Currently, there are many methods of cleaning metals, applicable both in the laboratory and at home. One of these methods is refining, until recently used exclusively at specialized enterprises using patented technologies.
What is refining
Usually, the concept of "refining" means obtaining a metal of high purity through a series of procedures to remove impurities. This process is carried out in several stages, each of which uses certain physico-chemical methods for separating interfering substances. Precious metals are often refined in this way.
The raw material for refining in this case can be jewelry scrap, "silver foam", sludge after electrical cleaning of the relevant substances and slip gold.
Silver refining
Often this cleaning method is used to obtain high-grade silver. In general, the procedure is no different from similar methods carried out for other noble, ferrous or non-ferrous metals. For example,the refining of gold and silver or any platinum metals can be the same. Only in some cases, the procedures differ.
Ways for refining
In processing technology, silver refining is presented in three different ways - the metal can be purified from impurities by chemical, electrolytic or cupellation methods. Removal of excess chlorine is rarely used. The choice of technique is determined by the amount of processed silver and its condition. The features of the production process also matter.
How the path is chosen
Electrolytic refining is used for initially high-grade silver. Usually, when using this method, there is a daily output. Electrolysis helps to obtain exceptionally pure silver through a redox interaction in which impurities do not enter at the time of purification.
In the case when argentum is in the form of a solution (insoluble sulfates and chlorides), the most economical and convenient method of metal deposition is the chemical (in some situations, electrochemical) method.
Low-grade alloys are most often separated using cupellation - in this case, it is easiest to increase the purity of the mixture.
Cupellation method
This type of refining requires an oven with a cup-like (assay) crucible. In the purification process, lead is used, the melt of which is oxidized with silver in the presence of oxygen. All impurities, including the solvent, are separated from the noblemetal, giving it relative purity: gold and platinum family metals remain in the alloy.
For refining, the oven must be preheated. A technical lead-silver mixture is placed in it, which is heated until completely melted. Streams of atmospheric air are launched into the furnace, causing the oxidation of the components of the contents. At the end of the heat treatment, the crucible is removed and poured into molds.
The inside of the furnace is lined with marl - one of the types of clay enriched with limestone and having a porous structure. It absorbs lead oxides formed during the refining process, since the latter are prone to evaporation when exposed to air currents. At the output, after the oxidation of impurities, an alloy with an iridescent iridescent surface is obtained. When it cracks, a bright silver sheen can be seen in the mixture, which indicates the completion of refining.
Cupellation is considered the roughest cleaning method due to the fact that not complete elimination of impurities is achieved: all noble metals in the alloy remain in place. Refining of gold, silver and platinum group metals for their separation is carried out by other methods.
Electrolysis method
Electrolysis as an affination method is carried out with the consciousness of a double electron layer: a contaminated silver fragment placed in a bag becomes the anode of the process, and thin plates formed from non-corrosive steel become the cathode. The electrodes are immersed in a solution of nitrate of the metal to be cleaned (concentrationions - up to 50 mg / ml), nitric acid with a density of 1.5 g / l is added, and an electric current is passed.
Undissolved silver fragments and impurities collect in the anode bags. In the cathode space, a pure sample is collected in microcrystalline form. The volume of released silver can grow towards the other pole of the system, which provokes a short circuit. To prevent such a situation, the grown crystalline fragments, when the solution is stirred, break off parallel to the electrodes near the location of the cathode. The resulting silver is recovered as a precipitate and subsequently cast into ingots. It is important to replace the electrolyte in time, because if copper is present as an impurity, at the end of the desired process, its deposition on the cathode over the noble metal will begin.
If the silver solution behaves like a galvanic cell, the electrolytic method is also the most effective for separating the metal. The anode can be graphite or non-corrosive (alloys), the cathode can be stainless steel. The voltage in the element is set at a level of no more than 2 V. The reaction itself is carried out until all the silver is deposited.
Chemical refining
Silver can be extracted from solutions of s alts or colloids by chemical technologies. The process is multi-stage. The procedure requires sodium sulfite, upon addition of which an exchange reaction occurs with the precipitation of a black precipitate of a new s alt of a noble metal. Upon completion of the interaction to the receivedammonia (ammonium chloride) or common s alt is added to the solution. The mixture is settled until a clear fractional separation - cloudy and transparent parts should form. Silver is considered completely precipitated if the addition of s alts does not cause clouding.
There are two ways to extract pure metal from chloride - dry and wet.
Carbonate method for separating silver from chloride
This technology involves obtaining pure silver from dried chloride - the substance combines with an equilibrium amount of sodium carbonate. In the crucible, the resulting mixture is heated (it is only necessary to fill the bowl halfway due to the increase in the volume of the contents due to the release of gas). After the formation of volatile products, the temperature of the process rises, reaching the values necessary for quiet melting.
After the system cools down, the silver is extracted and re-smelted, after which the product can be considered finished. A negative point may 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.
Reductive method for separating silver from chloride
To restore silver from a 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 sludge with a concentration of 0.2mass shares. You can add the solution in parts, controlling the degree of the reaction and topping up the residues at its completion. A qualitative sign of the interaction in this case is the release of hydrogen - the gas ceases to form at the moment of complete dissolution of the metal or the disappearance of the acid (its consumption can be evidenced by indicator paper).
Isolation of silver from s alt is completed when the system becomes similar in hue to lead. After that, acid is added to transfer the remaining fragments of unwanted metals into solution (large parts are manually removed). The remaining powder substance (the so-called silver cement) is cleaned with distilled water, dried and melted down.
Chlorine refining
The method is based on the assumption that silver and base metals react in a chlorine atmosphere faster than gold and the platinum family of elements. This allows you to separate the last substances from the purified one (in refining technology, the most laborious process is the separation of noble alloys).
Black gold in molten form is passed through gaseous chlorine. The interaction begins with impurity elements of a non-noble type, then silver passes into the form of the compound, which can subsequently be isolated by other refining methods. Chlorides in the mixture float to the surface due to the lower density of s alts compared to metals.
Refining 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 inshavings). Then base 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 products, the mixture is heated over an alcohol lamp or in a water bath. At temperatures below 50-60 degrees, it is possible to use glass or porcelain dishes. In the same way, the cleaned metal can be separated from nickel, tin or lead.
Refining silver at home
All the methods described above are theoretically suitable for home use, subject to special equipment and experience. For beginners, it is better to try the electrolytic method. Usually, silver is refined from contacts in this way.
The procedure consists of 3 stages. This is the dissolution of silver in nitric acid, its cementation and fusion, and directly the refining of silver at home by electrolysis.
Dissolving with nitric acid
Silver nitrate is prepared immediately for the entire process - usually 50 grams of metal is taken per liter of solvent (to obtain this ratio, 32 g of scrap is dissolved in 80 g of hydrogenated nitric oxide V). The acid must be diluted in equal proportions with water and mixed with a glass rod. It is possible to carry out refining of silver with nitrate by mixing ammonium nitrate with an electrolyte (with a reaction of the medium less than 7) to obtain the same HNO3. Pieces of silver are added to the resulting solution. The mixture should be left for 10-11 hours, as the transitionmetal in suspension will not happen immediately. Violent emission of red-brown gas is possible. If the solution becomes bluish or greenish, this indicates the presence of vitriol or iron impurities. Silver refining with nitric acid works better in cases where there is no intense staining.
Silver cement extraction
Bars of copper are added to the mixture to carry out a substitution reaction with silver. Almost immediately, a noble metal begins to precipitate on the surface of the red metal, which should be periodically shaken off into a solution to speed up the process. If the bars are completely dissolved, they need to be replaced with new ones. The end of the reaction in this case is the cooling of the solution and its fractional separation into silver-cement and bluish liquid parts.
Filtering
A funnel and filter paper are used to separate the metal from the solution. The solution with cement is poured into a specially prepared container: the copper s alt flows through the parchment layer, and the silver remains on the surface. Subsequently, it is required to wash the filtrate 5 more times with distilled water.
There is probably some remaining silver in the solution. To extract it, table s alt is added to copper s alt until a curd precipitate forms.
Silver cement being dried. Fusion is carried out in a crucible, which is not intended to be used for working with purer samples. The sample must be heated evenly to avoid scattering of silver or oxidized dust. Can surfacemelt, add baking soda and borax, mixed in equal proportions - the composition will create a vitreous film over the metal that protects against losses.
The resulting substance is base. For its more thorough purification, electrolysis of silver is required. Refining in this case is carried out according to the method already described above - for this it is convenient to melt the metal into granules.
Safety
It is important that the room is well ventilated. As protection, it is recommended to use gloves, a gown and protective goggles. In order to avoid spilling acid into the water, the concentrate itself is added, and not vice versa. Obtaining HNO3 by exchange reaction is the safest way by which silver can be refined. Ammonium nitrate in this case is mixed with the electrolyte (the reaction of the medium is less than 7). Chemical glassware should be tested for resistance to temperature, as the heat of the process can exceed 100 degrees. No more than a third of the vessel is filled with the solution to avoid acid splashing.
Results
Silver refining is not a difficult procedure with certain experience and equipment. If you follow the safety measures, you can carry it out in a non-laboratory environment.
To obtain the highest quality metal, it is convenient to use silver refining by electrolysis at home, as this method minimizes the risk of impurities due to the use of current.
