Currently, there are many methods for the purification of metals, applicable both in laboratory and at home. One of such methods is refining, which until recently was used exclusively at specialized enterprises using patented technologies.
What is refining?
Typically, the term “refining” means obtaining high purity metal through a series of procedures to remove impurities. This process is carried out in several stages, at each of which certain physical and chemical methods of separation of interfering substances are used. Often, precious metals are refined in this way.
In this case, the raw material for refining may be jewelry scrap, “silver foam”, sludge after electrical cleaning of the corresponding substances, and fine gold.
Silver refining
Often, this refining method is used to obtain fine 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 may be the same. Only in individual cases the procedures are different.
Ways of refining
In the processing technology, silver refining is represented in three different ways - the metal can be cleaned of impurities by chemical, electrolytic or cupellated methods. The removal of excess chlorine is rarely used. The choice of methodology is determined by the amount of silver processed and its condition. The features of the production process also matter.
How to choose a path
For initially high-grade silver, electrolytic refining is used. Usually, using this method, there is a daily output. Electrolysis helps to obtain silver of exceptional purity due to redox interaction, in which impurities do not enter at the time of purification.
In the case when the 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.
Base alloys are most often separated by cupellation - in this case, it is easiest to increase the purity of the mixture.
Cupellation method
For this type of refining, a furnace with a cup-like (assay) crucible is required. In the cleaning 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 noble metal, giving it a relative purity: gold and metals of the platinum family remain in the alloy.
For refining, the oven must be preheated. It contains a technical lead-silver mixture, which is heated until completely melted. Atmospheric air flows into the furnace, causing oxidation of the components of the contents. At the end of the heat treatment, the crucible is removed and poured into molds.
Inside 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 a iridescently iridescent surface is obtained. When it is cracked in the mixture, you can see a bright silver luster, which indicates the completion of refining.
Cupellation is considered the toughest cleaning method due to the fact that not complete disposal 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 a refining method is carried out with the consciousness of a double electronic layer: a contaminated silver fragment placed in a bag becomes the anode of the process, thin plates formed of non-corroding steel become the cathode. The electrodes are immersed in a solution of nitrate of the metal being purified (ion concentration up to 50 mg / ml), nitric acid with a density of 1.5 g / l is added, and an electric current is passed.
Insoluble fragments of silver, as well as dirt, are collected in the anode bags. A clean sample in microcrystalline form is collected in the cathode space. The amount of released silver can grow towards the other pole of the system, which provokes a short circuit. To prevent this situation, the grown crystalline fragments, when the solution is stirred, break off parallel to the electrodes near the cathode location. The resulting silver is recovered 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, at the end of the desired process, its deposition on the cathode over the noble metal will begin.
If a silver solution behaves like a galvanic cell, the electrolytic method is also most effective for metal recovery. The anode can be graphite or non-corrosive (alloys), the cathode can be stainless steel. The voltage in the element is set at no more than 2 V. The reaction itself is carried out until the moment of deposition of all silver.
Chemical refining
Silver can be extracted from chemical solutions of salts or colloids. The process is multi-stage. The procedure requires sodium sulfite, 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 settles until a clear fractional separation - a cloudy and transparent parts should form. Silver is considered completely precipitated if the additional addition of salts does not cause turbidity.
There are two ways to isolate pure metal from chloride - dry and wet.
Carbonate method for the separation of silver from chloride
This technology involves the production of 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 cup by half due to an increase in the volume of contents due to gas evolution). Upon completion of the formation of volatile products, the temperature of the process rises, reaching the values necessary for smooth melting.
After cooling the system, silver is removed 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.
Recovery method for the separation of 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 slurry with a concentration of 0.2 mass fractions. You can add the solution in parts, controlling the degree of reaction and topping up the residues at its completion. In this case, a qualitative sign of interaction is the evolution of hydrogen — gas ceases to form when the metal is completely dissolved or acid disappears (its consumption can be evidenced by indicator paper).
Isolation of silver from salt is completed when the system becomes tinted like lead. After this, the acid is added to transfer the remaining fragments of unnecessary metals into the solution (large parts are manually removed). The remaining powder substance (the so-called silver cement) is cleaned with distilled water, dried and re-melted.
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 cleaned (in the technology of refining the most time-consuming process is the separation of noble alloys).
Raw gold in molten form is passed through gaseous chlorine. The interaction begins with impurity elements of a non-noble type, then silver goes 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 salts compared to metals.
Other refining
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 as shavings). Then nitric and sulfuric acids can dissolve the base metal. Concentrated substances are used in cold or hot form (the reaction rate depends on this).
To remove the silver shell from the product, 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 metal being cleaned can be separated with nickel, tin or lead.
Silver refining at home
All the methods described above are theoretically suitable for home use, provided that special equipment and experience are available. Beginners should 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.
Nitric acid dissolution
Silver nitrate is prepared immediately for the whole process - usually 50 grams of metal per liter of solvent is taken (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 refine silver with nitrate by mixing ammonium nitrate with an electrolyte (with a reaction medium of less than 7) to obtain the same HNO3. Pieces of silver are added to the resulting solution. The mixture must be left for 10-11 hours, as the transition of the metal to a suspended state does not occur immediately. Perhaps the rapid release of reddish-brown gas. If the solution acquires bluish or greenish tones, this indicates the presence of vitriol or iron impurities. Silver refining with nitric acid is better in cases where intense staining is absent.
Silver Cement Extraction
Copper bars are added to the mixture to carry out the substitution reaction with silver. Almost immediately, a noble one begins to precipitate on the surface of the red metal, which should be periodically shaken off in a solution to speed up the process. If the bars are completely dissolved, they must be replaced with new ones. In this case, the end of the reaction is considered to be cooling of the solution and its fractional separation into silver-cement and bluish liquid parts.
Filtration
A funnel and filter paper are used to separate the metal from the solution . In a specially prepared container, the solution is merged with cement: the copper salt flows through a layer of parchment, and silver remains on the surface. Subsequently, it is necessary to rinse the filtrate 5 times with distilled water.
A certain amount of the remaining silver is likely to be present in the solution. For its extraction, the table salt is added to the copper salt until the curd is precipitated.
Silver cement is dried. Fusion is carried out in a crucible, the use of which for work with cleaner samples is not supposed. The sample must be heated evenly to avoid flying silver or oxidized dust. You can add baking soda and borax to the melt surface, mixed in equal proportions - the composition will create a glassy film over the metal that protects against losses.
The resulting substance is base. For its more thorough purification, silver electrolysis 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 precautions
It is important that the room is well ventilated. As protection, it is recommended to use gloves, a bathrobe and tread glasses. To avoid acid splashing, the concentrate itself is added to the water, and not vice versa. The preparation of HNO3 by an exchange reaction is the safest method by which silver refining can be carried out. Ammonium nitrate in this case is mixed with an electrolyte (the reaction of the medium is less than 7). Chemical glassware should be checked for resistance to temperature, since the heat of the process can exceed 100 degrees. The solution fills no more than a third of the vessel to avoid acid splashing.
Summary
Refining silver is not a complicated procedure with certain experience and equipment. If safety measures are followed, it can be carried out not in laboratory conditions.
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 ingress of impurities through the use of current.