The invention relates to copper metallurgy and can be used to recover copper from its sulfide compounds present in sulfide products, such as concentrates, mattes. The method of recovering copper from sulfide products is carried out in molten alkali with intensive mechanical stirring of the solid-liquid system with a paddle mixer. The process is carried out at a temperature of 450-480°C for 30-40 min while bubbling through the technical oxygen system, the consumption of which is 350-375% (wt.) of the mass of sulfur present in the original sulfide product. The technical result of the invention is the high speed of the copper metallization process with the exclusion of material sintering. 2 tab.
The invention relates to copper metallurgy and can be used to recover copper from its sulfide compounds present in sulfide products (for example, in concentrates, mattes, etc.).
A known method of obtaining metallic copper from a melt of its sulfides, at high temperatures, for example, when converting white matte (Complex processing of copper and nickel raw materials. Vanyukov A.V., Utkin N.I.: Chelyabinsk, Metallurgy, 1988, p. 204 , pp. 215-216), when in the process of blowing the melt with air, some of the copper sulfides are oxidized with the formation of its oxide oxygen compounds, which enter into redox reactions with copper sulfides with the formation of molten metal and a gaseous product - sulfur dioxide. The process is described by the following reaction equations:
During the interaction of copper sulfide and its oxide (reaction 2), sulfide sulfur acts as a copper reducing agent from oxygen and sulfide compounds. The reaction is thermodynamically possible and proceeds at a high rate at a temperature of 1300-1450°C with the formation of a melt of metallic copper and oxygen compounds of tetravalent sulfur with high vapor pressure. As a result of the conversion, blister copper is obtained with a content of the main element of 96-98%. The degree of copper metallization is 96-98%.
The disadvantages of the copper recovery method include:
Use of high temperatures (1300-1450°C);
Formation of gaseous sulfur-containing products.
Closest to the claimed is a method of recovering copper from sulfide compounds, when the sulfide copper material is charged with caustic soda in the ratio material: NaOH, equal to 1:(0.5÷2), and heated at a temperature of 400-650°C for 0, 5-3.5 hours. In this case, an alkaline melt is obtained containing dispersed particles of metallic copper and an alkali melt, concentrating all the sulfur present in the original sulfide material in the form of sodium sulfides and sulfates (Method of recovering copper from sulfide compounds. Patent RU 2254385 C1 , IPC S22V 15/00). As a copper reducing agent from sulfide compounds, its own sulfide sulfur acts, which, as a result of redox reactions, turns into elemental and, in an alkaline environment, disproportionates into sulfide and sulfate:
When recovering copper from synthetic sulfide compounds and those contained in industrial materials (“white matte” and copper matte separation concentrate), under the conditions of the prototype, sintering of dispersed particles of freshly reduced copper takes place at a temperature of 500 ° C and above with the formation of a monolithic metal sinter. The phenomenon of sintering slows down the process of delivering the reagent to the surface of unreacted sulfide grains, and there are also difficulties at the stage of unloading metallic copper from sintering apparatuses. When the temperature drops to 450°C, sintering is not observed, but the process of copper reduction from sulfides is greatly extended in time.
In accordance with the foregoing, the development task was to ensure a high rate of copper metallization from sulfide products (“white matte”, matte separation copper concentrate), while excluding material sintering.
To achieve the desired result, the reduction of copper from sulfide materials, it is carried out in molten alkali at a temperature of 450-480 ° C for 30-40 minutes with intensive mechanical stirring and bubbling through the melt of technical oxygen, at its consumption of 350-375% (wt. ) from the mass of sulfur present in the original sulfide product.
This technical solution is related to:
With active mechanical mixing of the alkali melt and the dispersed material containing copper sulfides introduced for recovery, which ensures efficient heat transfer in the system;
With the supply of technical oxygen to the melt, which ensures effective oxidation of the accumulating elemental and sulfide sulfur to sulfate.
The consumption of technical oxygen is 350-375% (wt.) of the mass of sulfur present in the original sulfide material. All forms of sulfur (S 2- ...S 5+) participate in the oxidation reactions with the formation of sulfate sulfur in the system. Redox reactions are completed within a few minutes, and, accordingly, the process of copper reduction is completed without the formation of cakes. The resulting metallic copper in the form of a suspension in the NaOH melt can be easily unloaded from the apparatus. In experiments according to the proposed method, the speed of the process increased several times compared to the implementation without the introduction of oxygen, and the duration of the process did not exceed 30 min with 100% copper metallization.
To exclude sintering of the resulting metallic copper, the process can be implemented in the temperature range of 450-480°C. The upper temperature limit ensures the exclusion of sintering of metallic copper particles, the lower one (450°C) is associated with the need to ensure high rates of sulfur oxidation reactions.
The proposed set of features: the introduction of a system of sulfide copper material - industrial oxygen alkali with a given flow rate - 350-375 wt.%, from the mass of sulfur present in the source material, active mechanical mixing of the melt and the implementation of the process in the temperature range of 450-480 ° C , provide high speed and completeness of copper recovery from sulfide raw materials. Increasing the oxygen consumption in excess of the specified amount can lead to oxidation of the freshly reduced copper surface.
When implementing the process with the participation of dispersed sulfide copper materials (concentrates, mattes), it is envisaged to prepare a charge at a ratio of alkali (NaOH): concentrate equal to 1.25÷1.5, and to moisten the materials to prevent ignition of sulfides. The mixture is dried and loaded into a steel cylindrical retort of a shaft electric furnace, with mechanical stirring with a paddle mixer. At a temperature in the retort of 450-480°C, technical oxygen is supplied to the melt for 30-40 minutes. The oxygen supply is stopped. Through the bottom valve of the retort, an alkaline melt containing metallic copper is poured into the mold. After cooling, the melt is pulped in water. The copper cake is separated from the alkaline solution by centrifugation.
The method is described in the examples.
Products containing copper sulfide compounds - "white matte" (68.8% Cu, 9.15% Ni, 17.3% S) and matte separation copper concentrate (66.8% Cu, 4.17% Ni, 18, 1% S), weighing 100 g was subjected to batch preparation with alkali (NaOH), the mass of which was 150 g, and moistened. The mixture obtained was loaded into a retort equipped with mechanical agitation, placed in a shaft electric furnace. With stirring turned on, the contents of the retort were heated to a predetermined temperature and stirred at this temperature for a certain time, after which the contents of the retort were unloaded into the mold and, after cooling, leached in water. The obtained copper-containing cakes were analyzed by X-ray phase method for the content of metallic copper.
Example 1 (by prototype)
Process temperature 450°C. The mixing time was 120, 180 and 240 minutes.
The results of the experiments are shown in table 1.
Example 2 (according to the proposed method)
The process temperature was changed in the range of 400-500°C. Upon reaching the desired temperature, technical oxygen was supplied to the melt in the amount of 300-400% (wt.) of the mass of sulfur in the original sulfide product. The supply of the above quantities of oxygen was carried out for 20-40 minutes. After a predetermined time, the oxygen supply was stopped.
The results of the experiments are shown in table 2.
| table 2 | ||||
| The results of experiments on the recovery of copper (example 2) | ||||
| experience number | Oxygen consumption, % of the mass of sulfur in the initial product | Temperature, °С | Mixing time, min | Degree of copper metallization, % |
| "White Matt" | ||||
| 1 | 360 | 450 | 20 | 83,7 |
| 2 | 360 | 450 | 30 | 100 |
| 3 | 360 | 450 | 40 | 100 |
| 4 | 300 | 450 | 40 | 81,3 |
| 5 | 350 | 450 | 40 | 100 |
| 6 | 375 | 450 | 40 | 100 |
| 7 | 400 | 450 | 40 | 100 |
| 8 | 350 | 400 | 40 | 81,1 |
| 9 | 350 | 480 | 40 | 100 |
| 10 | 350 | 500 | material sintering | |
| Copper matte separation concentrate | ||||
| 11 | 350 | 450 | 40 | 100 |
| 12 | 375 | 450 | 40 | 100 |
Table 2 shows that when the process is implemented under the stated conditions (temperature 450-480°C, oxygen consumption 350-375% (wt.) of the mass of sulfur in the original sulfide product, duration 30-40 min), it is possible to achieve 100% metallization of copper from the "white mat" (experiments Nos. 2, 3, 5, 6, 9) and copper concentrate of matte separation (experiments Nos. 11, 12). Lowering the temperature to 400°C (experiment No. 7), reducing the amount of oxygen supplied (experiment No. 4), as well as reducing the duration of phase contact (experiment No. 1) lead to a decrease in the yield of metallic copper. When the temperature was raised to 500°C, the material sintered in the retort.
As can be seen from the examples, the claimed method provides a deep recovery of copper from sulfide copper-containing products, but, unlike the prototype, when implementing the proposed method, this result is achieved at a lower temperature (450-480°C) and in a shorter period of time (30-40 min).
Copper metal products obtained during the processing of industrial materials (concentrates, mattes) are sent for hydrometallurgical refining from iron, nickel and cobalt using known methods, followed by anode melting and electrolytic refining to obtain high-quality sludge in terms of noble metal content.
Alkali solutions containing sulfate sulfur are fed to evaporation with the latter salted out and separated from the alkaline solution. Sodium sulfate is a commercial product of the technology. Alkali, after evaporation of water, returns to the process again.
CLAIM
A method for reducing copper from sulfide products, including heating in molten alkali at a temperature of 450-480°C for 30-40 minutes, characterized in that the reduction is carried out with intensive mechanical stirring and bubbling through the melt of technical oxygen at its consumption of 350-375 (wt .%) based on the mass of sulfur present in the original sulfide product.
PRESERVATION OF NON-FERROUS METALS
Often in archaeological sites come across non-ferrous metals: copper, silver, lead, tin, gold and their alloys. These metals were used in the manufacture of art objects, coins, ornaments, and various household items such as fasteners, navigational tools, kitchen utensils, and small hand tools. These metals are more noble than iron, and in an unfavorable environment are better preserved than iron samples. Perhaps for this reason, so much attention has been paid to their storage and a large number of methods have been developed for their preservation. However, the problems of oxidation of each of the metals in different environments are very different. Only techniques applicable to the problems of non-corrosive metals are considered here.
As already mentioned, non-corrosive metals are often surrounded by deposits. However, it is much thinner on non-ferrous metals than on iron. Of course, artifacts made of such metals are often surrounded by the same oxides as iron artifacts. Prior to processing metal artifacts, preliminary conservation steps must be taken, which include: 1) primary documentation 2) conservation 3) plaque removal, and 4) artifact evaluation. Handling of metals belonging to each of the groups, i.e. copper metals, silver and its alloys, tin, lead and their alloys, as well as gold and its alloys, are considered separately.
PRESERVATION OF NON-FERROUS METALS
In the sea it often happens to find a large number of artifacts from various metals stuck together with each other. In such cases, the material must be handled in such a way that the most brittle metal is fully protected, and at the same time, other metal or non-metal objects adhering to it are not harmed. Since iron artifacts are found most often, most attention is paid to the conditions for the preservation of iron. However, artifacts of gold, silver, tin, brass, bronze, copper, and lead, as well as pottery, stone tools, glassware, bone tools, cloth, and seeds, are often found together in various combinations. In some cases, storage in plain fresh water may be best. After the different materials have been separated, they are placed in the most appropriate storage environment for each material. While the minimum possible amount of iron artifacts should be kept in a sun-protected alkaline solution, such a solution is not necessary or even recommended for other metal artifacts. Copper is corroded by acidic solutions and concentrated alkaline solutions. In neutral or weak alkaline solutions, copper passivates, oxidation is noticeable by the oxide film formed on the surface. A 5% solution of sodium sesquicarbonate or sodium carbonate is recommended. A 5% sodium carbonate solution with an acidity (pH) of 11.5 will protect copper and silver. Silver is stable in aqueous solutions with any value of acidity and in air, since such an environment is devoid of oxidizing agents. Since chlorides do not attack lead or silver, once the oxides have been removed, they do not need to be placed in an aqueous solution and can be dried immediately. However, before removing adhering oxides, it is best to place them in a proper solution to prevent the oxides from hardening and make it difficult to remove them. It is perfectly safe to place silver objects in either 5% sodium sesquicarbonate or sodium carbonate, as is iron artefacts. When storing silver in chromate solutions, a brown Ag2O film is formed, which can be removed during conservation, but for this reason it is not recommended to place single silver artifacts in such solutions. Sometimes, the need to place silver in a chromate solution may arise when it is glued to an iron object. It is much easier to save lead, tin and their alloys. They can be kept dry, but as stated above, once the oxides on the metals have dried, it will be much harder to remove them. Therefore, they are placed in an aqueous solution. Lead is attacked by aqueous solutions that do not contain passivating agents, especially soft water, deionized water, or distilled water. Therefore, lead should never be kept in deionized or distilled water, both of which are slightly acidic and lack passivating agents. However, since lead is corrosion resistant in hard, bicarbonate (bicarbonate) water, since bicarbonate is a passivator, and tin and tin-lead alloy passivate in weak alkaline solutions, they can all be stored in tap water adjusted to an acidity of 8-10 by adding sodium sesquicarbonate. Both lead and tin-lead alloy can be placed in sodium carbonate with an acidity of 11.5, but this acidity is the boundary of the tin oxidation zone, so it should not be used to store tin. Tin will be resistant to oxidation in weak alkaline solutions that do not contain oxidizing agents, but at the same time will react in a completely opposite way in concentrated alkaline solutions. Therefore, any alkaline solution with an acidity of more than 10 is potentially dangerous. Generally speaking, tin can be safely stored in tap water. Lead, tin and tin-lead should not be kept in chromate solutions due to its oxidizing effect, which results in an orange chromate film on their surface that is difficult to remove. In the absence of a passivating agent, an oxidizing agent such as chromate can damage the sample.
COPPER AND COPPER ALLOYS
OXIDATION OF COPPER METALS
The term "copper metal" is used to define all metals composed of copper or copper alloys in which copper is the base metal, such as bronze (an alloy of copper and tin) or brass (an alloy of copper, zinc, and often lead). This term does not imply anything about the valence state, unlike divalent or monovalent copper. Copper metals are comparatively noble metals that often remain unscathed in harsh environments, including long exposure to salt water, which often completely oxidizes the iron. They react with the environment to form similar alternative products such as copper chloride (CuCl), cupric chloride (CuCl2), copper oxide (Cu2O), and the aesthetically pleasing green and blue copper carbonates, malachite, and azurite (Gettens 1964: 550-557). In marine (salty) environments, the two most commonly formed oxidation products of copper are copper chloride and copper sulfide. However, the mineral alternatives (changes) in copper alloys, bronze and brass, can be more complex than in plain copper. The first step in the electrochemical corrosion of copper and copper alloys is the formation of copper ions. They alternately combine with chloride in seawater to form copper chloride, the main component of the oxide layer.
Cu? -e? Cu+
Cu+ + Cl-? CuCl
Copper chlorides are highly unstable mineral compounds. Once copper objects are removed and exposed to air, they inevitably continue to chemically oxidize. This process is often referred to as "bronze disease". In this case, copper chloride hydrolyzes in the presence of moisture and oxygen to form hydrochloric acid and basic cuprous chloride (Oddy and Hughes 1970:188).
4CuCl + 4H2O + O2 ? CuCl2 . 3Cu(OH)2 + 2HCl
Hydrochloric acid gradually interacts with the unoxidized metal and forms more and more copper chloride.
2Cu + 2HCl? 2CuCl + H2¬
The reactions continue as long as there is metal. Preservation of chloride-containing copper objects requires that the chemical exposure to chlorides be stopped by eliminating the copper chlorides or converting them to harmless copper oxide. Otherwise, the artifact will collapse on its own after a certain amount of time.
Copper objects in seawater are also converted to copper sulfide and cuprous sulfide (Cu2S and CuS) by sulfate bacteria (Gettens (1964:555-556; North and MacLeod 1987:82). In anaerobic environments, copper sulfide products typically have the lowest oxidation state, as well as iron sulfide and silver sulfide.After extraction and exposure to oxygen, copper sulfide undergoes subsequent oxidation and increase in oxidation state, i.e. transformation into cuprous sulfide.The whole chemical reaction usually proceeds in the same way as in gland.
When marine deposits are removed, the copper and copper artifacts are inevitably covered in a varying thickness of black powdered copper sulfide, which has an unpleasant appearance. Sometimes, however, corrosion pits can form on the surface during corrosion, but this is more typical for copper alloys, where tin or zinc corrodes predominantly, leaving pits on the surface. The copper sulfide layer does not have a harmful effect on the object after it is removed from the sea, unlike chlorides - they mainly disfigure the shape and size of the object. Sulfide corrosion is easily repaired and does not cause significant problems for the conservator. See North and MacLeod (1987) for more information on the oxidation of copper, bronze and brass in marine (salty) environments.
COPPER METALS
The non-specific term "copper metals" is used here for copper and copper-dominated alloys such as brass and bronze, due to the difficulty in distinguishing copper, brass, and bronze objects from each other without analytical testing. In general, the exact composition of the alloy matters little, so they are usually handled in this way. Care should be taken only with a high percentage of lead or tin, as they are amphoteric metals and dissolve in alkaline solutions. There are many methods of chemical treatment of copper, bronze and brass, but most of them are not suitable for copper metals from the marine (salty) environment. For further information, refer to the bibliography.
In marine (salty) environments, the two most commonly formed oxidation products are copper chloride and copper sulfide. However, the mineral alternatives (changes) in copper alloys are more complex than in plain copper. Once the copper object is removed and exposed to air, it continues to oxidize, a process called "bronze disease". In "bronze disease", the copper chlorides in the metal become very unstable in the presence of moisture and oxygen. They hydrolyze to form hydrochloric acid and basic divalent copper chloride. Hydrochloric acid gradually interacts with the unoxidized metal and forms more and more copper chloride. The reactions continue as long as there is metal. Conservation of chloride-containing copper objects requires: 1) elimination of copper chlorides, 2) conversion of copper chlorides to harmless copper oxide, 3) prevention of chemical interaction of chlorides.
Neither copper chloride nor copper sulfide gives a pleasant patina to the surface of metals, so there is no reason to keep it. In fact, most copper, bronze, or brass has a dark coloration due to sulfide, which often gives the item the color of lead or an alloy of tin and lead. The stable copper sulfide only changes the color of the copper, giving the metal an unnatural color, and is easily washed off with commercially available cleaning solvents, formic acid, or citric acid. In some cases, it may be necessary to remove large oxides and corrosion products mechanically, to the surface of the preserved metal. This is easier to do with copper objects salvaged from the sea, as marine oxides form a dividing line between the surface of the object and the layering. Due to the fragility of the artifact or to avoid damage to the surface, after removing large oxides, adhering surface oxides are often deliberately left. Gentle mechanical brushing and rinsing with water is all that may be required to remove any remaining plaque. In other cases, any adhering oxides are removed by soaking in 5-10% citric acid with 1-4% thiourea added as an inhibitor to prevent metal etching (Plenderleith and Torraca 1968:246; Pearson 1974:301; North 1987 :233). Care should be taken as citric acid dissolves copper compounds. The artifact is completely immersed in the solution until the plaque is removed. This may take from an hour to several days. During this time, the solution should be stirred from time to time to evenly disperse the acid concentration.
When the sample is very thin, brittle, has fine detail, or is almost or completely mineralized, any exposure to acid can be detrimental to it. Therefore, the artifact can be immersed in a 5-15% sodium hexamethonium solution (Plenderleith and Werner 1971:255) to convert the insoluble calcium and magnesium salts into soluble salts that can be washed out.
By observing the necessary preliminary steps when preserving chloride-containing copper objects, it is necessary to prevent the harmful chemical effects of chloride. This can be done by:
1. eliminate copper chloride
2. conversion of copper chloride to harmless copper oxide
3. isolation of the sample coated with copper chloride from air. Possible alternative methods:
1. electroplating cleaning
2. cleaning by electrolytic reduction
3. alkaline dithionite
4. dry cleaning
a. sodium sesquicarbonate
b. sodium carbonate
c. benzotriazole
The first three methods will help remove copper chloride (CuCl) and return some of the corrosion products back to a metallic state. However, they are best used on objects with a metal core. With careful use, it is possible to bring the object to a stable state and obtain forms as close as possible to the original uncorroded appearance. If used incorrectly, they can strip the oxide layer down to bare metal. Jedrzejewska (1963:135) draws attention to the fact that the removal of oxides, especially by electrolysis, can destroy important archaeological information such as stamps, engravings, and decorative elements, as well as change the original shape of the object. Therefore, deposits of oxides on metal artifacts should never be removed without sufficient experience and knowledge. Treatment should be directed towards preserving their condition through the use of tightly controlled electrolytic reduction or the use of alkaline dithionite. These two chemical methods do not remove the oxide layer. Washing in a solution of sodium sesquicarbonate eliminates the chlorides, while benzotriazole and silver oxide isolate the copper chlorides from the air. Chemical treatment is applicable to large and strong objects, as well as fully mineralized objects.
GALVANIC CLEANING
This procedure is carried out in exactly the same way as for iron. Since this method I consider obsolete, and acceptable only under certain circumstances, there is no point in describing it further.
ELECTRIC RECOVERY CLEANING
The electrical reduction of copper metals is carried out in exactly the same way as for iron. As an electrolyte, 2% caustic soda or 5% sodium carbonate can be used. The latter is the most commonly used, although an acceptable result can be achieved using 5% formic acid as the electrolyte, following the directions given for silver processing. A mild steel anode can be used, but when using formic acid as an electrolyte, a 316 stainless steel or platinized titanium anode must be used. The same schemes are used for iron and silver.
The electrolysis time is shorter compared to comparable chloride-containing iron objects. For example, small items such as coins require only a few hours, while larger items such as cannons may take several months. Accurate data on the electric current density are not available. Plenderleith and Werner (1971:198) state that the current density should not fall below .02 amps per square centimeter to avoid deposition of an orange-pink copper film on the sample. In addition to these lines, Pearson (1974:301-302) rightly warns that when electrolytically cleaned, special care must be taken with mineralized bronze from the seabed to avoid damaging the surface when hydrogen gas is released. For various objects, a current density is usually applied within the limits given, as well as significantly exceeding them. North (1987:238) recommends using the energized hydrogen release method described for iron. In general, the same procedure applies to iron. The main difference is that copper metals require shorter processing times. After electrolytic and dry cleaning, copper metals must undergo several hot washes in deionized water. Since copper tarnishes in water, Pearson (1974:302) recommends washing it several times in denatured ethanol. When washed with water, dull oxide film can be removed with 5% formic acid or polished with sodium bicarbonate paste.
After washing, the copper objects are dehydrogenated in acetone, after which they are covered with a protective film, such as pure acrylic. Commercially available Krylon Clear Acrylic Spray No. 1301 is recommended for ease of application, shelf life and availability. Recommended is the procedure proposed by Pearson (1974:302) of mixing 3% benzotriazole in ethanol (when washing the object) as an inhibitor (retarder) to combat bronze disease, and then coating with pure acrylic containing a benzotriazole inhibitor (Incralac). The same protective composition can be prepared by adding 3% benzotriazole to a solution of polyvinyl acetate (V15) in ethanol.
ALKALINE DITHIONITE
This method was created to strengthen mineralized silver. Since then, it has also been found to be effective on copper objects. See the full description in the "Silver" section. The treatment destroys the patina, but effectively removes all chlorides in the shortest possible time, and also brings some copper corrosion products back to a metallic state.
CHEMICAL TREATMENT
Many copper specimens affected by chloride, such as heavily patinated bronzes with "bronze disease", highly mineralized bronzes with or without copper chloride, bronzes without a strong metal core, and bronzes with mineralized decorative parts, cannot be treated with any restoration techniques. For such objects, three procedures are used to stabilize the artifact, leaving the oxide layers intact. This is a treatment with: 1.sodium sesquicarbonate, 2.sodium carbonate, and 3.benzotriazole.
Sodium sesquicarbonate
Copper chloride elements in copper metal and its alloys are insoluble and cannot be removed by water washing alone. When bronze or other copper alloys are placed in a 5% sodium sesquicarbonate solution, the hydroxyl ions of the alkaline solution react chemically with insoluble copper chlorides to form copper oxides and neutralize any hydrochloric acid by-products formed during the hydrolysis process to form soluble sodium chlorides (Organ 1963b :100; Oddy and Hughes 1970; Plenderleith and Werner 1971:252-253). Chlorides are removed with each change of solution. Sequential washing continues until the complete removal of chlorides. Then the object must be washed in several baths of deionized water until the acidity in the last bath becomes neutral.
In practice, surface corrosion products are mechanically removed from the surface of metal objects before the object is sequentially placed in baths with 5% sodium sesquicarbonate mixed with tap water in the first baths, and with deionized water in subsequent baths. If chloride contamination is significant, tap water can be used until the Cl- level in the solution is equal to the Cl- level in tap water. The water should then be replaced with deionized water. This procedure is very economical in cases where objects require monthly processing.
At the beginning, the baths are changed weekly; then the interval increases. Chloride levels are monitored using the mercury(II) nitrate quantitative test described in the iron section, which allows the conservator to determine exactly how often to change the solution. Instead of the quantitative chloride test, the already described qualitative silver nitrate test (1) can be used to determine when the solution is free of chlorides. The cleansing process is slow and can take months, and in some cases even years.
The immersion in sodium sesquicarbonate is followed by a wash in several distilled or deionized waters until the acidity in the last bath is neutral. Then the object is dehydrated in acetone or an aqueous solution of alcohol, and coated with clear acrylic varnish or microcrystalline paraffin. To increase corrosion resistance, benzotriazole can be added to drying alcohol or even varnish.
Sodium sesquicarbonate treatment is often chosen because, unlike other cleaning methods, it does not remove the green patina on copper objects. However, side effects such as the formation of blue-green malachite deposits on the object's surface can enhance the color of the patina. If this happens, the object should be removed from the solution and the deposits removed. On some bronze objects, there is a noticeable surface darkening that hides the true green patina and is difficult to remove. This darkening is a sign of the formation of black copper oxide and is inherent in some copper alloys.
Washing in sodium carbonate
Washing in sodium sesquicarbonate, as described above, is a standard procedure for brittle copper artifacts affected by chloride, as well as for artifacts that have a patina that is desirable to preserve. However, in practice, conservators have noticed that it often enhances the color of the patina, causing it to take on a richer blue color. In other cases, it darkens or darkens the patina significantly. Recently, Weisser (1987:106) remarked:
Although the sodium sesquicarbonate treatment seems ideal because you don't have to remove the outer oxide layers while stripping the copper chloride, a number of drawbacks have been found with it. First, the processing can take over a year before the copper chloride is converted. This fact further exacerbates other shortcomings. It has been found that sodium sesquicarbonate (double carbonate) forms a complex (polyatomic) ion with copper and therefore preferentially removes copper from the remaining metal (Weisser 1975). Potentially, this can be structurally dangerous in the long run. A mixture of carbonates including chalconatronite, a blue-green hydrated sodium copper dihydroxocarbonate, has also been found to form on the patina and also appears to replace the copper salts in the patina (Horie and Vint 1982). This promotes a color change from green to blue-blue malachite, which is not desirable in many cases. On the objects examined by the author, a blue-green color was found in the cross section of the crust of external corrosion, going to the metal base, whereby Weiser (1987:108) concluded:
Stabilization of actively corroding archaeological bronze remains a difficult problem for conservators. Currently, there is no ideal processing tool. Sodium carbonate pre-treatment in conjunction with standard benzotriazole treatment gives the conservator facing bronze stabilization problems another option. Although this treatment has been successful where others have failed, it should be used with caution until the deficiencies identified can be more thoroughly investigated. Bronze that cannot be stabilized by this method should be stored or exposed in an environment with relatively low humidity. In general, it is recommended that all bronzes be stored in an environment with relatively low humidity whenever possible, as the long-term effect of treatment against "bronze disease" has not been proven. Weiser believes that if previous treatments with BTA (benzotriazole) have not been successful, then a treatment with 5% w/o sodium carbonate in distilled water should be carried out. Sodium carbonate eliminates copper chlorides and neutralizes hydrochloric acid in potholes. Sodium carbonate, in contrast to sodium sesquicarbonate, which is a double carbonate and acts as a complexing agent with copper, reacts with copper metals relatively more easily. However, in some cases some discoloration of the patina may occur.
Benzotriazole
The use of benzotriazole (BTA) has become commonplace in any copper metal preservation, following the stabilization process and anticipating the final insulation. In some cases this may be the only treatment, but in the conservation of marine copper objects, it is usually used as a final step in addition to other treatments such as electrolytic reduction or alkaline washing, which can remove almost all chlorides. In this purification method (Madsen 1967; Plenderleith and Werner 1971:254), benzotriazole forms an insoluble, complex compound with cuprous ions. The deposition of this insoluble compound on the copper chlorides forms a barrier against moisture which can activate the copper chlorides leading to "bronze disease". The treatment does not remove the copper chlorides from the artifact, but only forms a barrier between the copper chlorides and atmospheric moisture.
The process consists of immersing the object in 1-3% benzotriazole dissolved in ethanol or water. For artifacts that have been in fresh water, this may be the only treatment needed. It is carried out to prevent future corrosion or discoloration of the patina. Benzotriazole is usually dissolved in water, but ethanol can also be used. For more information see Green (1975), Hamilton (1976), Merk (1981), Sease (1978) and Walker (1979). Benzotriazole forms an insoluble, complex compound with divalent copper ions. The deposition of this insoluble compound on the copper chlorides forms a barrier against moisture which can activate the copper chlorides leading to "bronze disease". It has been found that when the artifact is left in benzotriazole for at least 24 hours, 1% benzotriazole mixed with deionized (D.I.) water performs as well as stronger solutions. For shorter treatment times, it is recommended to use 3% benzotriazole mixed with water or ethanol. The main advantage of ethanol is that it penetrates into ruts and cracks better than water. In cases of short-term treatment with benzotriazole, it is preferable to use ethanol. In most cases, the best results are obtained if the sample is soaked in the solution under vacuum for 24 hours. Upon removal, the object is wiped with a cloth soaked in ethanol to remove any residual benzotriazole. Then the artifact can be left in the air. If any fresh corrosion occurs, the process is repeated until the harmful reaction disappears. Tests at the British Museum (Plenderleith and Werner 1971:254) have shown that in the presence of active "bronze disease", all attempts to stabilize the object with benzotriazole may fail due to the widespread distribution of copper chloride CuCl in the oxide layers. It has been observed by many conservators that when processing copper artifacts found at sea, better long-term stability can be achieved by removing chlorides by either washing with sodium sesquicarbonate or sodium carbonate, followed by the application of benzotriazole and a final insulator such as Krylon Clear Acrylic 1301. It should be emphasized that treatment with benzotriazole does not remove copper chloride from the artifact, but only forms a barrier between copper chlorides and atmospheric moisture. Therefore, artifacts heavily affected by chloride, such as copper/brass/bronze objects found in the sea, should be treated in combination with the other procedures described above. Machining by this method alone is not always successful, but, in combination with other methods, is a standard part of the machining of copper or copper alloys. Benzotriazole is a carcinogen, so skin contact or powder inhalation should be avoided.
FINISHING AND INSULATION
After electrolytic or chemical cleaning, objects must undergo a series of washes in hot deionized water. Since copper tarnishes in water, Pearson (1974:302) recommends washing in several baths of denatured ethanol. When washed in water, tarnish can be removed with 5% formic acid or polished with a damp sodium bicarbonate paste (baking soda).
After washing, copper objects should be polished to the required level, treated with benzotriazole, dehydrated in acetone, and spray-coated with a protective layer of pure acrylic. Due to ease of application, durability and availability, Krylon Clear Acrylic Spray #1301, which is Acryloid B-66 in toluene, is recommended. For additional protection, benzotriazole can be mixed with Acryloid B-72 or polyvinyl acetate and applied with a brush to the artifact. Microcrystalline wax can be used, but in most cases it has no advantage over acrylics.
CONCLUSION
The processing methods described here are effective for all copper-bearing artefacts raised from the seabed. Each method is effective to a certain extent and is preferred for certain artifacts. Of the conservation methods discussed in this section, only electrical reduction, alkaline dithionite, and alkaline washing can remove copper chlorides. For this reason, they provide the most durable protection. The method of cleaning copper alloys, brass, and bronze objects by electrical recovery is often avoided, as it removes the beautiful patina and can contribute to discoloration due to the electrodeposition of copper contained in corrosive compounds onto the surface of the metal alloy. My experience and apparently successful application of electrical recovery to a large number of copper and bronze artifacts clearly shows that electrolysis is the fastest, most efficient and long lasting means of treating copper, brass and bronze objects from the marine environment. This statement is especially true for large objects such as cannons.
The use of sodium carbonate or sodium sesquicarbonate is hampered by extremely long processing times. Pre-treatment with sodium carbonate, and post-treatment with benzotriazole, may give satisfactory results, but further experiments should be carried out before a final conclusion can be made. It can also be said in advance that good results were obtained when using an alkaline dithionite solution in the processing of copper alloys. This method, as well as electrical reduction, has the property of reducing the return of corrosive copper products back to the metallic state, and, like alkali washing, eliminates soluble chlorides. This processing method can be useful for both copper and silver artifacts, for which it was originally developed. Regardless of the processing method, the application of benzotriazole is an integral part of the processing of copper metal artifacts. In most cases, if the artifact is effectively treated with any of the above benzotriazole treated methods, isolated with an acrylic such as Krylon 1301 Clear Acrylic, and stored under the correct conditions, the artifact will remain in a stable state.
How to clean copper? The relevance of this issue is explained by the fact that products made of this metal have been used by mankind for many centuries. For a long time, the value of this metal was so high that it was equated to gold. The development of technologies has led to the fact that it was possible to significantly reduce the cost of copper production. This made it possible to make not only jewelry, but also dishes and interior items from this metal. The high popularity of this metal and alloys based on it is explained not only by its decorative effect, but also by its unique characteristics - high ductility, thermal conductivity, corrosion resistance, etc.
Why copper products need to be cleaned regularly
Regular cleaning of copper utensils and other items made of this metal is necessary because during operation they quickly darken or become covered with a green coating - an oxide film. The most actively oxidized are those products made of copper and its alloys, which are often heated during operation or used outdoors. Dishes made of copper, with active use, quickly lose their original luster and fade, its surface may become black.
Copper jewelry behaves a little differently: they can first fade and lose their luster, and then return to their original appearance. Some people believe that the appearance of a copper piece of jewelry (such as a bracelet) is influenced by the well-being of the person who wears it all the time. However, this is most likely due to the fact that in the external environment with which such a product is constantly in contact, humidity, pressure and temperature are constantly changing. Meanwhile, many adherents of alternative medicine recommend wearing copper bracelets to people experiencing problems with the cardiovascular system.
Copper utensils, which our distant ancestors began to use, are still held in high esteem by many housewives. Such popularity is explained by the fact that in dishes made of copper, which is characterized by high thermal conductivity, all cooked products are heated evenly and in full, and such heating occurs in a short period of time. Meanwhile, with constant use, dishes made of this metal quickly lose their external attractiveness: they become covered with an oxide coating, grow dull, darken and lose their original luster.
If you do not clean it, it will release toxic substances, respectively, it will not be possible to use it for cooking. In the event that it is not possible to clean such dishes by all known means, it is better not to use it for its intended purpose, so as not to harm your health. You should also keep in mind that dishes with black or green oxide stains on the surface look unpresentable, so they will not decorate your kitchen.
Effective cleaning methods
There are many proven methods that allow you to clean copper products even at home. Let's get acquainted with the most effective of them.
Method #1One of the most affordable home remedies for cleaning objects made of copper is ordinary tomato ketchup. In order to clean copper with such a tool, it is simply applied to the surface to be treated and left on it for 1-2 minutes. After such an exposure, the ketchup is washed off with a stream of warm water. As a result of this procedure, the original luster and color brightness will return to the copper product.
You can also clean copper items at home, if they are not very dirty, using ordinary dishwashing gel. To do this, use a soft sponge on which detergent is applied. Rinse it off under running warm water.
This cleaning method is used if it is necessary to clean a large copper product that cannot be placed in any container. The surface of such an object is wiped with half a lemon. To increase the effect of lemon juice on copper, you can clean it with a brush with bristles that have sufficient elasticity.
Method #4A tool such as "vinegar dough" helps to give copper its former shine. Prepare it as follows. In a special container, wheat flour and vinegar are mixed in equal proportions, bringing the resulting mass to a homogeneous state. Then the dough is applied to an object made of copper and kept until completely dry. The crust formed after the mixture has dried is carefully removed, and the copper surface is polished to a shine with a piece of soft cloth.
Method #5
There is a radical and effective method for cleaning products made of copper, which is used if their surface is heavily contaminated and it has not been possible to clean them with other means.
- Vinegar is poured into a specially prepared stainless steel container, which is mixed with a small amount of table salt.
- The object to be cleaned is placed in the resulting solution and the container is put on fire.
- After the cleaning solution has come to a boil, turn off the fire under the container and leave it on the stove until it cools completely.
- After the solution has cooled down, the product to be cleaned is removed, washed under running warm water and its surface is wiped dry.
If you are cleaning copper with any of the above methods, be sure to strictly adhere to safety rules, do all work with protective gloves, and be sure to wear a respirator when working with acetic acid.
Copper Coin Cleaning
Coins made of copper are no longer produced in our time, and many of these items, which are in the hands of the population, are of antique value. That is why the question of how to effectively and at the same time carefully clean such coins is quite relevant.
There are several ways to restore the former attractiveness of copper coins. The choice of each of them depends on the nature and degree of pollution. So, depending on the color of the plaque formed on the surface of the old copper coin, you can clean it in one of the following ways.
- If there is a yellowish coating on the surface of the coin (this indicates that it has been in contact with a lead product), then it should be cleaned with a 9% vinegar solution.
- A plaque of a pronounced green color is cleaned with a 10% solution of citric acid.
- Coins made of copper may also have a reddish coating. They clean such a coin by dipping it in a 5% ammonia solution or in ammonium carbonate.
When extracting copper from pyrite cinders, waste from copper smelters, mine dumps, as well as from oxidized copper ores, dilute solutions of copper sulfate (or copper chloride) are obtained. Mine, formed in copper mines as a result of the slow oxidation of copper sulfide with atmospheric oxygen, also represent a weak solution of copper sulphate. Since the concentration of such weak solutions is not economical, copper is isolated from them by cementation70-71. This process consists in displacing copper from solutions with iron shavings and iron scrap:
Cu2+ + Fe= Fe2+ + C
The electrode potential of copper is much higher than that of iron - in M solutions containing Sc2+ or Fe^+ ions at ordinary temperature and hydrogen pressure 1 at it is equal to +0.34 V for C, -0.44 for Her V. Therefore, iron displaces copper from the solution in the form of a thin metal sludge called cement copper.
Cementation is carried out in a steel lined or lead-coated tank, where iron scrap cleaned of dirt and rust is loaded. Then a dilute solution of copper sulfate is fed into the tank. For complete precipitation of copper, the solution should not contain significant amounts of sulfuric acid. The optimal concentration of sulfuric acid is - 0.05% or about 5 Yu-3 g-mol/l 72. With such acidity, there is practically no dissolution of iron with sulfuric acid and the most complete removal of copper from the solution is ensured, up to a Cu2 + content of ~ 5 10-6 g-ions/l 73.
The dilute solution of iron sulfate formed as a result of cementation is drained into the sewer, and another portion of the initial solution containing copper is poured into the reactor. Processing of the same load of iron is carried out 10-12 times. After that, the remaining iron is removed and the cement copper that has settled to the bottom is unloaded, which is then washed from iron particles with 10-15% sulfuric acid with continuous stirring. After removal of iron, copper is washed with water until it is completely washed away from sulfuric acid. Washed cement copper is obtained in the form of a reddish-brown paste; it contains 65-70% Cu, up to 35% moisture and about 1% impurities, and is processed into blue vitriol using the same methods as copper scrap. The dispersity of cement copper increases with an increase in the pH of the solution and with a decrease in the concentration of CUSO4 and C1~74 in it. Cementation of copper can also be carried out in a fluidized bed of iron granules. A method has been developed for extracting cement copper by flotation78. Powdered copper can be obtained from acid solutions of copper salts by adding water-soluble polysaccharides (~1%) to them and treating them with a gaseous reducing agent under pressure, for example, hydrogen at 30 at and 140°76.
Copper can be recovered from dilute solutions of CuSO< обработкой их слабой аммиачной водой. При этом образуется осадок Си(ОН)г CuSO«, который после отделения от раствора можно растворить на фильтре серной кислотой для получения медного купороса. Если в растворе присутствуют, кроме меди, ионы железа и никеля (например, при переработке полиметаллических руд), возможно ступенчатое осаждение их аммиаком при нейтрализации раствора последовательно до рН = 3, затем 4,5 и б77"7*.
Methods have been developed for the extraction of copper from dilute solutions by extraction with organic solvents.
When sodium chlorite interacts with chlorine, sodium chloride is formed and chlorine dioxide is released: 2NaC102 + C12 = 2NaCl + 2 CIO2 This method was previously the main one for obtaining dioxide ...
On fig. 404 shows a diagram of the production of diammonitro - phosphate (type TVA). Phosphoric acid with a concentration of 40-42.5% P2O5 from the collector 1 is supplied by pump 2 to pressure tank 3, from which it is continuously ...
Physico-chemical properties Ammonium sulfate (NH4) 2S04 - colorless rhombic crystals with a density of 1.769 g/cm3. Technical ammonium sulfate has a grayish-yellowish tint. When heated, ammonium sulfate decomposes with the loss of ammonia, turning into ...