Currently, many cleaning procedures are knownmetals, applicable both in laboratory and at home. One such method is refining, which until recently was used exclusively at specialized enterprises for patented technologies.
Usually the concept of "refining" means obtainingmetal of high purity by carrying out a number of procedures for removing impurities. This process is carried out in several stages, each of which uses certain physico-chemical methods for separating the interfering substances. Often in this way precious metals are cleaned.
The raw material for affination in this case can be jewelry scrap, silver foam, sludge after the electrical cleaning of the relevant substances and schlich gold.
Often this cleansing method is used forobtaining high-quality silver. In general, the procedure does not differ from similar methods used for other noble, black or non-ferrous metals. For example, the refining of gold and silver or any platinum metals may be the same. Only in certain cases, the procedures for the conduct are different.
In the technology of processing silver refiningis represented by three different methods - the metal can be purified from impurities by chemical, electrolytic or cupred method. It is rare to remove excess chlorine. The choice of technique is determined by the amount of silver being processed and its state. The features of the production process also have significance.
For the original high-grade silver is appliedelectrolytic refining. Usually when using this method, daily production takes place. Electrolysis helps to obtain silver of exceptional purity due to oxidation-reduction interaction, in which impurities do not enter at the time of purification.
In the case where the argentum is in the form of a solution(insoluble sulfates and chlorides), the most economical and convenient method of metal deposition is chemical (in some situations - electrochemical) method.
Low-grade alloys are often separated by cupping - in this case, to improve the purity of the mixture is the easiest.
For this type of refining, a furnace is requiredwith a cup-like crucible. In the purification process, lead is used, the melt of which with silver is oxidized in the presence of oxygen. All admixtures, including the solvent, are separated from the noble metal, giving it a relative purity: the alloy retains gold and metals of the platinum family.
To carry out refining, the furnace is requiredpreheat. A technical lead-silver mixture is placed in it, which is heated until it melts completely. The furnace starts the flow of atmospheric air, causing the oxidation of the components of the contents. When the heat treatment is completed, the crucible is extracted and poured into molds.
From inside the oven is lined with marl - one of the speciesclay enriched with limestone and having a porous structure. It absorbs the lead oxides produced in the refining process, since the latter tend to evaporate when exposed to air currents. At the outlet, after the oxidation of impurities, an alloy with iridescent iridescent surface is obtained. When it is cracked, a bright silver shine can be seen in the mixture, which indicates the completion of the affinity.
Kupelirovanie is considered the most crude methodpurification due to the fact that not complete disposal of impurities is achieved: all noble metals in the alloy remain in place. The refining of gold, silver and metals of the platinum group for their separation is done by other methods.
Electrolysis as an affinity method is madeat the consciousness of a double electron layer: the anode of the process becomes a contaminated fragment of silver placed in a bag, cathode - thin plates formed of non-corroding steel. The electrodes are immersed in a solution of the nitrate of the metal being cleaned (ion concentration is up to 50 mg / ml), nitric acid with a density of 1.5 g / l is added, and electric current is passed.
In anode bags, undissolvedfragments of silver, as well as pollution. A pure sample is collected in the cathode space in microcrystalline form. The volume of released silver can grow toward the other pole of the system, which provokes a short circuit. To prevent such a situation, the grown crystal fragments with the mixing of the solution break off parallel to the electrodes near the location of the cathode. The resulting silver is recovered as a deposit and subsequently cast into ingots. It is important to replace the electrolyte in time, since if copper is present as an impurity, after its completion the deposition begins on the cathode over the noble metal.
If a solution of silver behaves likea galvanic cell, the electrolytic method is also most effective for the isolation of metals. The anode can be graphite or non-corroding (alloys), cathode - stainless steel. The voltage in the element is set at a level no more than 2 V. The reaction itself is carried out until the moment of precipitation of all silver.
From solutions of salts or colloids can be extractedsilver by chemical technologies. The process is multi-stage. The procedure requires sodium sulphite, with the addition of which an exchange reaction occurs with the precipitation of a black precipitate of a new salt of a noble metal. Upon completion of the interaction, ammonia (ammonium chloride) or common salt is added to the resulting solution. The mixture is settled until a clear fractional separation - a cloudy and transparent part should be formed. Silver is considered to be completely precipitated if additional salt addition does not cause turbidity.
There are two ways to separate pure metal from chloride - dry and wet.
This technology assumes the receipt of a cleansilver from dried chloride - the substance is combined with an equilibrium amount of carbonate sodium. In the crucible, the resulting mixture is heated (it is only necessary to fill the bowl by half because of the increase in the volume of the contents due to gas evolution). At the end of the formation of volatile products, the process temperature rises, reaching the values necessary for calm melting.
After cooling the system, silver is extracted andis subjected to repeated smelting, after which the product can be considered ready. A negative point can be the fact that technical soda has a negative effect on the state of the crucible. The main advantage of this method of chemical refining is its speed.
To restore silver from the solution, you can take different sets of reagents - sulfuric acid with zinc or iron or hydrochloric acid with the same metals, including aluminum.
One of the elements is introduced into the chloride medium. The selected acid is added to the resulting slurry at a concentration of 0.2 parts by weight. Adding the solution can be in parts, controlling the degree of reaction and replenishing the residue when it is completed. A qualitative sign of the interaction in this case is the evolution of hydrogen - the gas ceases to form at the time of complete dissolution of the metal or the disappearance of the acid (its consumption can be witnessed by indicator paper).
The separation of silver from salt is completed whenthe system becomes a shade similar to lead. After that, the acid is added to transfer the remaining fragments of unnecessary metals to the solution (manually removing the large parts). The remaining powdered substance (so-called silver cement) is purified by distilled water, dried and melted.
The method is based on the assumption that silverand non-precious metals react in the atmosphere of chlorine faster than gold and the family of platinum elements. This makes it possible to separate the last substances from the one being cleaned (in the technology of refining, the most time-consuming process is the separation of noble alloys).
Black gold in a molten form is skippedthrough gaseous chlorine. Interaction begins with impurity elements of a non-noble type, then silver passes into the form of the compound, which can later be separated by other affinity methods. Chlorides in the mixture float to the surface due to the lower density of salts compared to metals.
If there is a copper impurity in silverit is rational to speak not of an alloy, but of a mixture of metals (can be represented in the form of shavings). Then, a non-noble metal can be dissolved with nitric and sulfuric acids. Concentrated substances are used in cold or hot form (the reaction rate depends on this).
To remove the silver shell from the products mixtureheated over an alcohol lamp or in a water bath. At temperatures below 50-60 degrees, glass or china can be used. In the same way, it is possible to separate the metal to be purified with nickel, tin or lead.
All the methods described above, theoreticallysuitable for home use, subject to the availability of special equipment and experience. It is better for beginners to try the electrolytic method. Usually, silver refining is carried out from contacts in this way.
The procedure consists of 3 stages. This dissolution in silver nitric acid, its cementing and fusion, and directly silver refining at home by electrolysis.
Silver nitrate is prepared immediately for the whole process -usually takes 50 grams of metal per liter of solvent (to obtain this ratio 32 g scrap is dissolved in 80 g hydrogenated nitric oxide V). The acid must be diluted in equal proportions with water and mixed with a glass rod. Silver refining with nitrate can be carried out by mixing ammonium nitrate with an electrolyte (with a medium reaction of less than 7) to obtain the same HNO3. In the resulting solution, pieces of silver are added. The mixture should be left for 10-11 hours, since the transition of the metal into a suspended state will not occur immediately. Possible rapid release of reddish-brown gas. If the solution becomes bluish or greenish, this indicates the presence of vitriol or iron impurities. Silver refining with nitric acid is obtained better in cases when intensive staining is absent.
In the mixture are added bars of copper to conductsubstitution reaction with silver. Almost immediately on the surface of red metal begins to precipitate noble, which is periodically shaken into the solution to accelerate the process. If the bars completely dissolve, they need to be replaced with new ones. The end of the reaction in this case is cooling of the solution and its fractional separation into silver-cement and bluish liquid parts.
To separate the metal from the solution,funnel and filter paper. In a specially prepared container, the solution with cement merges: the copper salt flows down through the layer of parchment, and the silver remains on the surface. Subsequently, it is required to rinse the filtrate 5 more times with distilled water.
In solution, the presence of a certain amount of the remaining silver is likely. To extract it into the copper salt add cooked before the precipitation of cheesy precipitate.
Silver cement is dried. Fusion is carried out in a crucible, the use of which is not supposed to work with cleaner samples. Heat the sample evenly to avoid the scattering of silver or oxidized dust. You can add baking soda and borax to the surface of the melt, mixed in equal proportions - the composition will create a vitreous film over the metal, which protects against loss.
The obtained substance is base. For its more thorough purification it is required to conduct electrolysis of silver. Refining in this case is carried out according to the method already described - for this it is convenient to remelt the metal into granules.
It is important that the room is goodventilation. As a protection, it is recommended to use gloves, a robe and protective glasses. To avoid spilling acid into the water, concentrate is added, and not vice versa. The production of the HNO3 exchange reaction is the safest method by which silver refining can be carried out. Ammonium nitrate in this case is mixed with the electrolyte (the reaction of the medium is less than 7). Chemical utensils should be checked for resistance to temperatures, since the heat of the process can exceed 100 degrees. The solution is filled with not more than a third of the vessel to avoid spattering of the acid.
Silver refining is not a complicated procedure with certain experience and equipment. If you follow the safety measures, you can conduct it not in the laboratory.
To obtain the metal of the highest sample, it is convenient to use silver refining by electrolysis at home, since this method minimizes the risk of getting impurities by using a current.