Classifications

• • C—CHEMISTRY; METALLURGY
  • C02—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F—TREATMENT OF WATER, WASTE WATER, SEWAGE, OR SLUDGE
  • C02F5/00—Softening water; Preventing scale; Adding scale preventatives or scale removers to water, e.g. adding sequestering agents
  • C02F5/08—Treatment of water with complexing chemicals or other solubilising agents for softening, scale prevention or scale removal, e.g. adding sequestering agents
• • C—CHEMISTRY; METALLURGY
  • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
  • C23G—CLEANING OR DE-GREASING OF METALLIC MATERIAL BY CHEMICAL METHODS OTHER THAN ELECTROLYSIS
  • C23G1/00—Cleaning or pickling metallic material with solutions or molten salts

Definitions

• Operational scale deposited in steam-generating equipment usually consists of oxides of iron, often magnetite (Fe O .FeO) together with some red iron oxide (Fe O
• Fe O magnetite
• Fe O red iron oxide
• the unit is fabricated in part from alloys of copper, e.g. for the condensers, the scale will be found to contain copper as well, in the form of the elemental metal and sometimes as cuprous and cupric oxides.
• Such scale is generally tightly adherent and low in porosity. Its gradual build-up reduces heat transfer and water circulation to the point where, usually every one to four years, it must be removed. Since the shutdown of a high capacity boiler can easily represent losses of thousands of dollars per day, it is obvious that the most rapid and effective cleaning is desirable.
• a further object is a process which is safe to both per- Patented Jan. 8, 1963 sonnel and equipment, and which may be used even with very high pressure units containing austenitic alloys.
• Another object is a process which is rapidly efiective even where tightly adherent magnetite scale predominates. Still another object is a cleaning process which efiiciently utilizes relatively lowconcentrations of inexpensive, readily available cleaning ingredients.
• the first step of the new process consists of contacting the scaled surfaces with an aqueous solution of citric acid adjusted to a pH between about 2.5 and about 5 with a nitrogen base. While other acids, such as gluconic, tartaric, oxalic, lactic, glucoheptonic, glycollic, saccharic or malic acids, or mixtures thereof, may be effective in varying degree in the first step of my new process, citric acid possesses the virtues of high effectiveness, low corrosion rate, non-toxicity, ready availability and low cost. Mineral acids are not suitable.
• the nitrogen base which I employ for the pH adjustment may be ammonia, an ethanolamine, or an aliphatic hydrocarbon amine.
• an ethanolamine I mean ethanolamine, diethanolamine or triethanolamine. Any aliphatic hydrocarbon amine, primary, secondary or tertiary, may be employed, provided that it is water-soluble.
• Such amines include trimethylamine, 'diethylamine, the various primary butyland amylamines, ethylamine, propylamine, dimethylamine, triethylam% and the like. Ordinarily, ammonia will be preferred for its low cost and high effectiveness.
• citric acid for high capacity and rapid cleaning I employ the citric acid in a concentration of at least about 1% W./v. Any concentration above 1% up to saturation may be used,but ordinarily concentrations above 10% w./v. offer no added advantage and are unnecessary. Concentrations between about 1.5 and about 3% w./v. are generally preferred.
• the expression w./v. means grams per 100 milliters; of page xiii, Merck Index, 6th edition, 1952.
• the solution is light or freshly deposited, room temperature treatment is adequate.
• temperatures from about 140 F. up to the boiling point of the solution are most efiective. If desired, temperatures above the atmospheric boiling point may be employed by operating under pressure. However, this expedient is generally unnecessary. It is noteworthy that the process is so safe that no corrosion inhibitor is normally required, even at the higher temperatures.
• the progress of the cleaning is conveniently followed by withdrawing liquid samples periodically and assaying for iron according to any of the standard procedures. When the iron content of the solution becomes substantially constant the first phase of the operation may be regarded as substantially complete. Generally this will be found to occur after about one to three hours at temperatures of about 140-160 F.
• Alkaline step.--In this step I utilize the same cleaning solution, merely raising the pH with one of the nitrogen bases already described, without even draining the boiler.
• Triethanolamine is a particularly convenient base to Work with, since its low vapor pressure avoids fumes at the higher pHs.
• ammonia is eminently suitable, and economy may often dictate its use.
• the base is conveniently introduced in water solution, which may be pumped into the boiler. This will displace a small portion of the cleaning solution to waste, but the loss is inconsequential.
• the pH should be adjusted to between about 8 and about 10, preferably about pH 9. If mineral acid had been used in the first step, adherent iron oxides would reprecipitate upon thus raising the pH to the effective range.
• oxidizing agent is often beneficial in increasing the reaction rate.
• a Wide range of oxidants are suitable. Air or oxygen are eminently suitable and are readily introduced by injection into the solution through a sparger or bubbler during alkaline cleaning.
• gaseous oxidants which may be employed in the same manner is nitrogen tetraoxide.
• liquid or solid oxidants such as potassium permanganate or nitrophenylsulfonic acid sodium salt
• Particularly convenient and effective are watersoluble inorganic salts, especially persulfates, perchlorates, bromates or nitrites.
• Suitable salts include, for example, the sodium, potassium and ammonium salts. Very little of such oxidant is required for the desired beneficial effect, ordinarily about one part by Weight for each part by Weight of copper to be removed.
• the temperature for this step is not critical. Room temperature (or even lower) is quite suitable, but temperatures up to boiling may be used if desired.
• the solution at pH 8-10 is best circulated as before, until standard analysis of liquid samples indicates that the copper content is substantially constant. With air or other added oxidant one to two hours will usually suffice.
• the cleaning solution may be drained to waste and the unit rinsed with water.
• the treated surfaces are left silvery grey in color, scale-and copper-free, and ready for return to service.
• the entire operation usually requires only about 5-8 hours, even for a large unit.
• Example I A conventional controlled circulation boiler having a capacity of 1.5 million lbs. steam per hour at about 2000 psi. is shut down to remove operational deposits from the inner surfaces of the steel boiler, steam drum, risers and downcomer. The deposits are estimated to contain about 1000 lbs. of Fe O and about 50-100 lbs. of copper picked up from the condenser.
• the boiler water temperature is allowed to cool to 200 While a solution of 3000 lbs. citric acid in about 3000 gallons of Water is prepared in a separate tank and adjusted to about pH 2.7 with aqueous ammonia. This solution is then pumped into the bottom of the boiler, displacing an equal amount of water which is permitted to overflow to waste from the steam drum.
• the diluted cleaning solution in the boiler has a volume of about 22,000 gallons and a pH of approximately 3.7.
• the boiler pumps are operated to slowly circulate the solution through the system (about 1 ft./sec. in the tubes) while periodic samples are withdrawn for iron analysis. After about two hours the iron content of the cleaning solution has become substantially constant at a value corresponding to about 900 pounds of dissolved iron, and the solution temperature has dropped to about F.
• the solution is slowly circulated by means of the boiler pumps and aeration is continued without applying heat. After an hour the copper content Of the cleaning solution has leveled off at a value corresponding to approximately 80 pounds of dissolved copper.
• Example II A mild steel boiler tube sample, coated with a scale which proves upon analysis to contain magnetite, Fe O copper, and oxides of copper, is immersed in a 1% w./v. aqueous citric acid solution previously adjusted to pH 2.5 with triethanolamine, and the solution is heated at about 140 F. with gentle agitation for about 3 hours. At this time the iron content of the solution is substantially constant at a level corresponding to the original molar concentration of citric acid. The surface is free of scale and coated with copper.
• Example III A number of scaled tube samples similar to that of Example II are immersed in 1.5% citric acid solutions which have been adjusted to pH 5 with a series of amines:
• Trimethylamine Diethylamine Diethanolamine Isobutylamine n-Amylamine Each solution with its immersed plate is boiled until the scale is removed and the plates covered with a light copper film. The molar iron content of the solutions corresponds closely to the citrate introduced. Each pH is now adjusted to about 8 with the amine previously employed. Air is then bubbled into the solutions with gentle stirring'until the copper has dissolved and clean metal surface exposed.
• Example V A number of scaled boiler tube samples similar to those of Examples II and IV are immersed in 3% w./v. citric acid solutions adjusted to pH 4 with ammonia.
• the various solutions are heated at F. with gentle agitation until the copper plating has dissolved.
• a process for the removal of copper-containing ir n oxide scale from metal surfaces which comprises contacting said surfaces with an aqueous solution containing at least about 1% w./v. citric acid together with sufficient base to provide a pH between about 2.5 and about 5, said base being selected from the group consisting of ammonia, an unsubstituted ethanolamine and a water-soluble aliphatic hydrocarbon amine, continuing said contact until the iron content of said solution becomes substantially constant, subsequently adjusting the pH of said solution with one of said bases to a value between about 8 and about 10, and maintaining the pH within said alkaline range while continuing said contact until the copper content of said solution becomes substantially constant.
• said salt is selected from the group consisting of persulfates, perchlorates, bromates and nitrites.
• a process for the removal of copper-containing iron oxide scale from metal surfaces which comprises contacting said surfaces with an aqueous monoamrnonium citrate solution at a concentration equivalent to between about 1.5 and about 3% w./v. citric acid and a temperature between about 140" F. and the boiling point of said solution until the iron content of said solution becomes substantially constant, sub equently adjusting said solution with ammonia to a pH between about 8 and about 10 at a free citrate concentration equivalent to at least about 0.5% w./v. citric acid, and maintaining said alkaline solution in contact with said surfaces while injecting air until the copper content of said solution becomes substantially constant.

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Organic Chemistry (AREA)
• Environmental & Geological Engineering (AREA)
• Hydrology & Water Resources (AREA)
• Water Supply & Treatment (AREA)
• Life Sciences & Earth Sciences (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)

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Cited By (39)

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Citations (9)

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• 1961
  • 1961-02-14 US US89116A patent/US3072502A/en not_active Expired - Lifetime
  • 1961-06-08 GB GB20707/61A patent/GB904515A/en not_active Expired
• 1962
  • 1962-02-05 BE BE613507A patent/BE613507A/fr unknown
  • 1962-02-10 DE DEP28782A patent/DE1264351B/de not_active Withdrawn
  • 1962-02-13 FR FR887882A patent/FR1314838A/fr not_active Expired

Patent Citations (9)

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