Silver refining: at home

At present, many methods of metal purification are known, applicable both in laboratory and at home. One such method is refining, which until recently was used exclusively at specialized enterprises for patented technologies.

What is refining?

Usually the term "refining" means obtaining a high purity metal by carrying out a number of procedures to remove 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 purified.

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.

Silver refining

Often, this purification method is used to produce 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 vary.

Ways of carrying out refining

In the processing technology, silver refining is represented in three different ways: 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.

How to choose the path

For initially high-grade silver, electrolytic refining is used. 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 way of depositing the metal 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.

Method of cupping

For this type of refining, a furnace with a cup-like crucible is required. 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 must be preheated. A technical lead-silver mixture is placed in it, which is heated until it melts completely. The furnace starts the flow of atmospheric air, which causes the oxidation of the components of the contents. When the heat treatment is completed, the crucible is extracted and poured into molds.

From the inside, the furnace is lined with marl, one of the clay types enriched in 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 method of purification 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.

Method of electrolysis

Electrolysis as a method of affinity is carried out under the consciousness of a double electron layer: an anode of the process becomes a contaminated fragment of silver placed in a bag, cathode - thin plates formed of non-corroding steel. Electrodes are immersed in a solution of the nitrate of the metal being purified (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 the anode bags are collected undissolved silver fragments, as well as impurities. A pure sample is collected in the cathode space in a microcrystalline form. The amount 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 extracted as a precipitate and subsequently cast into ingots. It is important to replace the electrolyte in time, since if copper is present as an impurity, after the desired process begins, it will precipitate on the cathode over the noble metal.

If the silver solution behaves like a galvanic cell, the electrolytic method is also most effective for the isolation of metal. The anode can be graphite or non-corroding (alloys), cathode - stainless steel. The voltage in the element is set at a level not more than 2 V. The reaction itself is carried out until the moment of precipitation of all silver.

Chemical refining

From solutions of salts or colloids, it is possible to extract silver 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 sodium chloride is added to the resulting solution. The mixture is settled until the moment of 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.

Carbonate method for the isolation of silver from chloride

This technology assumes the production of pure silver from the dried chloride - the substance is combined with an equilibrium amount of carbonate sodium. In the crucible the 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). After the formation of volatile products, the temperature of the process rises, reaching the values necessary for calm melting.

After the system is cooled, silver is extracted and re-smelted, 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.

Recovery method for the isolation of silver from chloride

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. A selected acid with a concentration of 0.2 parts by weight is added to the resulting slurry. Adding the solution can be done in parts, controlling the degree of reaction and adding the residue at its completion. 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 release of silver from the salt is completed, when the system becomes a shade similar to that of 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.

Chlorine refining

The method is based on the assumption that silver and base metals react in the chlorine atmosphere 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 laborious process is the separation of noble alloys).

Black gold in a molten form is passed through gaseous chlorine. Interaction begins with impurity elements of a non-noble type, then silver forms 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.

Refinancing in other cases

In the case of the presence of a copper impurity in silver, it is rational to speak not of an alloy, but of a mixture of metals (can be represented in the form of chips). 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, the mixture is heated 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.

Silver refining in the home

All the methods described above are theoretically suitable for home use, provided that special equipment and experience are available. It is better for beginners to try the electrolytic method. Usually, silver refining is carried out in this way from contacts.

The procedure consists of 3 stages. This dissolution in silver nitric acid, its cementing and fusion, and directly silver refining at home by electrolysis.

Dissolution with nitric acid

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. The refining of silver with nitric acid is better when there is no intensive staining.

Extraction of silver cement

In the mixture, copper bars are added to carry out the substitution 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.

Filtration

A funnel and a filter paper are used to separate the metal from the solution . In a specially prepared container, the solution with cement merges: the copper salt flows 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 some amount of the remaining silver is likely. To extract it into the copper salt, cooked before the precipitation of the curdled sludge is added.

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.

Safety precautions

It is important that the room is well ventilated. 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 cookware 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.

Results

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 affinity for electrolysis at home, since this method minimizes the risk of contamination by using current.