US 3438811 A
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United States Patent "ice 3,438,811 REMOVAL OF COPPER CONTAINING INCRUSTA- TIONS FROM FERROUS SURFACES Lester W. Harriman, Angleton, and Paul E. Muehlberg, Jackson, Tex., and Fred N. Teumac, Charlotte, S.C., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Continuation-impart of application Ser. No. 387,481, Aug. 4, 1964. This application Oct. 18, 1965, Ser. No. 497,530
Int. Cl. B08b 3/08 US. Cl. 1342 9 Claims This application is a continuation-in-part of US. patent application Ser. No. 387,481, now abandoned.
This invention concerns the removal of plated copper from a ferrous surface. 7
It has long been desired to be able to remove plated copper from a ferrous suf-ace, e.g., steel, without also oxidizing excessively the iron thereof. This problem has been especially difficult and economically costly in the removal of plated copper from the internal metal surfaces of steam generating equipment, particularly high pressure steam generating equipment, which is operated in connection with a condenser, the condensing surfaces of which are of copper alloy.
In the operation of high pressure steam generating equipment (over 600 pounds per square inch steam pressure) in which the feed water is largely returned condensate from a copper alloy condenser, incrustations are usually produced upon the steam generating surfaces of the steam generator despite the fact that the feed water is substantially pure. These incrustations oftentimes contain copper, both in metallic form and combined with oxygen, corroded from the copper alloy condenser by the action of the condensed steam which carries the copper to the steam generator.
Attempts to remove such incrustations, as by the use of conventional acidizing procedures, are not wholly suc cessful. Tests have shown that by acidizing incrusted steam generating surfaces of the usual high pressure steam generator, having copper in the incnustations, some of the copper is removed from the incrustations and some of the copper so-removed is redeposited elsewhere on the surfaces of the steam generating equipment during the acidization so that only a partial net removal of copper from the incrusted surfaces results. Insofar as is known, there is no completely satisfactory method commercially available for treating the internal ferrous metal surfaces of high pressure steam generating equipment subject to deposition of copper-containing incrustations so as to free the surfaces of the incrustations and the copper. Accordingly, it is an object of the invention to provide a method fulfilling this need. Other objects and advantages will become apparent as the description of the invention proceeds.
It has now been discovered that an aqueous solution of a ferric chelate of a polycarboxylic acid chelating agent alone or together with some free polycarboxylic acid chelating agent when adjusted to an alkaline pH, i.e., in excess of 7 and up to about 10, by combination with ammonia, an amine or a hydroxyalkylamine or one or more of the preceding in amount of at least 50 mole percent in combination with up to 50 mole percent of an alkali metal hydroxide, is quite effective in dissolving plated copper from ferrous surfaces, e.g., of high pressure steam boiler surfaces.
3,438,811 Patented Apr. 15, 1969 The chelating agents used in the practice of this in vention as their ferric chelates and, if desired, in combination with ammonium and amine salts of the aforementioned polycarboxylic acid chelating agents are those of alkylene polyamine polyacetic acid (APAPAA), e.g., ethylenediaminetetraacetic acid -(EDTA), N hydroxyethylethylenediaminetriacetic acid (NHEDTA); nitrilotriacetic acid (NTA) and N-2hydroxyethyliminodiacetic acid (OHEtIDA); diethylenetriaminepentaacetic acid (DTPA); and mixtures thereof, hereinafter referred to broadly as polycarboxylic acid chelating agents.
In practice, a ferrous metal surface on which copper has plated out, e.g., that of a high pressure boiler, is heated at a temperature above room temperature and up to about 300 F. in the presence of an aqueous solution of a ferric chelate of a polycarboxylic acid chelating agent, if desired also containing free chelating agent, which solution is adjusted to an alkaline pH up to about 10 with ammonia and/or an amine or alkanolamine and with up to a 50 mole percent proportion of an alkali metal hydroxide, if desired. The resulting salts will here inafter be referred to as ammonium and amine salts of said chelating agents. A solution containing a total of about 0.5 weight percent polycarboxylic acid as iron chelate and, if desired, as ammonium and/or amine salt of said polycarboxylic acid up to a saturated solution thereof can be used. The weight proportion of iron chelate of the total of free and chelated chelating agent, i.e., degree or percent spentness, can be varied from between about 60 to about percent.
It appears that the solution containing ferric chelate oxidizes the copper metal to copper ions (Cu++ and Cu which react with the resulting ferrous chelate or with the free, i.e., uncomplexed or salt-form, chelating agent therein to form a copper chelate, and after a sufficient reaction time, as determined by analysis of the treating solution, the plated copper is dissolved. However, we
do not wish to be bound by this theory. As reaction proceeds, the ferric chelate isreduced to a ferrous chelate. This reduction would progressively slow the copper-dissolving reaction. In order to maintain a useful level of ferric chelate, i.e., some of the iron chelate must be in the ferric form, it has been found necessary to add an oxidizing agent to the copper-dissolving ferric chelate-containing solution so that ferrous chelate formed when the plated copper is oxidized to copper ions is reoxidized to ferric chelate for continued oxidation and subsequent dissolution of copper. This may be done by continuously or periodically monitoring or analyzing the copper-dissolving solution and adding an oxidant such as hydrogen peroxide, water-soluble salts such as alkali metal or ammonium nitrites, permanganates, persulfates, or perchlorates; or such gaseous oxidants as nitrogen tetraoxide, oxygen or air, advantageously by a sparger, in amount suflicient to maintain some of the iron chelate in the ferric chelate form. Of these oxidants, air is preferred, since it does not substantially affect pH and it introduces no extraneous matter.
The ferric chelates of polycarboxylic acid chelating agents useful in the practice of the present method are advantageously made by reacting iron, iron oxide or hydroxide or magnetite with a polycarboxylic acid chelating agent which has been adjusted to an alkaline pH up to about 10 with ammonia and/or an amine as stated above or with a mixture of ammonia and/or an amine and an alkali metal hydroxide, in proportions as specified, so that an average of not more than one free carboxylic acid group remains per mole of chelating agent, at least about half of the carboxylic acid groups of the chelating agent are in the ammonium or amine salt form and provided that an average of not more than half of the carboxylic groups are in the alkali metal salt form. Alternatively, the corresponding ferrous chelates are made and oxidized, at least partially to the ferric chelate form in the manner previously described, advantageously in use. It is not required that pure ferric and/or ferrous chelates be used. On the contrary, a commercially attractive iron chelate-chelating agent solution can be prepared by dissolving iron-containing scale from ferrous surfaces, e.g., those of oxide scaled ferrous boiler tubes, by the reaction with an aqueous solution of a polycarboxylic acid chelating agent adjusted to an alkaline pH with ammonia and/or an amine or mixture thereof or with ammonia and/or an amine and with an alkali metal hydroxide, thereby forming iron chelate containing both ferrous and ferric chelate. Such a method is described in copending US. patent application Ser. No. 296,464, filed July 22, 1963 now United States Patent 3,308,065.
The more preferred ammonium and/or amine salts whose ferric chelates are used in the process of this invention are those of the APAPAAs of the formula where n and m may each independently be 1, 2, 3 or 4, up to two of the carboxymethyl groups may be replaced with a p-hydroxyethyl group and one or more of the carboxymethyl groups may be replaced by carboxyethyl groups.
Since no two ferrous surfaces are likely to have the same amount of copper plated out thereon, it is advantageous that the copper-dissolving ferric chelate-containing solutions can be varied in concentration. The stoichiometry of polycarboxylic chelating agents is wellknown and can be used to calculate the requirements for copper solution. In the case of EDTA, for instance, one mole is required to solvate one mole of copper. Thus, as the plated copper is oxidized to copper ions, it reacts with free chelating agent present as a salt or as an iron chelate.
The degree or precent of spentness of iron chelatechelating agent solution is defined by:
Weight iron-complexed chelating agent Free chelating agent is determined analytically by a standard colorimetric or visual titration with strontium chloride to a constant turbidity after first filtering the sample solution. From this analysis, the percent by weight of unchelated chelating agent can be determined. Dissolved copper is analytical determined by a chlorimetric or visual determination using diethyldithiocarbamate sodium salt as follows. Transfer ml. of sample solution to a 250 ml. volume flask and dilute to volume with water. Mix solution and transfer 5 ml. thereof to a beaker or flask. Add 1 ml. of aqueous one percent diethyldithiocarbamate sodium salt and dilute to exactly 200 ml. volume with 2B ethanol. Mix solution thoroughly and take a reading on a colorimeter or take frequent samples and use the previous sample as a comparative blank to a constant visual end point. Total dissolved iron and copper can be determined by X-ray emission spectroscopy.
Most generally, optimum conditions for the removal 9 4 from the ferrous metal surfaces, e.g., of boiler tubes, according to a procedure previously indicated.
Degree of spentness determines the corrosion rate at any given temperature. The rate can be modified with iron-oxidation inhibitors. At each temperature, there is a degree of spentness above which there is no further corrosion. At about 140 F., it is about percent spentness, and at about 180 F., it is about 88 weight percent spentness, in the latter case, if ca. 0.05 weight percent thioethylamine iron-oxidation inhibitor is present. At F., the degree of spentness can be reduced to 67 percent without any practical difference in corrosion rate. Theoretically, a solution containing more ferrous EDTA should require more oxygen. The efficiency of copper oxidation is increased, however, and the same amount or less air is required to oxidize the copper in a more highly spent solution.
If the solution is 67 percent spent or more, no iron oxidation inhibitor is required at 140 F. At or F., the solution should be 91 percent or more spent if no iron inhibitor or only an inhibitor as disclosed in US. Patent 3,077,454, is used.
The solubilized copper appears to be stabilized by the formation of cupric chelate. Therefore, in highly spent solutions, the stripping of copper is accompanied by a reduction in the dissolved iron to give a colloidal ferric hydroxide precipitate.
After plated copper and iron oxide are removed from the treated ferrous surfaces, rising is accomplished, e.g., by draining the boiler and refilling with water, all with air agitation. This facilitates the removal of suspended undissolved solids and causes better rinsing. Finally, the rinse water is drained off. If the magnetite is not completely removed, small areas of copper are protected from oxidation. In contact with air and water, these areas develop tiny ant hills of corrosion products, i.e., red rust.
The following examples describe completely representative specific embodiments and the best mode contemplated by the inventors of practicing the invention. They are not to be taken as limiting the invention other than as defined in the claims. Parts and percentages therein are given by weight.
Example 1 A formulated spent solution of ammoniated EDTA was prepared by adding iron powder, in amount sufficient to saturate, to an aqueous 7.6 weight percent solution of ammoniated EDTA originally adjusted to a pH of about 9 with free ammonia and maintained at a reaction temperature of about 95 C. for 30 minutes in the presence of a nitrogen atmosphere so that the ferrous chelate of EDTA was formed. This solution was used to prepare a series of 3.8 percent total EDTA solutions having various percentages of spentness, e.g., by mixing with ammoniated EDTA adjusted to a pH of about 9 with free ammonia and with water, the percent of spentness being measured as described above. About one-half gallon of such solutions, some of them modified with a small percentage, up to ca. 0.1 weight percent of an iron-oxidation inhibitor, were then placed in a simulated high pressure boiler containing 2.3-2.5 grams of plated copper on a square foot of inside surface. Simulated boiler heaters were turned on to give various operating temperatures. Until a temperature equilibrium was reached, a constant flow of nitrogen through the simulated boiler was maintained to get circulation and exclude air. The nitrogen purge was then changed to air, using a pressure regulator and a valve to meter air through a coarse frit at the bottom of the boiler tubes. Time and rate of air flow were measured. The solutions were sampled for subsequent iron analysis, total iron and copper being determined by X-ray emission spectroscopy. One or more steel coupons were suspended in the boiler tubes for corrosion data.
At pre-determined times, the steel coupons were removed and dried for determination of corrosion data and limited samples of the aqueous solutions were taken. At the completion of each run, heat was discontinued and the solution was drained from the bottom of the boiler. Distilled water was then added and the boiler tubes were air agitated for several minutes. The rinse water was drained and the boiler tubes removed and examined. Operational data and results are summarized in the following table.
TABLE I.-SUMMARY OF AIR-BLOWING DATA Percent Air-Blow Copper Inhibitor 1 A. EDTA Temp,
Spent 2 F. Total, C.F.M. Required Avg, Percent min. Mins. mils Stripped Run No 1 11-124 plus ThEA 88 180 40 07 100 2 .(10 89. 5 180 035 100 98. 5 180 40 035 100 91 180 004 100 83 140 125 004 100 140 180 004 100 67 140 180 004 100 67 140 210 004 100 85 140 40 035 100 140 40 035 91. 0 180 40 035 100 90 160 40 035 100 79 80 80 035 100 68. 5 140 240 004 100 67. 0 140 30 .035 100 70. 0 140 40 035 99+ 86. 0 180 40 035 100 70.0 140 30 035 100 82. 0 180 40 035 100 90. 2 180 40 035 100 86. 5 180 40 035 100 81. 2 145 035 100 Final Gone. in Solution (Theoretical) O ColrRrosion ate, Percent Cu Percent Fe Required Used Percent lbs/itfl/day (lb. moles) 00035 None None None None 0005 00034 00013 00005 0002 None 0054 None None None None 0189 None 0330 0089 0403 0233 1 A-124, an iron-oxidation inhibitor disclosed in U.S. Patent 3,077,454, is used in amount of 0.1%. ThEA is thloethylamine used in amount of 0.05%.
2 Ammoniated EDTA, pH ca. 9.
3 The calculated theoretical no. oi mins. at the designated flow rate to convert the ferrous EDTA to ferric EDTA and Cu to Cu ion.
Elemental analysis of the solutions in runs 1, 2 and 3 indicated that the copper was being complexed by the EDTA. In the case of highly spent solvent (98.5 percent) the formation of tferric hydroxide was obvious. In other runs, the colloidal sediment could only be detected by the difierence in iron analysis in filtered and unfiltered solutions.
Free ammonia content is :not critical. At the completion of run 3, for example, the pH was almost neutral.
Runs 9 and 10 established the corrosion rate at F. when A124 was the only inhibitor employed. The data indicate that a spentness of 6-7 percent gave a negligible iron corrosion rate.
Run 13 indicated that, although there was no iron corrosion at 80 F. and 79 percent spentness, the copper stripping rate was slow.
Example 2 In accordance with the method described in Example 1, the following named polycarboxylic acid chelating agents were prepared as 80 percent spent solutions and surfaces. Analytical procedures were the same as in Example 1.
Polycarboxylic Acid Time to Strip Oil 100% of Scale, mins.
Example 3 through the solution in contact with a 0.25 mil copperscale on a ferrous surface.
Conc. EDTA, percent: Copper-strip 38 Yes.
2 Yes. 0.5 Yes. 0.25 Partial.
At the 0.25 percent level, corrosion of the substrate was accelerated at stress areas.
Example 4 The following listed amines have been found to be operable in the practice of this invention for the purpose of adjusting the pH to the proper range and thereby forming the salts or partial salts of polycarboxylic acids and/or their partial salts, thereafter spending at least a portion of the so-formed solution of chelating agent by adding iron to form iron chelate. These highly spent solutions are operable in removing copper-containing scale from ferrous surfaces while having little or no corrosive effect on the substrate, they also gave a passivated ferrous surface.
Amines: Used with: polycarboxylic acid Ethanolamine EDTA.
Ethanolamine Ethylenediaminetetrapropionic acid.
Ethylenediamine N,N-di-([i-hydroxyethyl) glycine.
Ammonia Tetramethylenediamine- N,N,N,N'-tetraacetic acid.
Ammonia (2-hydroxyethylimino) diacetic acid.
What is claimed is:
1. A process for removing copper from a ferrous metal surface containing copper thereon by contacting said surface with an aqueous alkaline solution wherein the solution employed contains as an essential constituent at least one member of the group consisting of ferric chelates of polycarboxylic acid chelating agents and mixtures 'of ferric and ferrous chelates of polycarboxylic acid chelating agents in amount suflicient and for a time sufficient at a reaction temperature above about 68 F. and up to about 300 F., to dissolve said copper.
2. A process as claimed in claim 1 wherein the said solution may also contain a salt of the group consisting of ammonium, amine and hydroxyalkylamine salts of polycarboxylic acid chelating agents.
3. A process as claimed in claim 2 wherein the total iron chelate originally present ranges between about and about 100 weight percent of total salt form and iron chelate and the solution originally contains a total of between ca. 0.5 Weight percent and up to a saturated solution of salt form and iron chelated chelating agent.
4. A process as claimed in claim 3 wherein some iron chelate is maintained in the ferric form by the addition of a water-soluble compatible oxidizing agent.
5. A process as claimed in claim 3 wherein some iron chelate is maintained in the ferri form by bubbling air through said aqueous alkaline solution.
6. A process as claimed in claim 3 wherein the iron chelate and chelating agent salt are those of ethylenediaminetetraacetic acid.
7. A process as claimed in claim 3 wherein the reacted ferrous surface is rinsed with rinse water containing a water-soluble oxidant.
8. A process as claimed in claim 3 wherein the reacted ferrous surface is rinsed with rinse water while air is bubbled therethrough.
9. A process for removing copper from a ferrous metal surface containing copper thereon by contacting said surface with an aqueous alkaline solution wherein the solution contains as an essential constituent ammoniated ethylenediaminetetraacetic acid adjusted to a pH of about 9 with free ammonia and iron chelate thereof wherein the iron chelate originally present ranges between about 60 and ca. 100 weight percent of total salt form ethylenediaminetetraacetic acid and iron chelate thereof and the solution originally contains between about 0.5 weight percent and up to a saturated solution of total salt form and iron chelate of ethylenediaminetetraacetic acid while air is being bubbled therethrough at a reaction temperature above about 68 F. and up to about 300 F. in amount suificient and for a time sufficient to dissolve said copper, removing said solution from said ferrous metal surface, contacting said surface with rinse water while bubbling air therethrough and removing said rinse water.
References Cited UNITED STATES PATENTS 2,396,938 3/1946 Bersworth 1342 2,959,555 11/1960 Martin et al. 134-41 3,308,065 3/1967 Lesinski 25282 L. DEWAYNE RUTLEDGE, Pl'ilfllll'y Examiner.
T. R. FRYE, Assistant Examiner.
U.S. Cl. X.R.