Classifications

• • 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
  • C23G1/02—Cleaning or pickling metallic material with solutions or molten salts with acid solutions
  • C23G1/10—Other heavy metals

Definitions

• Silver and many of its alloys have the property of forming silver compounds, in particular silver sulfide, in contact with the environment or surface layers. Objects made of silver will darken over time and, because this discoloration is usually undesirable, must be cleaned at certain intervals.
• a quicker and gentler cleaning can be achieved with so-called silver immersion baths, in which the discoloration is removed by chemical means without mechanical work. Used in one form of these baths the possibility of dissolving poorly soluble silver compounds by complex binding of the silver ions; alkali metal cyanides or thiourea are generally used as complexing agents. Because of the simple and quick handling - short immersion and rinsing are sufficient - such immersion baths have found wide use in technology and household. The content of highly toxic cyanide or thiourea, which is also toxicologically unacceptable, is disadvantageous because it requires special precautionary measures when storing and using these baths.
• Another type of silver immersion bath uses the reducing properties of certain base metals in alkaline solutions to remove the annoying deposits on the silver surfaces by electrolytic means.
• a method is described in the Swiss patent specification 48 976 and similarly in the German patent application 19 32 337, in which the objects to be cleaned are brought into conductive connections with aluminum pieces in strongly alkaline solution.
• the use of aluminum powder in alkaline media is described as particularly effective in the Austrian patent specification 63 365 and the European patent application 39 193. In this process, the cleaning process is linked to the dissolution of the aluminum, ie the corresponding baths must be freshly prepared from the components before use. A sufficiently quick cleaning requires the use of very strong alkalis.
• a disadvantage is that the active components of the baths are also in shape Only allow a dry mixture to be stored for a short time, since the use of a mixture instead of the separate dosing of individual components, especially in the household sector, would be very useful to rule out dosing errors.
• the invention thus relates to a process for cleaning silver and silver alloys from disruptive surface-adhering silver compounds, which consists in bringing the surfaces into contact with the dilute aqueous solution of an acid suitable for dissolving zinc, the acid bath simultaneously containing zinc in metallic form contains, and if necessary rinse and dry the surfaces after removing the deposits.
• the invention further relates to solid agents which contain a mixture of a solid water-soluble acid and zinc and can be used to prepare corresponding silver immersion baths.
• the process conditions can be varied within a wide range: Since the cleaning speed increases primarily with the bath temperature and with the strength and concentration of the acid used, it can easily be adapted to different surface coverings.
• Usual bath temperatures are in the range from 15 to 80 ° C, but there is a particular advantage of the process in that a quick removal of the deposits is possible in the range around room temperature from 15 to 30 ° C. Moving the bath or the silver surfaces is beneficial but not necessary.
• the strength and concentration of the acid used influence the cleaning speed within wide limits.
• readily water-soluble acids with a dissociation constant of over 10 -5 mol / liter (in water), for example acetic acid, citric acid, monosodium dihydrogen phosphate, potassium hydrogen sulfate, phosphoric acid, amidosulfonic acid and sulfuric acid.
• acetic acid for example acetic acid, citric acid, monosodium dihydrogen phosphate, potassium hydrogen sulfate, phosphoric acid, amidosulfonic acid and sulfuric acid.
• As an acid concentration in the bath 10% by weight is always sufficient, even with heavy deposits; Far lower concentrations of 5% by weight and below are common.
• cleaning times of a few minutes at 25 ° C can be achieved with concentrations down to 0.001% by weight.
• Very dilute acid solutions however, exhaust themselves too quickly during use, so that preferably acid concentrates between 0.005 and 2.5% by weight, in particular between 0.02 and 2.5% by weight, are used.
• the method according to the invention requires the presence of metallic zinc during the cleaning process, without which cleaning using such diluted, largely harmless acids would not be possible.
• the application form of the metal is not critical, but the reaction rate increases within certain limits with the degree of distribution of the metal.
• zinc is therefore used as a granular or finer material, preferably as a powder.
• the amount of metal is generally chosen so that acid remains after its complete dissolution, preferably 1 gram atom of metal to 3 to 1000, in particular 8 to 300, equivalents of acid.
• the aqueous baths can contain other auxiliary substances, in particular surfactants for better wetting of the silver surfaces and soluble neutral salts, which serve as adjusting agents in the preparation of the baths and which can enhance the cleaning action of the acid baths through electrolyte effects.
• Suitable surfactants are primarily water-soluble anionic or nonionic types, such as alkylbenzenesulfonates, fatty acid ester sulfonates, alkanesulfonates and adducts of ethylene oxide and / or propylene oxide with long-chain alcohols or alkylphenols.
• Low-foaming, in particular nonionic, surfactants are particularly preferred, since excessive foaming would make it difficult to observe the cleaning effect.
• the readily water-soluble alkali and ammonium salts are used as salts, for example sodium citrate, potassium chloride, sodium sulfate and ammonium phosphate. When choosing the salts, care must be taken to ensure that the effect of the acid used is not undesirably weakened by buffering.
• the implementation of the process does not require any complex equipment, since it is generally possible to work without heating and without a stirrer. It is sufficient to hang the silver objects in the baths, for example with grippers or in baskets, and to remove them after a certain time, to rinse and dry them if necessary. Bath vessels with non-metal surfaces such as ceramic or plastic are preferred as there are no corrosion problems here, but suitable precautions, metallic materials can also be used.
• the process itself can be carried out not only batchwise, but also continuously, for example by transporting the surfaces to be cleaned on suitable conveying devices through the baths and drying devices.
• the preparation of the cleaning bath also does not require any special measures. It is advisable to first dissolve all soluble components in the water before adding the zinc, because from the time the metal is added, the baths slowly lose their effectiveness, even if they are not used for cleaning. Exhausted baths can be regenerated to a certain extent by adding acid and metal.
• agents which are suitable for the simple preparation of silver immersion baths, can be offered, for example, in a scatterable or pourable form, but also in the form of larger compactates which easily disintegrate in water.
• these agents are produced by mixing the components, but can also be carried out, for example, by joint granulation or pressing.
• acids described above can be used as acid in these agents, for example citric acid, maleic acid, succinic acid, ammonium dihydrogen phosphate, sodium hydrogen sulfate and amidosulfonic acid.
• Citric acid, amidosulfonic acid and sodium hydrogen sulfate have proven particularly useful.
• the metallic zinc is preferably used in granular form or in the form of a powder.
• the ratio of acid to metal is usually chosen so that at least that to dissolve the metal the necessary amount of acid is present, ie at least two equivalent weights of acid are used for one gram atom of metal. In general, however, significantly more acid is used to increase the reaction rate and to keep it approximately constant over the period of metal dissolution.
• the upper limit of the acid to zinc ratio is largely determined by the economic considerations, since the excess acid is generally not used by small consumers. Usual acid to metal ratios are therefore 3 to 300, preferably 8 to 200 equivalents per gram atom.
• the acid content in the agents is preferably between 4 and 99.9% by weight, the metal content is preferably between 0.1 and 10% by weight.
• the agents can contain other auxiliaries. So it is useful if the agents are to be used predominantly for the production of very dilute baths, to incorporate adjusting agents in order to arrive at quantities which can be metered more easily.
• the leveling agent content can make up to 95% by weight of the agent itself.
• Water-soluble neutral salts are particularly suitable as adjusting agents.
• the agents contain tensides in amounts of up to 2% by weight.
• non-hygroscopic substances are preferably used here.
• substances which improve the scatterability can be added to the compositions in amounts of up to 5% by weight.
• Such substances are in particular water-insoluble substances with a high surface area, for example silicates, cellulose powder and various types of silica gel. Aerosil®, a fumed silica, has proven particularly successful.
• the agents are generally not allowed to be exposed to high levels of humidity for a long time, they are usually stored in moisture-tight, reclosable containers.
• the containers can be provided with metering aids, for example graduated measuring cups.
• a portion pack is another consumer-friendly form of offer.
• the dosage of the agents depends in particular on the desired cleaning time and the intended duration of use of the bath. If agents with a high acid-metal ratio are used, cleaning times of less than 1 minute at 25 ° C can be achieved with just 10 mg of metal per liter of bath liquid, but such baths very quickly become unusable. Agents that have a low acid to metal ratio usually get higher from the start, i.e. with at least 100 mg metal per liter, dosed to achieve sufficiently short cleaning times. Similar considerations determine the choice of bath composition if the components are not used in a pre-assembled form, but are dosed individually.
• test material used was commercially available silver spoons with a silver coating of 90 g, which were first cleaned with an abrasive agent and cleaned of grease with a common dishwashing detergent. By placing them in an aqueous solution of ammonium sulfide containing 0.08% by weight, a fairly uniform layer of silver sulfide was then produced on the spoons.
• the spoons were cleaned at room temperature (25 ° C.) in a 10 liter plastic bowl; at other temperatures glass vessels were used, which were in a tempering bath. In all cases, the required amount of water was initially introduced, the desired amount of acid and other auxiliaries were added, and the zinc was only stirred in after the tempering. The test spoons (usually 10) were then placed or hung in the bath and left in until the deposits were completely removed. In most cases, cleaning was followed by a rinse cycle with warm water and a dry cycle. Regardless of the process conditions, the spoons returned to the original appearance of the freshly cleaned silver after the treatment; only the cleaning time was different in the individual process variants.
• Table 2 shows the composition of 7 different agents according to the invention. These agents were produced in a slow-running mixing drum in which the individual components were introduced one after the other. The nonionic surfactant was sprayed into the running drum as a melt. After sufficient mixing time, recognizable by the even gray color of the batch, the free-flowing products were filled into plastic bottles.

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• Chemical & Material Sciences (AREA)
• Chemical Kinetics & Catalysis (AREA)
• General Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
• Materials Engineering (AREA)
• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Cleaning And De-Greasing Of Metallic Materials By Chemical Methods (AREA)
• Detergent Compositions (AREA)

Applications Claiming Priority (2)

Publications (2)

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Family Applications (1)

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Cited By (2)

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• 1983
  • 1983-12-02 DE DE19833343640 patent/DE3343640A1/de not_active Withdrawn
• 1984
  • 1984-11-24 EP EP84114213A patent/EP0146010A3/fr not_active Withdrawn
  • 1984-11-30 DK DK571784A patent/DK571784A/da not_active Application Discontinuation

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