 
30 porous solid of substantial strength. While the support 
can take many forms for maximum efficiency and utili- 
zation of the membrane, the support is generally in the 
form of a tube, the membrane being formed on either 
the inside or the outside of the tube. It is to be under- 
35 stood however that the invention is not limited to any 
particular form and can be used in the coating of irregu- 
lar surfaces, many of which cannot be uniformly coated 
with a filter aid by prior art in situ coating methods. The 
use of sintered powdered metallurgy to provide tubes, 
40 plates and other such porous forms is well known and 
need not be described in detail here. By control of pow- 
der size, initial forming compaction, and sintering con- 
ditions, the tubes or other formations may be produced 
having controlled porosity. Formations, e.g., tubes, 
45 having a nominal porosity of about 2 to about 30% and 
pore ratings (based on absolute particle retention) of 
from about 0.2 to about 20 microns in diameter are 
usually satisfactory for purposes of the invention. A 
range of from about 0.2 to about 10 microns is also 
In forming the membranes of the present invention, 
an aqueous nitrate solution of a suitable polyvalent 
metal and a soluble alkali metal nitrate is employed. 
Examples of typically useful salts are the nitrates of tri 
55 and tetravalent metals such as iron, zirconium, tin (IV) 
and thorium. Particularly preferred as the membrane- 
forming metal is zirconium, zirconium nitrate (also 
known as zirconyl nitrate) being an especially preferred 
salt. In general, metallic nitrates such as those noted 
60 above which are capable of forming hydrous metal 
oxide membranes are desired. The aqueous nitrate solu- 
tion of the membrane forming metal contains, in addi- 
tion, a soluble metal nitrate such as an alkali metal ni- 
trate, e.g., sodium or potassium nitrate. Generally 
65 speaking, the nitrate salt of the membrane forming 
metal will be present in an amount of from about 0.01 to 
about 0.10 grams per liter while the soluble metal ni- 
trate, e.g., sodium or potassium nitrate, will be present 
15 
25 
50 useful. 
3 
4,762,6 1 9 
in amounts of from about 0.01 to about 0.10 moles per 
liter. The pH of the aqueous nitrate solution has a pH of 
below about 5 and is generally from about 3 to about 5, 
a pH of about 4 being particularly preferred. 
Although not necessary, a filter aid can be used with 5 
advantage in the method of the present invention. Suit- 
able, non limiting examples of filter aids which can be 
used include diatomaceous earth, perlite, asbestos fi- 
bers, cellulose fibers, dried-silica gel and carbon. Pow- 
dered carbon has been found to be an especially desir- 10 
able filter aid. 
While useful membrane formation is accomplished, 
according to the present invention, using only the aque- 
ous nitrate solution of hydrous oxide forming metals 
described above, a particularly desirable membrane can l5 
be formed by the addition of certain organic polymers 
such as for example, polyacrylic acid. When a poly- 
meric material such as polyacrylic acid is employed, the 
aqueous nitrate solution is further acidified, preferably 
with nitric acid, to avoid the presence of chlorine and 2o 
the products of chlorine corrosion of stainless steel, to 
reduce the pH to between 1.5 and 2.5, preferably about 
2. A suitable amount of the organic polymers, as for 
example from about 20 to about 100 and preferably 5 0  25 
ppm, is then added to the aqueous nitrate solution. The 
pk of the solution is thenslowly brought to neutral 
using sodium hydroxide or some other suitable basic 
material. 
In the method of the present invention, the stainless 
steel support is contacted with the aqueous nitrate solu- 
tion, the nitrate solution being circulated past and 
through the porous stainless steel member, e.g., the 
tube, either in the presence or absence of the inert filter 
aid. Contact between the support and the aqueous ni- 
trate solution is maintained until the flow rate through 
the support diminishes indicating that the hydrous metal 
oxide membrane has formed. The time may range from 
seconds to hours depending on the applied pressure, the 
pore size, the concentration of the aqueous nitrate solu- 
tion and other such parameters. 
To further illustrate the invention, the following non- 
limiting example is presented. 
EXAMPLE 
An aqueous solution of approximately 0.025 molar 
sodium nitrate and containing about 0.04 grams per liter 
of zirconium nitrate at a pH of 4 was circulated past and 
through a porous stainless steel tube prepared from 
sintered, powdered stainless steel. Flow rate through 
the tube diminished to an average velocity of from 
about 10-4 m/sec. to 5 X  10-3 m/sec. at an applied 
pressure of 4x 10 6 to 7x 106N/m2. At this point the 
solute rejecting membrane of zirconium oxide had 
formed on the stainless steel tube. The electrical con- 
ductivity of the throughflow was significantly lower 
than the feed flow conductivity indicating substantial 
salt rejection by the membrane. At this point nitric acid 
was added to lower the pH to about 2 and approxi- 
mately 50 parts per million of polyacrylic acid was 
added to the solution. The pH was slowly brought to 
neutral by the addition of sodium hydroxide to the 
solution to complete the formation of a zirconium ox- 
ide-polyacrylic acid membrane. It was found that the 
electrical conductivity of the throughflow or filtrate 
from the tube was typically from 5 to 10% of the.con- 
ductivity of the feed solution at a throughflow velocity 
in the range of from about 3 to about 5~ 10-5 m/sec. 
30 
35 
40 
45 
50 
55 
60 
4 
As the above example demonstrates, the method of 
the present invention permits the formation of a highly 
efficient solute rejecting membrane directly on the sin- 
tered stainless steel tube. As noted above, the produc- 
tion of iron salts formed by normal chloride chemistry 
deposition processes prohibits membranes of good qual- 
ity. The use of the nitrate salts allows formation of the 
membrane without production of the iron salts which 
would normally occur with acid chloride solutions 
reacting with the stainless steel tube. It will be readily 
appreciated that because the supports employed are 
made of stainless steel, the present invention permits 
fabrication of membrane systems capable of high reli- 
ability in many severe mechanical environments such as 
might be associated with flight conditions. 
The dynamic membrane formed according to the 
present invention can be used in numerous applications 
where it is desirable to separate contaminants from 
aqueous solutions by reverse osmosis. For example, the 
membranes may be used in the dyeing process in the 
textile industry where separation of the dyeing material 
from its aqueous solution permits recovery and reuse of 
the dye. 
From the above description, it is apparent that nu- 
merous modifications may be made in the method of the 
present invention without departing from the spirit or 
scope thereof. Accordingly, it is intended that the in- 
vention be limited only by the appended claims. 
We claim: 
1. A method of preparing a hyperfiltration member 
forming a porous stainless steel substrate from pow- 
dered stainless steel using suitable particle size, 
compression and sintering conditions to yield a 
porous stainless steel substrate having a nominal 
porosity of from about 2% to about 30% and an 
absolute particle retention pore rating of from 
about 0.2 micron to about 20 microns in diameter; 
flowing through and about and in contact with the 
porous substrate, an effective amount of an aqueous 
nitrate solution comprising zirconium nitrate, and 
also a material selected from the group consisting 
of soluble sodium nitrate and soluble potassium 
nitrate to form a solute rejecting membrane of 
zirconium oxide on said stainless steel substrate; 
continuing the flow of the.aqueous nitrate solution 
through and about the porous substrate at a flow 
rate and pressure and for period of time effective to 
form the zirconium oxide membrane on the porous 
substrate; 
adding to the aqueous nitrate solution nitric acid 
suficient to lower the pH to between 1.5 to about 
2.5; 
adding to the aqueous nitrate solution polyacrylic 
acid in the amount of from about 20 to about 100 
parts per million; 
slowly adding to the aqueous nitrate solution sodium 
hydroxide until the aqueous nitrate solution is neu- 
tral to form a zirconium oxide-polyacrylic acid 
membrane on said substrate; 
performing all steps in the absence of chlorine or 
chlorine compounds. 
2. A method of claim 1 wherein said aqueous nitrate 
including the steps of: 
65 solution is applied to said support in the presence of an 
3. The method of claim 2 wherein said filter aid mate- 
inert, non solute-rejecting filter aid material. 
rial comprises powdered carbon. 
4,762,619 
5 
4. The method of claim 1 wherein said porous stain- 
less steel substrate has a pore rating, based on absolute 
particle retention, of about 0.2 to about 10 microns. 
5. The method of claim 4 wherein said support com- 
prises a tube. 5 
6. The method of claim 1 wherein the aqueous nitrate 
solution contains zirconium nitrate in the amount of 
about 0.4 grams per liter. 
7. The method of claim 1 wherein the material se- 
lected from the group is approximately 0.025 molar 10 
sodium nitrate. 
8. The method of claim 1 wherein the aqueous nitrate 
solution contains zirconium nitrate in the amount of 
about 0.04 grams per liter, the material selected from 
the group is approximately 0.025 molar sodium nitrate, 15 
the nitric acid is added until the pH is about 2, and 
approximately 50 parts per million of the polyacrylic 
acid is added to the aqueous nitrate solution. 
9. A method of filtering an aqueous solution by form- 
ing a membrane on a substrate, comprising the steps of: 20 
circulating an aqueous nitrate solution at a pH of 
about 4 past and through a porous stainless steel 
6 
tube prepared from sintered, powdered stainless 
steel, the aqueous nitrate solution containing ap- 
proximately 0.025 molar sodium nitrate and further 
containing about 0.04 grams per liter of zirconium 
nitrate, at a flow rate and pressure and for a period 
of time effective to form a solute rejecting mem- 
brane of zirconium oxide on the stainless steel tube, 
adding nitric acid sufficient to lower the pH of the 
aqueous nitrate solution to about 2; 
adding approximately 50 parts per million of poly- 
acrylic acid to the aqueous nitrate solution; 
slowly adding sodium hydroxide to the aqueous ni- 
trate solution to bring the pH to neutral to form a 
zirconium oxide-polyacrylic acid membrane on the 
stainless steel tube; 
performing all steps in the absence of chlorine; and 
filtering the aqueous nitrate solution by flowing it 
through the stainless steel tube at a throughflow 
velocity in the range of about 3 to about 5 x 10-5 
m/sec. * * * * *  
25 
30 
35 
40 
45 
50 
55 
60 
65 


Method of forming dynamic membrane on stainless steel support

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