US3547697A - Method of removing copper containing iron oxide scales from iron - Google Patents

Description

Lil- I- United States Patent ABSTRACT OF THE DISCLOSURE The redeposition and precipitation of copper during the removal of copper-containing iron oxide incrustations from ferrous metal surfaces, with an acidic solution, is prevented by contacting the ferrous metal surface with an aqueous acid solution containing a copper complexer from the group consisting of hexahydropyrimidine-Z- thione, N-(2-hydroxyethyl)-ethylene thiourea, diethanol thiourea, 4-methylimidazolidine-Z-thione and mixtures thereof.

This is a continuation-in-part of our copending application Ser. No. 584,339 filed Oct. 5, 1966, now abandoned, the disclosure of which is incorporated herein by reference BACKGROUND OF THE INVENTION Iron oxide scales containing copper and copper oxide are frequently encountered in industrial cleaning. Normally, methods of removing the scale require two stages, a copper removal stage and an iron oxide removing stage. The iron oxide is removed with an aqueous solution of an acid such is hydrochloric acid. Even if a copper removal stage is first used, there will normally be copper present in the iron oxide incrustation when it is treated with hydrochloric acid. During the dissolution of the iron oxide, copper will also dissolve but it will be redeposited from the acid solution and plated back onto the iron surface being cleaned.

A method of removing iron oxide incrustations containing copper, using an acid solution, is described in US. Pat. No. 2,959,555. According to that patent, copper redeposition is prevented by treating the copper-containing iron oxide incrustation with a suitable .acid solution in which is dissolved a urea derivative selected from the group consisting of thiourea, 1,3-dimethy1 thiourea, ethylene thiourea, 1,3-diethyl thiourea, and 1,3-diisopropyl thiourea. Although the use of such thioureas aids in preventing copper redeposition, it exhibits one property which is highly disadvantageous. In field use, the amount of thiourea used is often insufiicient to completely complex the copper in the boiler scale. In such cases, a precipitate which is a thick, curdy mass, sometimes described as a cottage cheese precipitate, maybe formed. The formation of this heavy precipitate is particularly troublesome when scale is being removed from the inside of a pipe, for example, because various means must be used to flush or mechanically scrape this heavy precipitate out of the pipe or other vessel in which it is formed.

SUMMARY OF THE INVENTION It is accordingly an object of the present invention to provide a method of removing copper-containing iron oxide incrustations from ferrous metal surfaces.

Another object of this invention is to provide a one step operation wherein copper-containing iron oxide incrustations may be removed from ferrous metal surfaces without redeposition of copper.

It is a further object of the present invention to provide ice a method of removing copper-containing iron oxide incrustations on ferrous surfaces using a copper complexing material which will not form a heavy, difiicult-to-remove precipitate.

Other objects and advantages will become apparent from the following description of the invention.

Briefly, the present invention comprises removing incrustations, e.g., boiler scales containing copper, by treating a ferrous surface having incrustations with an acid solution containing a complexing agent selected from the group consisting of hexahydropyrimidine-Z-thione, N-(2-hydroxyethyl)-ethylene thiourea, diethanol thiourea, 4-methylimidazolidine-2-thione and mixtures thereof.

DESCRIPTION OF PREFERRED EMBODIMENTS The complexing agents of this invention include hexahydropyrimidine-Z-thione (also known as N,N'-propylene thiourea) having the formula:

N-(2-hydroxyethyl)-ethylene thiourea having the formula:

8 H-N-Pl-N-CHzCHzOH Hr-C 2 diethanol thiourea having the formula:

5 H-N- NH HOCHICH CHzCHzOH 4-methylimidazolidine-2-thione having the formula:

and mixtures thereof.

e act so utton containing the complexing agent may be contacted with the surface to be cleaned by any suitable means, e.g., soaking, pouring, spraying, etc. The invention is particularly suitable for cleaning the inside of vessels of complex shape where formation of a precipitate can present difficult removal problems. Normally, the area to be cleaned is contacted by filling the vessel with the acid solution containing the complexing agent of this invention. It has been found that copper removal can be greatly enhanced by stirring or other suitable means of agitation during the removal step.

Because of the corrosiveness of the incrustation-removing acid solution with respect to the ferrous surface,'it is frequently necessary to use a corrosion inhibitor in the acid treatment step. Any suitable inhibitor, including nitrogen-containing corrosion inhibitors, may be used.

The present invention provides a one step method of removing both iron and copper in mi. incrustation found on a ferrous surface. Under some circumstances, however, it is first desirable to treat the incrusted surface with an alkaline solution containing an oxiding agent. This initial alkaline treatment will tend to remove a substantial portion of the copper in the incrustation and thus insure that a subsequent acid treatment using the complexing agent of this invention will result in essentially complete removal of all iron and oxide and remaining copper. An effective method for removal of copper is to treat the copper-containing surface with a heated ammonia solution. The

solution may contain oxidizing agents such as sodium bromate, ammonium persulfate, sodium chlorate, mixtures of the foregoing, etc. The treatment is particularly effective at temperatures of about 120 to 180 F.

Any temperature may be used for treatment of the ferrous surface with the acid solution of this invention. In general, a higher rate of removal is achieved at higher temperatures. Thus, the rate of removal of the incrustation may be enhanced by using elevated temperatures, usually in the range 120 to 160 F. v V

Any acid capable of dissolving the iron oxides and copper-containing incrustations on a ferrous metal surface may be used. Suitable acids include hydrochloric acid, sulfuric acid and phosphoric acid. The acid is used normally as an aqueous solution with suitable concentrations being in the range of from about 3% to about 30%. The concentration of the complexing agent of this invention may vary over a wide range. However, a ratio of at least about 6 moles per mole of copper is preferred for maximum precipitation inhibition. Much smaller amounts are effective, however. Thus, if about 2 moles of N-(2-hydroxyethyl)-ethylene thiourea is present per mole of copper, eifective copper removal may be achieved.

The complexing materials of the present invention provide highly effective prevention of copper redeposition without the attendant heavy precipitate experienced in the use of thiourea when the thiourea concentration is not carefully controlled vis a vis the amount of copper.

liter, 1.8 grams concentrated H 80 (i.e., 1 ml. per liter) per liter, and 0.5 gram per liter of Armohib 31, a quaternary ammonium, sulfuric acid inhibitor, manufactured by Armour Chemical Co. Copper was allowed to plate onto these pipe sections at room temperature for 3 hours, after which the plating solutions were analyzed for remaining copper. The diiferences between the original concentration and the final concentration of these solutions, divided by 4, represented the amount of copper plated onto the pipe section. Next, a solution of 5% HCl and 0.1% of Rodine 213, a rosin amine ketone, hydrochloric acid inhibitor manufactured by Amchem Co., was prepared, and 250 ml. portions of this solution containing the various weights of thiourea (TU) and N-(2-hydroxyethyl)-ethylene thiourea (HEETU) shown below were added to each sample. The samples were'then placed into a 150 F. bath and were allowed to react for 6 hours. After the reaction period, the samples were emptied of their contents and rinsed carefully with distilled water. The remaining copper on each sample was removed with ammoniacal solution of (NHQ SO; and analyzed by rinsing the liquid into a 100 ml. volumetric flask. HCl (1:1) was added until the color disappeared. Then, N11 (1:4) was added until the color returned. At this point, 10 ml. of CDTA solution was added and the solution was diluted to the mark with distilled water. An analysis was made for percent transmittance with a Hach colorimeter using a red filter No. 2408. The results are illustrated in Table I.

Whereas the precipitate formed when using thiourea is heavy and thus diiiicult to flush or scrape, a smaller amount of precipitate is formed using the complexers of the present invention and any precipitate which does form TABLE 1 Copper Copper Iron Wt. ratio Molar ratio re covered removed present (complexerzCu) (eomplexenCu) (mg. Cu) (mg. Cu) (mg. Fe)

4. 8:1 4:1 49 37 950 4. 9: 1 4:1 51 23 98( 72:1 6:1 39 47 1,150 7.2:1 6:1 33 49 1,180 2. 4:1 2: 1 b8 26 1, 130 2. 4:1 2: 1 26 1, 050 9. 2:1 4:1 62 26 1, 180 9.2:1 4:1 63 21 1,180 13.8:1 6:1 61 23 1,050 13.9:1 6:1 60 26 1,200 4. 6:1 2:1 64 24 950 4. 7: 1 2: l 65 21 9B0 EXAMPLE 2 The procedure in Example 1 was followed except that the samples were constantly stirred throughout the 6 hour reaction. The results are illustrated in Table II.

TABLE 11 Wt- Copper Copper plated Wt. oomplexer Wt. ratio Molar ratio recovered removed (mg. C (gms) (eomplexer:Cu) (eomplexer: Cu) (mg. Cu) (mg. Cu) 76.- 1.05 HEETU 13. 9:1 6:1 36 40 96 1 33HEETU 13.9:1 6:1 59 37 116 1 09 HEETU 9.3:1 4:1 84 32 74 0 35 HEETU 4.7:1 2:1 23 51 117 TU 4.8:1 4:1 66 61 is a light finely divided solid which can be readily re- EXAMPLE 3 moved from the area which is being cleaned.

The following examples are presented in order to more fully illustrate the invention.

The procedure of Example 1 was duplicated except that the treating temperature was varied in order to determine the efiect of temperature on copper removal. The

results are illustrated in Table 111.

TABLE 111 Copper Copper Wt. Cu iated Wt. oomplexer Wt. ratio Molar ratio recovered removed Temp. (mg. Cu (gms) (oomplexer:Cu) (complexerzCu) (mg. Cu) (mg. Cu) F.) 117 1.09 HEETU 9 3:1 4: 86 31 120 1.09 HEETU 9.3:1 A 4:1 87 28 130 119 .09 HEETU 9.3:1 4:1 86 33 140 118 .661U 4.8:1 4:1 73 45 120 114 0.56 TU 4811 4:1 69 47 116 0.56 TU 4.8:1 4:1 69 45 EXAMPLE 1 EXAMPLE 4 A section of 2 inch pipe was cut into 4 inch sections. Twelve of these sections were filled with 250 ml. of a copper solution containing 512 mg. of cupric ions per The procedure of Example 1 was duplicated except that hexahydropyrimidine-Z-thione was tested in place of HEETU. The results are indicated in Table IV.

TABLE IV Copper Cu not Wt. eomplexer Molar ratio removed removed Percent initial (eomplexer Cu) (mg. Cu) (mg. Cu) Cu removed EXAMPLE 5 EXAMPLE 8 The procedure of Example 1 was duplicated except that Lmethylimidazolidine-Z-thione was tested in place of HEETU. The results are indicated in Table V.

This example illustrates the cleaninng of a steam boiler using the process of the present invention. The portion TABLE V Copper Cu not Wt. eomplexer Molar ratio removed removed .Pereent initial (gms) (oomplexerzCu) (mg. Cu) (mg. On) On removed EXAMPLE 6 30 The procedure of Example 1 was duplicated except the mixtures of HEETU and hexahydropyrimidine-Z-thione (HPT) were tested. The results are indicated in Table VI.

of the boiler treated in the first stage included the side wall tubes, the headers and the steam drum. The boiler,

This example illustrates the heavy precipitates formed by the prior art thiourea compounds and the essential absence of a precipitate according to the present invention. The method of Example 1 was repeated except that no analysis was made for the amount of copper removed. The results are illustrated in Table VII. It can be seen that only a light cloudiness was obtained when copper was removed using a hydrochloric acid solution of N-(2- hydroxyethyl)-ethylenethiourea. On the other hand, thio- TABLE VI Wt. complexers Wt. Cu (gms.) Molar ratio Copper Cu not plated (eomplexer: removed removed Percent initial "'(mg. Cu) HPT HEEIU Cu) (mg. Cu) (mg. Cu) Cu removed 0.18 1.15 1P,5H l 43 36 0.37 0.92 2P,4H l 51 68 43 0.55 0.69 3P,3H 1 73 48 00 0. 73 0.46 4P,2H 1 92 29 76 0.92 0.23 5P,1H l 103 18 0.18 0.69 1P,3Hl 25 98 an 0.37 0.46 213,2 1 23 23 0.55 0.23 3P,1H 1 47 76 38 0.18 0.92 1P,4H 1 29 94 24 0.37 0.69 2?,31-11 39 86 31 0. 55 0.46 3P,2H l 53 72 42 0.73 0.23 4P,1H l 87 38 70 EXAMPLE 7 50 which-had a volume of 28,000 gallons, was filled with a solvent prepared according to the following formula:

Sodium bromate: 400 lbs. Ammonium bicarbonate: 650 lbs. Aquarammonia (30%): 624 gals.

The boiler was filled over a period of about 3 hours. The boiler.= was then allowed to remain filled over night and was drained the next day. The total copper removal was 401 pounds.

The second cleaning stage which included side walls urea resulted in a heavy precipitate both at 150 F. and 60 and headers, the steam drum and primary superheater,

at room temperature.

having a total volume of 35,000 gallons, comprised treat- TABLE VII Wt. copper Wt.eomplexer Molar Appearance oi prectpl- Appearance of precipitate when cooled to (mg.) (mtg) ratio tate at 150 F. room temperature 50 1:1 Heavy precipitate Heavy precipitate. 50 2:1 do D0- 3:1 Do. 4:1 D0. 5:1 do Do. 360 TU 0:1 Moderateprecipitate D0. 50 HEETU 1:1 Light cloudiness Llggtkefiildineas (thin film on bottom 0! er 2:1 D0- 3:1 D0. 4:1 D0. 5:1 D0. 6:1 Do.

ment with a solvent prepared according to the following formula:

The boiler containing the above composition was filled and allowed to stand for about 2 hours. The boiler was then drained with the removal of 138 pounds of copper and 2,670 pounds of iron as magnetite.

In a .third stage, the boiler was treated for 3 hours with a citric acid which resulted in a copper removal of 40 pounds and an iron (magnetite) removal of 407 pounds. The total copper removed in all stages was 579 pounds and the total iron (magnetite) removed was 3,014 pounds.

The foregoing examples are merely illustrative of this invention and should not be construed as limiting the scope thereof. Thus, the complexing agents may be used in any process involving removal of incrustations with acid solutions when copper is present in the incrustation. As can be seen from the foregoing disclosure, a wide range of concentrations and treatment conditions may be employed. This invention, therefore, is limited only by the lawful scope of the appended claims which follow.

We claim: 7

1. A'method for removing copper and iron oxide containing incrustations from a ferrous metal surface without forming a heavy precipitate comprising: contacting the surface with an aqueous solution of a complexing agent selected from the group consisting of [N-(2-hydroxy)- ethylene thiourea, diethanol thiourea,]hexahydropyrimidine-Z-thione, 4-methylimidazolidine-Z-thione and mixtures thereof and an acid capable of dissolving iron oxide and copper.

2. The method of claim 1 wherein the solution is agitated during contact.

3. The method of claim 1 wherein the temperature is in the range of 120 to 160 F.

4. The method of claim 1 wherein the acid is selected from the group consisting of hydrochloric, phosphoric and sulfuric acid.

5. The method of claim 1 wherein the solution contains at least about 2 moles of the complexing agent per mole of copper in the incrustation.

5. The method of claim 1 wherein the surface to be treated is on the inside of a vessel.

7. The method of claim 1 wherein the complexing agent is 4-methylimidazolidine-2-thione.

8. The method of claim 1 wherein the complexing agent is hexahydropyn'midine-Z-thione.

9. The method of claim 1 wherein the aqueous solution also contains a corrosion inhibitor.

10. The method of claim 9 wherein the inhibitor is a nitrogen containing inhibitor.

11. A method of removing copper and iron oxide containing incrustations from a ferrous metal surface without forminga heavy precipitate comprising: first contacting the surface with an aqueous alkaline solution containing an oxidizing agent and thereafter contacting the surface with an aqueous solution of a complexing agent selected from the group consisting of [N-(2-hydroxy)-ethylene thiourea and diethanol thiourea,]hexahydropyrimidine-Z- thione and 4-methylimidazolidine-2-thione and mixtures thereof and an acid capable of dissolving iron oxides and copper.

12. The method of claim 11 wherein the oxidizing agent is selected from the group consisting of bromates,

persulfates, chlorates and mixtures thereof.

References Cited UNITED STATES PATENTS 2,485,529 10/1949 Cardwell 252-149 2,959,555 11/1960 Martin 252--l49 3,074,825 1/1963 Gardner 252-149X MORRIS O. WOLK, Primary Examiner S. MARAN'IZ, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION patent 3, 547,697 Dated December 15, 1970 Inventm-(S) Jackie Gene Frost and Billy Bob Arnold It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 7, lines 34-35, delete "[N- (Z-hydroxyO-ethylene thiourea, diethanol thiourea, Column 8, lines 23-24, delete "[N (2-hyroxy)ethylene thiourea and diethanol thiourea.]"

Signed and sealed this 26th day of October 1971 SEAL) Attest:

EDWARD M.FLE'FGHER,JR. ROBERT GOT'ISCHALK Atbestzing O ficer Acting Commissioner of Patent