A silver nitrate recovery method

Abstract

The invention relates to a silver-nitrate recovery method comprising the process steps of preparing a nitric-acid solution in a reactor tank and feeding silver sludge into the prepared acid solution to obtain an intermediate product. The silver-nitrate recovery method further comprises introducing N2 onto the intermediate product, sweeping the resulting NOx gases with the introduced N2 and reacting them with H2O2 so as to eliminate them, and collecting the silver nitrate formed in a vessel.

Description

[0001] A SILVER NITRATE RECOVERY METHOD

[0002] Technical Field

[0003] The present invention relates to the recovery, in the form of silver nitrate, of the silver sludge that arises on a production line.

[0004] Background Art

[0005] In the technical field, the silver chemical product silver nitrate is the raw material and the basis for other high-value-added silver products. It is widely used in many areas such as mirror manufacture, medicine, electroplating and ultra-fine silver powder.

[0006] The most common route to prepare high-purity silver nitrate is a reaction process that comprises adding nitric acid to dissolve silver followed by impurity removal and the successive operations of filtration, evaporation and concentration, crystallisation, centrifuging, drying and packaging to obtain silver. Adding nitric acid to dissolve silver generates large amounts of nitrogen-oxide exhaust gas (NOx) during the reaction. Nitrogen oxides, mainly NO and NO2, are an important cause of photochemical smog and acid rain and pose a serious threat to human health and the ecological environment.

[0007] EP0568259 A1 concerns a method for preparing silver nitrate. It discloses a process for producing a silver-nitrate solution without releasing nitrogen oxides, in which metallic silver is reacted with nitric acid and from 20 % to 30 % excess hydrogen peroxide over the stoichiometric requirement.

[0008] Brief Summary of the Invention

[0009] An object of the invention is to provide an environmentally friendly silver-nitrate recovery method for recycling silver sludge.

[0010] In order to achieve above objective, the invention relates to a silver-nitrate recovery method that comprises the process steps of preparing a nitric-acid solution in a reactor tank and feeding silver sludge into the prepared acid solution to obtain an intermediate product. The silver-nitrate recovery method further comprises introducing N2onto the intermediate product, sweeping the resulting NOx gases with the introduced N2and reacting them with H2O2so as to eliminate them, and collecting the silver nitrate formed in a vessel. In this way, most of the nitrogen-oxide waste gas generated in the reaction between the silver sludge and nitric acid is reconverted into nitric acid with hydrogen peroxide for reuse, providing an environment-friendly solution.

[0011] In a preferred embodiment of the invention, the reactor tank is temperature-controlled and its temperature is maintained in the range of 55 °C to 95 °C, whereby the reaction temperature can be controlled via the reactor tank and kept constant at the temperature suitable for the reaction.

[0012] In a preferred embodiment of the invention, the initial silver-sludge concentration is in the range of 50 to 600 gpL depending on the desired concentration of the silver-nitrate solution. Adjusting the initial concentration can limit unnecessary waste generation and reduce environmental harm; it can also help make recycling processes more efficient when necessary.

[0013] In a preferred embodiment of the invention, the initial amount of nitric acid is between 15 L and 75 L. Determining the nitric-acid usage in this way enables the formation of harmful by-products such as NOx to be identified and eliminated and reduces the quantity of harmful gases emitted to the atmosphere.

[0014] In a preferred embodiment of the invention, the amount of nitric acid recovered with hydrogen peroxide is between 7 L and 40 L, thereby recovering nitrogen-oxide gases as nitric acid and providing an environmentally beneficial recovery method.

[0015] In a preferred embodiment of the invention, nitrogen gas is introduced at a rate of 0.5 - 2.5 L min-1. This allows the amount of nitrogen introduced into the reactor tank to be regulated according to the amount of nitrogen-oxide gas produced in the tank.

[0016] In a preferred embodiment of the invention, water vapour released in the reactor tank is collected in a collection vessel. Collecting the water vapour leaving the reactor reduces waste generation and can potentially lower treatment or waste-management costs.

[0017] In a preferred embodiment of the invention, the silver-nitrate solution is passed through a filter to collect impurities produced in the reactor as a result of the reaction, thereby removing undesirable impurities from the silver-nitrate solution and increasing the purity of the final product. In a preferred embodiment of the invention, the silver-nitrate solution is sent to a crystallisation unit for purification. Crystallisation ensures that the silver nitrate is obtained at a high degree of purity.

[0018] In a preferred embodiment of the invention, the silver-nitrate crystals obtained are purified so that their purity is in the 95 % - 100 % range. High-purity silver nitrate yields high-quality end products in the specific field in which it is used. In mirror manufacture in particular, the high purity of the silver nitrate used directly affects the quality of the final product: because the silver, which converts to the metallic form after application, contains fewer impurities, it is less prone to oxidation, and the product quality therefore depends on the silver nitrate being sufficiently pure.

[0019] Detailed Description of the Invention

[0020] In this detailed description, the invention is explained, solely in order to illustrate the subject more clearly, with reference to examples without implying any limitation.

[0021] In a preferred silver-nitrate recovery method — especially for recovering the silver nitrate formed on a mirror-production line — a nitric-acid solution is prepared in a reactor tank. The prepared nitric-acid solution is fed with silver sludge into a temperature-controlled jacketed steel reactor. The desired Ag concentration of the solution is controlled by the nitric-acid concentration in the feed. Nitrogen gas (N2) is supplied to the reactor tank to sweep the NOx gases generated therein. The resulting NOx gases are washed and eliminated by reaction with hydrogen peroxide. The hydrogen peroxide is in liquid form in a glass-packed column. The best NOx-removal results are obtained when 20 % - 30 % more hydrogen peroxide than the stoichiometric requirement is used. By appropriately controlling the residence time of nitric acid and hydrogen peroxide in the glass column, a silver-nitrate solution free of free acid can be obtained without generating any trace of NOx in the process. The water volatilised during the reaction is condensed and collected in a condenser and a collection vessel. After the reaction, a filter press is used to collect the impurities originating from the mirror line that did not participate in the reaction, and the AgNO3aqueous solution is accumulated in a collection vessel and sent to the crystallisation unit for purification. Silver-nitrate crystals obtained in the crystallisation unit are ensured to have a purity of 98 % - 100 %.

[0022] In an exemplary silver-nitrate recovery method, 15 L of HNO3solution is retained in the reactor tank. The reaction is started by feeding 500 gpL of silver sludge — depending on the desired concentration of the silver-nitrate solution — into the reactor tank. Nitrogen gas is introduced into the reactor tank at 1 .5 L min-1to sweep the NOx gases. In the glass-packed column H2O2reacts with NOx, washing and eliminating the gases. At the end of the method, 750 -800 g of silver-nitrate solid is obtained.

[0023] It has been ascertained that mirrors produced with the silver nitrate recovered by the method exhibit satisfactory performance in mirror-quality tests. In the rapid CASS test on a sample mirror produced by the recovery method, no deterioration was observed after 8 h. In the ETUV test, no deterioration was observed on the sample mirror kept at 60 °C for 3 days. When the sample mirror produced by the recovery method was subjected to a ferric-chloride test for 48 h, no deterioration was observed on the mirror after 24 h and 48 h.

[0024] In the table above, the test results of the mirror produced by the recovery method under a D65 light source show a reflection value L of 97.18, a colour value a of -1 .8058 and a colour value b of 2.1433. Under EN 410 the daylight value is 93 % and the solar-energy value is 84.1 %. Paint-thickness testing determined that the mirror produced by the recovery method had a paint thickness of 56 pm. Pull-off testing established that the mirror detached in red paint at a failure pressure of 4.22 - 3.87 MPa. Pendulum testing of the mirror produced by the recovery method found the swing time to be 179 s in 179 swings. When the mirror produced by the recovery method was subjected to a 20-day humidity test it was deemed suitable for use.

Claims

CLAIMS1 . A silver-nitrate recovery method comprising the process steps of preparing a nitric-acid solution in a reactor tank and feeding silver sludge into the prepared acid solution to obtain an intermediate product, characterised in that the method further comprises introducing N2onto the intermediate product, sweeping the resulting NOx gases with the introduced N2and reacting them with H2O2so as to eliminate them, and collecting the silver nitrate formed in a vessel.

2. The recovery method according to claim 1 , wherein the reactor tank is temperature-controlled and its temperature is maintained in the range of 55 °C - 95 °C.

3. The recovery method according to any preceding claim, wherein the initial silver-sludge concentration is in the range of 50 - 600 gpL depending on the desired concentration of the silver-nitrate solution.

4. The recovery method according to claim 3, wherein the initial amount of nitric acid is between 15 L and 75 L.

5. The recovery method according to any preceding claim, wherein the amount of nitric acid recovered using hydrogen peroxide is between 7 L and 40 L.

6. The recovery method according to any preceding claim, wherein nitrogen gas is introduced at a rate of 0.5 - 2.5 L min-1.

7. The recovery method according to any preceding claim, wherein the water vapour generated in the reactor tank is collected in a collection vessel.

8. The recovery method according to any preceding claim, wherein the silver nitrate is passed through a filter to collect impurities produced in the reactor tank as a result of the reaction.

9. The recovery method according to any preceding claim, wherein the silver-nitrate solution is sent to a crystallisation unit, silver-nitrate crystals are obtained and the purification step is carried out.

10. The recovery method according to claim 9, wherein the purification is performed such that the purity of the silver-nitrate crystals obtained is in the range of 95 % - 100 %.

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