Analysis of key patents of the regeneration of acidic cupric chloride etchant waste and tin stripping waste

Resources, Conservation and Recycling 49 (2007) 217–243

Review

Analysis of key patents of the regeneration of acidic cupric chloride etchant waste and tin stripping waste Tiina Keskitalo a,∗ , Juha Tanskanen a , Toivo Kuokkanen b a

Department of Process and Environmental Engineering, Chemical Process Engineering Laboratory, University of Oulu, PO Box 4300, FI-90014 Oulu, Finland b Department of Chemistry, University of Oulu, PO Box 3000, FIN-90014 Oulu, Finland Received 14 December 2005; received in revised form 2 May 2006; accepted 24 May 2006 Available online 11 July 2006

Abstract Spent acidic cupric chloride etchant waste and tin stripping waste from printed circuit board (PCB) industry are classified as hazardous wastes. They contain significant amounts of metals and acid. Usually, spent cupric chloride and tin stripping solutions are shipped off-site for reclamation. At the moment, a large proportion of acidic cupric chloride etchant waste and tin stripping waste are treated by neutralization, resulting in metal-bearing sludge that often ends up in special landfills. This wastes valuable natural resources, which could otherwise be recycled. This paper presents a review of some patented methods developed for the regeneration of acidic cupric chloride etchant waste and tin stripping waste. These methods are based on electrowinning, cementation, solvent extraction, precipitation and membrane technology. The advantages and disadvantages of these developed processes are summarized. It can be seen that there is still a need to develop more efficient and economical processes for the regeneration of acidic cupric chloride etchant waste and tin stripping waste. © 2006 Elsevier B.V. All rights reserved. Keywords: Etching; Etchant waste; Cupric chloride; Tin stripping; Regeneration

∗

Corresponding author. Tel.: +358 40 5712523; fax: +358 8 553 2304. E-mail address: [email protected] (T. Keskitalo).

0921-3449/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resconrec.2006.05.001

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Contents 1. 2.

3.

4. 5.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Regeneration of acidic cupric chloride etchant waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Electrolytic regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Regeneration based on cementation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Regeneration based on solvent extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Regeneration based on the precipitation of copper oxide . . . . . . . . . . . . . . . . . . . . . . . 2.5. Regeneration to yield basic copper chloride micronutrient additive . . . . . . . . . . . . . . 2.6. Summary of the methods of regenerating acidic cupric chloride etchant waste . . . . Regeneration methods of tin stripping waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Electrolytic regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Regeneration based on solvent extraction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Sustainable technologies for the regeneration of tin stripping waste . . . . . . . . . . . . . 3.4. Summary of the methods for the regeneration of nitric acid-based tin stripping waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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1. Introduction Etching is a process whereby the unwanted copper foil on a printed circuit board (PCB) is oxidized and then dissolved into a liquid carrier called an etchant. The parts of the foil intended to act as trackwork are protected from dissolution during etching by being covered with an etch resist (Fitzpatrick-Brown, 1985). The etch resist must not be affected by the etchant and vice versa. Resists are available in two different classes. The first class consists of organic resists based on an organic chemical or mixture. The second class consists of metallic resists, which are based on a pure metal. When etching innerlayers, the etch resist is usually an organic compound (photoresist), and when etching outerlayers, the etch resist is usually a metallic layer. There are several etchants available. The choice of etchant depends on many things, such as the structure of the PCB and the availability of suppliers. Cupric chloride (CuCl2 ) became the main etchant for PCBs with non-metallic resists in the 1960s. Up until then, ferric chloride had been the most common etchant for non-metallic resist PCBs due to its fast etch rate and high metal holding capacity. Cupric chloride superseded it in popularity because it is capable of continuous chemical regeneration and can be operated in a steadystate condition. CuCl2 also became desirable because it could use the copper from the circuitry as an additional etchant without a need to introduce other metals into the etching bath (Gurian et al., 2000). Cupric chloride etchant is an acidic solution where hydrochloric acid keeps the copper soluble. The reason why copper can be etched by using a solution of copper itself is due to the fact that copper can be easily transformed from one oxidation state to another. In cupric chloride etching, elemental copper reacts with cupric copper (Cu2+ ) to form cuprous copper (Cu+ ), as it can be seen in reaction (1) (Chemcut corporation, 2002): Cu0 + Cu2+ Cl2 → 2Cu+ Cl

(1)

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Copper accumulates in the solution during the etching process, while the copper ions responsible for etching are reduced. In order to maintain the etch rate, cuprous copper must be converted back to the cupric state, so that it can etch more metal. This is done through chemical regeneration of the etchant. From a chemical point of view, the simplest replenisher is chlorine gas (Cl2 ) (Sedlak, 1995). The reaction between cuprous chloride and chlorine gas can be seen in the following equation: 2Cu+ Cl + Cl2 → 2Cu2+ Cl2

(2)

However, there are practical difficulties when using Cl2 due to safety issues and legislation. Therefore, oxidizing chemicals, such as hydrogen peroxide (H2 O2 ) or sodium chlorate (NaClO3 ), are normally used in the chemical regeneration of the etchant because they react with hydrochloric acid, producing the chloride ions needed to oxidize Cu+ ions to Cu2+ ions, as it can be seen in reactions (3) and (4). If sodium chlorate is used as a replenisher, sodium chloride (NaCl) is a by-product of the regeneration reaction, which means that there will be an extra component in the etching bath. For continuous operation of etching, also the etched copper should be removed as it accumulates (Adaikkalam et al., 2002). The concentration of copper is maintained by occasionally withdrawing a portion of etchant from the etching bath and replacing it with a fresh solution. The disadvantage is that the excess etchant must be disposed of in an environmentally acceptable manner. 2Cu+ Cl + H2 O2 + 2HCl → 2Cu2+ Cl2 + 2H2 O

(3)

6Cu+ Cl + NaClO3 + 6HCl → 6Cu2+ Cl2 + NaCl + 3H2 O

(4)

Analysis of the recovery of ammoniacal etchants was not the target of this paper, but since they are sometimes processed together with acidic cupric chloride etchant wastes, some chemistry of ammoniacal etching needs to be examined as well. Ammoniacal etchant is the other approach to etching of innerlayers. Ammoniacal etchant is a mixture of cuprous chloride Cu(NH3 )x Cl and cupric ammonium chloride Cu(NH3 )x Cl2 (Queneau and Gruber, 1997). Similarly to the cupric chloride etching, the copper that is already in solution in the ammoniacal etchant, dissolves the copper metal on the circuit board. However, cuprous (Cu+ ) compounds are not soluble, except in the presence of ammonia, which is why etching is facilitated by the ammonia and chloride in the etchant. The overall reaction is shown in the following equation: Cu0 + Cu2+ (NH3 )4 Cl2 → 2Cu+ (NH3 )2 Cl

(5)

The cuprous salts are then oxidized by the oxygen in the air, which is being pulled through the etcher, as it can be seen in the following reaction: 2Cu+ (NH3 )2 Cl + 2NH3 + 2NH4 Cl + 1/2O2 → 2Cu2+ (NH3 )4 Cl2 + H2 O

(6)

When making an outerlayer, the etch resist is usually a metallic layer of pure tin or tin/lead alloy. The metal coating protects the copper tracks on the PCB during etching. After etching, the metallic etch resist must be removed by a process called tin stripping. Nowadays, the most common tin stripping solutions are nitric acid-based solutions that involve a source of ferric ions.

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Tin has three different oxidation states, i.e. elemental tin, stannous tin (Sn2+ ) and stannic tin (Sn4+ ). In the stripping process, tin is oxidized to its stannic form. The reaction for nitric acid-based tin stripping is shown in equation (7) (McKesson, 1999): 2Sn0 + 2HNO3 → 2Sn4+ O2 + N2 O + H2 O

(7)

Once tin has been oxidized, it must be dissolved into the solution. However, most stannic salts are insoluble in water. In addition to being insoluble in water, the stannic oxide (SnO2 ) formed during the stripping reaction is also insoluble in nitric acid. Therefore, to prevent the stannic oxide from precipitating out, it must be dispersed in the liquid phase by a suspending agent. Some other additives are also added into commercial tin strippers for different purposes, for example, to help to protect the underlying copper surface from the solution. Spent tin stripper is a milky white acidic solution, and stannic oxide is extremely difficult to remove from it since it does not form a filterable precipitate (Kerr and Coultard, 2004). At some point, the tin stripping bath becomes exhausted and needs to be treated and replaced with a fresh solution. The tin stripping process can be operated either batchwise or continuously. In batch type operation, there might be as much as 200 g/dm3 of metal in the final stripping solution. Alternatively, the chemistry may be used on a ‘feed-and-bleed’ basis, which means that it is kept at a steady level by bleeding out a certain amount of solution and replacing it with fresh chemistry. When these etching solutions are disposed of, they typically contain over 100 g/dm3 of metal. Both spent cupric chloride etchant and tin stripping solutions are usually stored in tanks and shipped off-site for reclamation. They are classified as hazardous wastes, which are becoming increasingly more expensive to dispose of. Usually spent tin stripping solutions and often also spent cupric chloride etchant wastes are treated by neutralization, resulting in metal-bearing sludge, which sometimes has to be discarded in special landfills. Neutralization consumes a lot of caustic solution and fails to solve the waste problem. Neutralization and disposal of the formed precipitate result in a costly waste of raw materials because spent acidic cupric chloride and tin stripping solutions contain a lot of valuable components that should be recovered economically. Therefore, at the present, tin stripping waste and cupric chloride etchant waste constitute a big problem for PCB industry. There is clearly a need to develop technically and economically feasible regeneration processes for these wastes. In this paper, some methods developed for the regeneration of spent acidic cupric chloride etching and tin stripping solutions are described. Certain advantages and disadvantages of these methods are also discussed.

2. Regeneration of acidic cupric chloride etchant waste 2.1. Electrolytic regeneration Since the copper concentration is relatively high in spent cupric chloride etching solutions, electrolytic regeneration seems an appealing alternative. There are inventions that utilize a regeneration process which reverses reaction (1) in such a way that the copper

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metal etched into the system is continuously removed, and the evolution of chlorine and hydrogen gas is avoided. Since electrolytic recovery from solutions containing chloride ions by avoiding Cl2 evolution is difficult, there are also methods that involve electrolytic recovery of copper and use of the generated chlorine gas in the chemical regeneration of the etchant. However, the use of Cl2 is forbidden in many areas, which makes such methods of regeneration useless. Several attempts have been made to develop electrolytic processes for the regeneration of acidic cupric chloride etchant waste. Those methods can be operated in a closed-loop cycle with the etching process. Many of them, however, suffer from various drawbacks. A few of the developed methods will be described below. If regeneration is done by reversing the etching reaction, Cu+ is oxidized to Cu2+ at the anode and the Cu metal is electrodeposited via the simultaneous reduction of Cu2+ to Cu+ and the reduction of Cu+ to Cu metal at the cathode (Oxley, 1995). This is accomplished by exceeding the limiting current for the reduction of Cu2+ to Cu+ , forcing the subsequent reduction of Cu+ to Cu metal to take place. At the same time, the limiting current for the anodic reaction must not be exceeded, so that the next electrochemical oxidation process, chlorine evolution, can be avoided. Similarly, in order to avoid H2 evolution at the cathode, the limiting current for combined cathodic reactions must not be exceeded. H2 or Cl2 evolution can be avoided by careful control of solution mass transfer and current density. The desired reactions can be achieved by arranging to have a much higher real current density at the cathode than at the anode. If the real current densities were the same, Cu+ would be oxidized to Cu2+ at the anode and Cu2+ reduced to Cu+ at the cathode without any net regeneration. Some of the first efforts to develop a commercial electrolytic system for the regeneration of acidic cupric chloride etchant waste are the methods patented by Bell Telephone Labour Inc. (Garn and Sharpe, 1960) and Western Electric Co. (Parikh and Willard, 1974). In those methods, the real current density is made higher at the cathode than at the anode by using smaller cathodes than anodes. Copper is recovered as loose, granular deposits that can be mechanically scraped off of the cathodes. In the former patent, the cathodes have a planar plating surface area with sharp corners. The sharp corners, however, lead to variable current density gradients, resulting in uneven copper build-up and looseness or hardness of the copper on the cathodes. It is also difficult to avoid chlorine gas evolution, especially at the solution interface, when relatively high operating currents are employed. The latter method involves an arrangement where the cathode consists of a bundle of cylindrical rods, whereas the anode is planar graphite. By having no major planar surfaces or sharp edges, the current density gradient variations at the cathode can be minimized. However, because the anode and cathode areas remain different, the problem of uneven current distributions is still not totally solved. The uneven current distribution translates directly into an uneven potential distribution, also resulting in Cl2 evolution at the anode (Oxley, 1995). The Electricity Council (Hillis, 1984) patented an electrolytic regeneration method which employs separate anolyte and catholyte flow loops with different electrolyte compositions such that the catholyte is approximately 10 times more dilute in copper than the anolyte. The electrolytic cell is provided with a cell divider slowing down the diffusion of ions between the anode and cathode compartments. In this method, the need for different anode and cathode areas can be avoided because the limiting current for the reduction of Cu2+ to Cu+ is exceeded by the difference in electrolyte concentrations. The metallic copper is

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produced at the cathode in a dendritic form, and it readily falls off the cathode to the bottom of the compartment, from where it can be removed without a need to withdraw the cathode. Some problems of the aforesaid processes are described in the method patented by Oxley (1995). One of the problems is the uneven current distribution caused by the different anode and cathode areas. The uneven current distribution leads to uneven potential distribution, which may make it impossible to avoid parasitic Cl2 evolution. The methods also suffer from waste heat generation stemming from IR losses as a direct consequence of high operating voltages (5–8 V). That leads to a need for heat exchangers, which is an additional operating cost burden. In the method patented by Oxley (1995), current density is arranged to be higher at the cathode than at the anode by using a porous flow-through anode instead of a flow-by anode. A flow-through anode provides the necessary increased surface area internally, so that the cross-sectional area of the anode and the cathode can remain the same. By using a porous flow-through anode and a planar flow-by cathode, mass transport and real current density can be controlled by adjusting flow rate and electrode thickness. In the process, the coulombic efficiency for copper deposition can be maximized at low flow rates, while anode efficiency is maximized at high flow rates. This method is less costly because of the lower operating voltages and the improved on-line process control, operating efficiency and reliability (Oxley, 1995). The etching process involves some oxygen ingress, which will consume both CuCl and HCl, resulting in the formation of CuCl2 and water. That has the effect of driving the redox potential more positive. It also leads to a net growth in the solution inventory similarly to chemical regeneration, which electrolytic regeneration seeks to avoid. This effect could be alleviated to some extent by removing part of the spent etchant and disposing of it for off-line waste treatment. Alternatively, to reverse the effect of oxygen ingress, Oxley (1995) suggests an auxiliary cell to be placed before the main electrolytic cell. In the auxiliary cell, the anode reaction would be oxygen evolution and the cathodic reduction of Cu2+ to Cu+ . The latter is not difficult to achieve, but it is impossible to exclude chlorine evolution at the anode, particularly at higher current densities, even though certain materials favour oxygen evolution. Although the above method seems advantageous, there is still a need for a more efficient and productive recovery process. Oxley Research Inc. (Oxley et al., 1998) patented another electrolytic process for the regeneration of acidic CuCl2 etchant waste, which also includes two electrolytic cells. In the first electrolytic cell, the spent etchant containing a high fraction of Cu2+ ions is converted into a solution containing a high fraction of Cu+ ions. In the second electrolytic cell, the monovalent form of the metal is converted into metallic copper in a marketable slab form. It has been found that plating copper from a solution containing a high fraction of Cu+ ions is the key to producing an even, dendrite-free copper deposit. In both cells, the anodic reaction consists of the oxidation of Cu+ to Cu2+ . Another advantage of the method is that plating copper from a solution containing a high level of Cu+ ions allows intermittent operation since the already plated copper will not redissolve when the plating current is turned off and the cathode remains submerged in the catholyte. Especially for smaller PCB shops, this is a major advantage. The above methods seem attractive because the etchant can be regenerated in a closedloop system integrated with the etching bath. The electrolytic regeneration methods, however, seek to avoid chemical regeneration, which is very common these days. In practice,

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too, it may be very difficult to avoid especially chlorine evolution whenever chloride ions are present in the solution. There are also some regeneration methods that include chlorine gas evolution and use the formed Cl2 for the chemical regeneration of the etchant. However, as earlier mentioned, the use of chlorine gas is limited, which makes these regeneration methods less relevant. Therefore, only one regeneration method of that kind is briefly described here. Nittetsu Mining Co. (Mikami et al., 1995) patented a method that includes electrolytic treatment of either cupric chloride or ferric chloride etchant waste, using a diaphragm to withdraw deposited copper in a cathode cell and supplying the generated chlorine gas in the anode cell into the etching bath to oxidize the etchant. The used etchant is conducted first to the cathode cell, from where the solution is conducted to the anode cell in order to oxidize the Cu+ ions to Cu2+ ions along with the generation of chlorine gas. Chlorine is supplied to the absorbing tower, from where it can be introduced into the etching bath. The process is operated in a closed system. 2.2. Regeneration based on cementation Cementation reaction is a spontaneous electrochemical reaction that involves electrochemical precipitation of a noble metal from solutions of its salts on a more electronegative metal, which correspondingly dissolves. The common chemical equation is shown in the following reaction: Me1 (s) + Me2 2+ (aq) ↔ Me1 2+ (aq) + Me2 (s)

(8)

Cementation is a feasible method when the concentration of the metal needed to be recovered is not very high. A few methods based on cementation have been developed for the regeneration of cupric chloride etchant waste. By cementation methods, copper ions can be replaced with ions of a metal other than copper. Copper can be recovered in the form of metallic copper precipitate. A valuable chloride of a metal other than copper can be produced as well. For example, Densan KK (Watanabe and Mori, 1988) patented a method where metallic aluminium is added into the spent cupric chloride etching solution. The precipitate that forms is a mixture of metallic copper and copper oxide, which can be separated by filtration. The aluminium chloride filtrate can be utilized as a starting material for polyaluminium chloride (PAC) production or for other purposes. Also Nikko Fine Product Co. (Narisawa et al., 1994) patented a method where metallic copper powder and a useful chloride of a metal other than copper are recovered from spent acidic cupric chloride etchants. First, the spent etchant is mixed with active carbon to remove adsorbable impurities from the solution, followed by removal of active carbon. Then, a powder of an additive metal that has a larger ionization tendency than copper (for example, manganese, zinc, cobalt, nickel or tin) is mixed with the solution to cause replacement of copper ions with ions of the additive metal in such a way that elementary copper is formed. Besides the cementation reaction, another reaction takes place between the additive metal and free hydrochloric acid with hydrogen gas evolution. These reactions define the amount of additive metal to be added into the waste solution. The amount of additive metal should be as closely as possible equivalent to the total amount of chloride

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ions in the solution. If the amount is too small, part of the copper remains unconverted and remains as an impurity in the chloride of the additive metal. On the other hand, if the amount is too large, part of the additive metal remains unutilized and remains as a metallic impurity in the recovered copper precipitate. The temperature should be between 40 and 100 ◦ C. If the reaction temperature is too low, the reaction proceeds very slowly. It is important to agitate the reaction mixture, to facilitate rapid removal of the precipitate of metallic copper deposited on the surface of the additive metal, so that it will not inhibit the progress of the reaction. The finely divided copper particles can be removed by, for example, filtration followed by washing and drying of the product. The aqueous filtrate contains the chloride of the additive metal in a quite high concentration and purity. The solution can be used as such for the intended application. For example, zinc chloride can be used as a constituent of dry batteries, cobalt chloride as an ingredient in drying agents and tin chloride, nickel chloride and manganese chloride as ingredients in catalysts, etc. The metal chloride solution can also be concentrated by evaporating part of the water, or optionally, the metal chloride can be crystallized. However, metal chloride by-products do not necessarily have a sufficient market. For example, some regeneration methods based on cementation involve replacement of copper ions by the addition of iron or aluminium powder. The aqueous solution of iron or aluminium chloride can be utilized as an inorganic flocculant in water treatment. However, no further increase can be expected in the demand for these flocculants, because they have been under continuous replacement in the recent years with flocculants based on organic polymers (Narisawa et al., 1994). Also, it has not been taken into account that sodium, mainly in the form of sodium chloride, is also very often present in the etchant waste nowadays if sodium chlorate has been used as an oxidizer in the etching bath. Since sodium is more electronegative than any of the suggested metals used in cementation, it will not be reduced but stays as an impurity in the chloride solution. That will further decrease the use of by-product chloride solution. 2.3. Regeneration based on solvent extraction Copper recovery by solvent extraction (SX) has progressed from a technology with limited applications to a technology with broad applicability for a variety of solutions. Several methods for the regeneration of spent acidic cupric chloride etching solution based on solvent extraction have been developed. In most cases, the final copper recovery is done by electrowinning (EW). Solvent extraction utilizes the differences in the solubilities of the components. Since solubility depends on chemical properties, extraction exploits chemical differences. Two phases must be brought into good contact to permit transfer of material and then be separated. By combining several extraction stages, the desired component can be concentrated into the other phase. The phase leaving a liquid–liquid contactor is called the extract. The raffinate is the liquid phase left from the feed after being contacted by the other phase. A successful copper extractant has certain properties. It should, for example, extract copper selectively from the other species present, be soluble in an inexpensive diluent (usually kerosene), not transfer acid from strip to extraction and be safe to use (Kordosky, 1992). Jensen (1981), for example, patented a method for selective extraction and ultimate recovery of copper from spent cupric chloride etchant. A flow chart of the process is shown in

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Fig. 1. A flow chart of acidic cupric chloride regeneration based on solvent extraction (Jensen, 1981, modified from Fig. 1).

Fig. 1. In the copper extraction units, the copper-containing solution and the organic extractant flow countercurrently between the extraction stages. In each stage, the active organic extractant selectively extracts copper from the solution, resulting in a copper-loaded organic phase and a raffinate from which copper has been removed. In the first extraction unit, copper is extracted as copper chloride by a copper chloride extractant. The extractant is dissolved in an inert organic solvent, such as kerosene. After extraction, the loaded organic extractant is transported into the first stripping unit, where it is brought into contact with a suitable aqueous stripping solution, which actively strips copper chloride from the loaded extractant. The stripped extractant is recycled into the first copper extraction unit, and the pregnant stripping solution containing copper chloride is transported into the second copper extraction unit. In that unit, the solution is contacted with a hydrogen ion exchange extractant, which selectively extracts copper. Hydroxy oximes, tertiary amines and beta diketones are examples of active and selective organic extractants. As the extractant selectively extracts copper from the stripping solution by hydrogen ion exchange, hydrochloric acid is also formed. The acid can be neutralized by the addition of a suitable base. The resulting spent stripping solution can be recycled into the first copper-stripping unit. The loaded hydrogen ion exchange extractant is transported into the second copper-stripping stage, where the loaded extractant is contacted with an aqueous, acidic stripping solution, such as sulphuric acid. The copper can be recovered from the stripping solution by electrowinning as metallic copper or by crystallization as copper sulphate. However, the above process suffers from a complicated sequence of steps involving repeated extraction and stripping stages under different operating conditions. A simpler approach to the recovery of metals from chloride solutions was patented by Inst Injenerna Chimia (Kyuchoukov et al., 1989). In that method, copper is first extracted with an organic extracting agent from the chloride solution as a copper chloride complex. After the extraction, the raffinate obtained is separated from the organic phase. The organic phase is freed from chloride ions by washing it with water, an aqueous ammonia solution or an aqueous ammoniacal ammonium sulphate solution. After that, the copper is stripped from the organic phase by contacting the organic phase with aqueous sulphuric acid to yield an aqueous solution of a copper sulphate. The organic phase can be returned to the extraction stage after regenerating it with a chloride ion-containing aqueous solution. Copper can be recovered from the copper sulphate solution as copper sulphate by crystallization or as metallic copper by electrowinning. This regeneration method is simpler to carry out and

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does not require many repeated stages of extraction and stripping, but it is still chemical consuming, which might make the process economically less feasible. Another interesting method based on solvent extraction was patented by Phibrotech Inc. (Edelstein, 2002). The method involves the use of cupric chloride etchant solution in a solvent extraction electrowinning copper refinery to improve the recovery of copper and to reduce material costs. In a SX-EW copper refinery, copper is obtained by first mining acidsoluble copper-containing ore and piling the ore into a heap. Then, in the acid leaching step, usually sulphuric acid is sprayed over the heap, dissolving copper in the ore and forming a copper-containing solution. Acid leaching is followed by solvent extraction and, finally, electrowinning. The patented method has shown that an acidic cupric chloride etchant solution can be an effective substitute for sulphuric acid in the acid leaching step, which reduces the need for purchased sulphuric acid and thereby reduces the costs. At the same time, it is possible to improve the copper yield without additional ore mining. Typically, SX plants cannot use spent acidic cupric chloride etchant solutions because the EW process has a low tolerance for chloride ion concentration. Therefore, the SX step should include one or more water washing stages to prevent or to minimize entry of chlorides into the EW step. After the extraction, the copper-free acid phase is returned to the acid leaching step. This method sounds tempting. However, it was developed to improve copper recovery and to reduce material costs in a SW-EX copper refinery and not as a method for the regeneration of acidic cupric chloride etchant waste. Therefore, on a larger scale, this method could not be a solution for the regeneration of acidic cupric chloride etchant waste. Racing raw material costs have accelerated efforts to produce inexpensive low-grade feedstock from secondary sources. Queneau and Gruber (1997) discuss the production of major copper chemicals from secondary sources in their article ‘The U.S. production of copper chemicals from secondary and by-product sources’. In the United States, the major copper chemicals produced are derived almost entirely from by-product sources, including spent etchant solutions. Some of these processes include regeneration based on solvent extraction. The companies that use etchants as feedstock, process usually spent ammoniacal etchants together with spent acidic cupric chloride etchants. Final products of such processes are copper sulphate and reconstituted ammoniacal etchant. In those regeneration methods, ammoniacal etchant is processed at ambient temperature by SX. The organic extractant is dissolved in an inert organic solvent and copper is extracted almost completely from the NH4 Cl raffinate. Free ammonia is formed during extraction (pH ∼ 8), as it can be seen in reaction (9) (Queneau and Gruber, 1997). Therefore, sufficient amount of acidic cupric chloride etchant waste can be added to the first extraction stage to convert the acidic cupric chloride etchant to ammoniacal etchant. The loaded organic phase is scrubbed with weak acid. Scrubbing the loaded organic phase removes loaded or physically entrained NH3 as (NH4 )2 SO4 . The copper sulphate stripping solution can then be crystallized. Cu(NH3 )4 Cl2 + R2H+ → R–Cu2+ + 2NH4 Cl + 2NH3

(9)

Spent copper etchant recyclers can actually produce more NH4 Cl than the circuit-board industry needs. The excess of recovered ammoniacal etchant is related to the quantity of processed acidic cupric chloride etchant waste (see reaction (9)). Therefore, in the long run,

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Fig. 2. The principle for the regeneration of cupric chloride etchant waste based on precipitation of copper oxide (Allies et al., 1996).

there is still a need to develop more advantageous processes for the regeneration of acidic cupric chloride etchant waste. Moreover, the capital costs of these kinds of SX plants are high and some carryover of the organic phase into the aqueous phase is likely to occur. 2.4. Regeneration based on the precipitation of copper oxide Some methods of regenerating acidic cupric chloride etchant waste based on copper oxide (CuO) precipitation have also been developed. For example, Training ‘N’ Technology Inc. (Allies et al., 1996) patented a process whereby cupric chloride etchant waste is converted into non-hazardous material consisting of copper oxide and saline solution. The principle of the method is shown in Fig. 2. A preheated spent cupric chloride etchant stream is combined with a preheated caustic solution stream. When the streams are mixed, an exothermic neutralization reaction begins, producing copper hydroxide precipitate and saline solution. The reaction continues and becomes endothermic as the formed blue copper hydroxide is converted to black copper oxide in the saline solution. Fine copper oxide can be separated by filtration, for example. The copper oxide can be used as feedstock for mining operations, and the saline solution can be discharged for standard waste waster treatment. The disadvantage of this method is the big caustic solution consumption, which makes the process economically less viable. Additionally, fine copper oxide slurry is difficult to filtrate. Another method where copper oxide precipitation is utilized in the recovery of waste cupric chloride etchant is a method patented by Myung Jin Chemical Co. (Seo et al., 2003). Compared to the previously described method, the caustic solution is slowly added to the etchant waste, while keeping the temperature relatively low, at about 10–30 ◦ C. Since waste etchant contains free hydrochloric acid, the neutralization reaction generates a large amount of heat, which leads to an instant increase of reaction temperature, even up to 80 ◦ C. The formed copper hydroxide is instantly dehydrated at high temperatures to form black copper

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oxide. If the temperature is not kept at a low level, the produced copper oxide is platy-shaped and therefore more difficult to filtrate than, for example, needle-shaped copper oxide. If the temperature is kept at a low level during neutralization, a blue-coloured copper hydroxide slurry is obtained first. Subsequently, when the slurry is heated and sintered at 100–400 ◦ C for 1–3 h, the slurry is converted to copper oxide, which has needle-form crystal morphology and can thus be easily filtered. Even though the copper oxide formed by this method has better features than the copper oxide produced with the previously described method, a lot of caustic solution is still consumed. Also, energy consumption is higher in the latter method due to the cooling of the copper hydroxide slurry and the energy needed for sintering the copper oxide precipitate. The advantage of these two precipitation methods is that it does not matter if there is also sodium chloride present in the etchant waste since sodium chloride is formed in the neutralization reaction, anyway. There are also similar methods that apply cupric oxide recovery by adding caustic solution into spent ammoniacal etchant. In addition, approximately 20 vol.% acidic cupric chloride etchant can be processed together with the ammoniacal etchant. Aqueous caustic soda is added to oxidized ammoniacal etchant (‘Ammo’) at above 85 ◦ C. Also limited quantities of oxidized acidic cupric chloride etchant (‘CCE’) can be added to the reactor together with ammoniacal etchant. The final product is black CuO. In addition, aqueous sodium chloride and gaseous ammonia by-products are formed. The ammonia is usually absorbed by using hydrochloric acid to produce aqueous ammonium chloride. The process is introduced in Fig. 3 (Queneau and Gruber, 1997). This kind of method, however, could not be a solution for the regeneration of acidic cupric chloride etchant waste on a larger scale, since the amount of acidic cupric chloride etchant waste processed by these methods has to be limited. If too much acidic cupric chloride is added, the formed product is useless, slimy hydroxide-contaminated precipitate. Furthermore, still a relatively high amount of caustic solution and energy are consumed by the regeneration process. 2.5. Regeneration to yield basic copper chloride micronutrient additive There are also inventions that relate to manufacturing of copper-based micronutrient additives for feed products from waste etchant streams. Those inventions incorporate ‘basic copper chloride’ Cu(OH)x Cl(2−x) as a mineral supplement for feed products. The products are valued both for their biological availability to animals and for their low reactivity with organic constituents in the feed mix. Often copper sources suffer from a variety of problems, including low bioavailability or destabilizing effects on vitamins in feed mixes. Furthermore, in manufacturing copper-based micronutrient additives for feed products, controlling the particle size of the additive might present problems. Also background salts may contribute, for example, to poor physical characteristics of the micronutrient additive. Heritage Environmental Services Inc. (Steward, 1995), for example, patented a method that provides a process for producing basic copper chloride from a source of copper and chloride ions. Spent etchant streams are suitable copper and chloride ion sources. In this method, spent etchant streams are regenerated to yield basic copper chloride and reusable ammonium chloride liquor which can be converted into fresh ammoniacal etchant by additional processing. A spent cupric chloride etchant stream and a spent alkaline etchant stream

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Fig. 3. The caustic boiling of spent ammoniacal and acidic etchant (Queneau and Gruber, 1997).

are combined to form copper-containing slurry. The self-neutralization reaction is shown in equation (10). From the copper-containing slurry, basic copper chloride can be recovered for use as a micronutrient supplement or as a copper source in other products. Cu(NH3 )4 Cl2 + CuCl2 + HCl + 3H2 O → Cu2 (OH)3 Cl + 4NH4 Cl

(10)

A flow chart of the process is shown in Fig. 4. First feed stream is spent copper-containing alkaline etchant solution and second feed stream is spent copper-containing acidic etchant solution. Quality assurance procedures, such as checking for acidity/alkalinity, copper and

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Fig. 4. A flow chart of the regeneration to yield basic copper chloride (Steward, 1995, modified from Fig. 1).

trace metallic impurities and specific gravity, must be performed before the etchant is pumped from the storage tanks. If the spent etchant streams contain high levels of soluble arsenic, they can be treated separately in pre-treatment reactors before being fed to the primary reactor. By adding calcium compounds and magnesium compounds into the alkaline etchant solution, arsenic can be precipitated in the form of low solubility calcium magnesium arsenates. To reduce the levels of soluble arsenic in the acidic etchant solution, the pH of the stream can be raised to the point where precipitation begins to occur. After precipitation, the streams are fed to the primary reactor where the residence time varies. The reaction products, which are basic copper chloride and soluble background salts, are pumped from the primary reactor to settler. The settler separates the reaction products into a supernatant brine and copper-containing slurry. The brine consists primarily of ammonium chloride liquor and dissolved copper, whereas the slurry comprises basic copper chloride along with a variety of background salts. A certain amount of the slurry is withdrawn for use as a seed stream for seeding the crystallization of basic copper chloride in the reactor. It is important to maintain an appropriate concentration of feed slurry in the reactor because seeding affects to the final product particle size distribution. The supernatant brine from the settler passes directly to a finishing operation, whereas the remaining portion of the slurry is pumped to a drying operation, which includes a filter, a dryer and a sieve. A water wash is provided to assist in removing ammonium chloride from the solids. The effluent wash water is sent to disposal. Appropriately sized fractions pass through the sieve as final products to be used as a micronutrient supplement or as a copper source in other

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products. According to the patent, the product is substantially free of background salts and contaminants and that the product has high bioavailability. Ammonium chloride liquor recovered from the product filter is fed together with the supernatant ammonium chloride liquor to polishing operation, which includes a polishing reactor and a filter press. A metal scavenger is fed to the polishing reactor to reduce the levels of dissolved copper and other metals. The by-product cake can be processed by a copper smelter for the recovery of copper values. The clear ammonium chloride can be finished into fresh ammoniacal etchant. The method seems appealing since the spent etchants can be recovered as valuable products. The disadvantage is the careful control of the process such that the quality of the final product is appropriate. 2.6. Summary of the methods of regenerating acidic cupric chloride etchant waste There are both advantages and disadvantages in the methods developed for the regeneration of acidic cupric chloride etchant waste. The advantages and disadvantages are summarized in Table 1. It can be seen that there is still an obvious need to develop more efficient and economical recovery processes for treating acidic CuCl2 etchant waste solutions.

3. Regeneration methods of tin stripping waste 3.1. Electrolytic regeneration Because the metal concentration is relatively high in spent tin stripping solutions, electrolytic regeneration would seem reasonable. However, electroplating is possible only from electrolyte systems that are true solutions. Because of the peculiar nature of tin dispersion, tin values cannot be electroplated from this system without further treatment of the spent tin stripper. There are several developed methods that are based on electrolytic regeneration. For example, Scott et al. (1997) studied electrochemical recycling of all metals from spent nitric acid-based tin stripping solutions as well as the combination of electrochemical deposition of copper and precipitation of tin and lead, followed up by furnace recycling. Their study turned out to have two limitations in the attempts to recycle commercial stripping solutions. Firstly, tin tends to deposit with both copper and lead to form alloys, which makes clear separation of metals impossible. Secondly, since there are additives in commercial stripping solutions, the current efficiency for the deposition of metals is much lower from commercial stripping solutions than from stripping solutions consisting exclusively of nitric acid and metals. Also, the deposition potentials relating to commercial stripping solutions are more negative, leading to higher energy consumption. Scott et al. (1997) used a simple aqueous nitric acid stripping solution in their experiments because it was more compatible with the electrochemical recycling process. Nitric acid concentration was also lower (∼1.4–2.1 M) in their experiments than in commercial nitric acid-based tin stripping solutions (∼5 M). They found that a more dilute stripping solution still selectively stripped tin from the PBC

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Table 1 Summary of the advantages and disadvantages of the methods for the regeneration of acidic CuCl2 etchant waste Advantages

Disadvantages

By electrochemically reversing the etching reaction

+ Reversing the etching reaction and restoring the etching capacity

+ Recovery of metallic copper

+ Avoiding waste formation

+ Possibility to regenerate the etchant in a closed-loop system with the etching process

− Several problems related to electrowinning; due to the difficulty of avoiding chlorine gas evolution, etc.

With chlorine gas evolution

Restoring the etching capacity

Recovery of metallic copper

Possibility to regenerate the etchant in a closed-loop system with the etching process

Using the generated chlorine gas for regeneration of the etchant

Safety, health and environmental issues; use of chlorine gas is forbidden in many area

Regeneration based on cementation

Recovery of metallic copper

Metal chloride other than copper chloride is formed as a by-product

Regeneration based on solvent extraction

Copper is eventually recovered as metallic copper or copper sulphate

In the methods where spent ammoniacal etchants are processed together with spent acidic CuCl2 etchants, ammoniacal etchant can be recovered

Electrolytic regeneration

The amount of additive metal has to be controlled carefully; if the amount is too small, part of the copper remains unconverted, and if the amount is too large, part of the additive metal remains unutilized There is often quite a complicated sequence of steps involving repeated extraction and stripping units under different operating conditions

− High operating voltages, leading to waste heat generation

− Electrolytic regeneration seeks to avoid oxidizing chemicals that are used in chemical regeneration

− Not reliable or efficient enough

There is not necessarily a sufficient market for the metal chloride produced

The case with sodium present in the etchant waste has not been taken into account. Since sodium is highly electronegative, it will not be reduced but remains as an impurity in the chloride solution

Relatively high chemical consumption

Some carryover of the organic phase into the aqueous phase is likely to occur

High capital costs

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Regeneration method

Copper is recovered as copper oxide

Sodium can be present in the etchant waste since sodium chloride is formed in the neutralization reaction

Regeneration to yield basic copper chloride micronutrient additive

Copper is recovered as basic copper chloride to be used as a micronutrient supplement or as a copper source in other products

Spent acidic CuCl2 etchant and ammoniacal etchant are processed together; ammoniacal etchant can be recovered

In the methods where a limited amount of spent acidic CuCl2 etchant can be processed together with ammonical etchant, ammonical etchant can be recovered

Acid is not recovered, and a lot of saline solution discharged into standard waste water treatment is formed

Process has to be controlled carefully such that the quality of the final product is sufficient

A lot of caustic solution is consumed in neutralization

Platy-shaped copper oxide is difficult to filter

A lot of energy is required in the regeneration process if needle-shaped copper oxide with better features than platy-shaped copper oxide is produced

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Regeneration based on precipitation of copper oxide

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at a considerably high speed and with very little attack on the copper underneath the metal resist layer. After the stripping process, stannic oxide precipitated out from the solution. Complexing agents were not considered necessary to keep the tin in solution. In their experiments, Scott et al. (1997) first separated stannic oxide from the stripping solution by filtration. The precipitate obtained could be dissolved in HCl when heated. Since no Cl2 was found to escape from the solution, it could be assumed that tin was in the form of H2 SnCl6 in the solution. Tin can be electrodeposited onto stationary stainless steel electrodes. However, the process of recovering tin is somewhat complicated and energy consumption is considerably higher than that for copper. After stannic oxide is filtered, the solution contains mainly copper and also lead, if a tin/lead alloy has been used as a metallic resist. Copper can be electrodeposited with high current efficiency at an acidity level as high as 2.5 mol/l, because the reduction potential of nitrate ion (NO3 − ) is much more negative than that of copper. The deposition of copper is also highly selective in the presence of lead because the electrodeposition potential of lead is much more negative than that of copper. High purity of the copper deposit can be achieved. After the recovery of tin and copper, lead can be electrodeposited onto stationary stainless steel electrodes. However, the current efficiency to electrodeposit lead is much lower because the powdered lead deposit tends to fall off from the electrode and redissolve in the solution. One option would be to separate the deposit from the substrate continuously. There is also a drawback in the chemical recycling of metals from the waste tin stripping solution because Pb(II) tends to form an oxide deposit on the anode. Therefore, an ionic exchange membrane must be applied to separate the cathodic and anodic compartments when copper or lead is electrodeposited, which increases the costs. The principle of the process is shown in Fig. 5. As an alternative, Scott et al. (1997) studied a process that incorporates electrochemical recycling of copper and furnace recycling of tin and lead. In this procedure, tin and lead are precipitated from spent stripping solution as stannic oxide and lead sulphate. The precip-

Fig. 5. The principle of electrochemical recycling of tin, copper and lead (Scott et al., 1997).

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itation of tin and lead can be combined in one step or be done separately, depending on the final use of the precipitate. The precipitate can, for example, be sent to furnace refineries specialized in secondary metals. Scott et al. (1997) found that the precipitation of stannic oxide is enhanced by aeration, especially when the nitric acid concentration in the stripping solution is low. Lead can be precipitated as PbSO4 by adding an appropriate amount of H2 SO4 into the solution. After the precipitation, copper can be recovered by electrodeposition. In copper electrowinning, the use of an ionic-exchange membrane is not necessary because lead can be removed before copper recovery. This method is relatively simple and also economical, but needs to be integrated to a furnace refinery that accepts the formed precipitate. The problem with the above methods is that Scott et al. (1997) studied stripping wastes with lower nitric acid concentrations than those in commercial stripping solutions. Also, there were no additives in the stripping solution used by Scott et al. (1997) in their experiments. In order to arrange recycling in line with the described schemes, it would be necessary to change the commercial stripping solutions. However, it would be much more reasonable to develop a recycling process for spent commercial tin stripping solutions. RD Chemical Company (McKesson et al., 2001) also patented an electrolytic recovery method for treating tin stripping waste. The principle of the method is that tin is made soluble by adding a strongly caustic solution, such as sodium hydroxide, into the spent tin stripping solution. By adding the caustic solution, the insoluble stannic oxide is converted to soluble stannate form. In the reaction, the tin remains in the Sn4+ oxidation state. By adding complexing agent, the other metal ions can be kept in the solution, and their precipitation can be avoided. The complexing agent should complex with metal ions other than tin. A suitable complexing agent would be gluconate. The treated waste solution can be used in an electroplating system to recover metals. Plating speed and efficiency can be improved by heating the treated waste solution (70–80 ◦ C) during electroplating. After the electrolytic treatment and pH adjustment, the final solution can be generally disposed of by discharging it into most sanitary sewage systems. The method seems appealing since all the metals can be recovered. The disadvantage is that nitric acid cannot be recovered by this method. Also a large volume of chemicals is consumed. Electrowinning was not described in detail, but as pointed out earlier, it will not be simple. Amia Co. and Persee Chemical Co. (Chen et al., 2004) patented a regeneration methodbased partially on electrolytic treatment. A flow chart of the method is shown in Fig. 6. First, copper ions are electrolytically reduced to metallic copper at as low a temperature as possible without ice formation (−5 to 40 ◦ C). This will suppress the hydrogen and nitrate ions being reduced to hydrogen gas and nitrogen oxides. Tin is mostly present in spent tin stripping solutions in a Sn4+ form, but some Sn2+ ions might also be present. After copper recovery, Sn2+ and Pb2+ ions are electrolytically oxidized to Sn4+ and Pb4+ at a high temperature (about 100 ◦ C) to form solid tin and lead oxides and hydroxides. The temperature should be as high as possible, but yet such that no vigorous boiling of nitrogen oxides occurs. If there is lead present in the tin stripping waste and the efficiency of the oxidization of Pb2+ is poor, sulphate ions can be added to the solution during or after electrolytic oxidation to form lead sulphate precipitate. The formed precipitate can then be filtrated. The resulting filtrate can be used to prepare a fresh tin stripping solution. The separated tin and lead oxides

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Fig. 6. Flow chart of the electrolytic method to treat spent nitric acid-based tin stripping solution (Chen et al., 2004).

and hydroxides and optional lead sulphate can be dissolved in a strongly alkaline or acidic solution, from which metallic tin and lead can be recovered by electrolytic reduction at a high temperature (50–105 ◦ C). The advantage of the above method is that all the metals in the solution can be recovered. Also, a fresh tin stripping solution can be prepared from the filtrate. However, electrolytic reduction is difficult to carry out directly on spent stripping solutions in such a way that hydrogen gas and nitrogen oxides will not be generated. Also, if the tin and lead oxide and hydroxide precipitate are dissolved in hydrochloric acid, it is difficult to prevent the formation of chlorine gas in electrolytic recovery. Furthermore, the big temperature elevations and the need for refrigeration consume a lot of energy. 3.2. Regeneration based on solvent extraction Lee et al. (2003) studied the regeneration of nitric acid-based tin stripping waste based partially on solvent extraction. Besides the solvent extraction, they also applied stripping, electrowinning, precipitation and cementation in the recovery process. In their experiments, Lee et al. (2003) used a synthetic solution chemically similar to the actual waste. First, nitric acid can be selectively extracted by tri-n-butyl phosphate (TBP) dissolved in kerosene. TBP is known to have selectivity for nitric acid over metal ions. Ninety-five percent extraction of nitric acid was possible by 50% TBP in five counter-current stages at a threefold volume of the organic phase compared to the aqueous phase. After extraction, by using distilled water as a stripping agent, most of the nitric acid could be stripped from the loaded organic phase. After nitric acid recovery, copper can be recovered by electrowinning. Electrowinning should be performed after nitric acid recovery because it is impossible to electrodeposit metal ions directly from spent stripping solutions due to their high acidity. Very pure copper metal can be selectively reduced at the cathode, whereas electrowinning of iron, tin and lead is difficult from the thermodynamic point of view because their reduction potentials are lower than the reduction potential of hydrogen ion. According to Lee et al. (2003), after copper has been recovered, the pH of the solution can be adjusted to 1.5

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with Pb(OH)2 , whereupon most tin ions will be precipitated as Sn(OH)2 . In order to prevent co-precipitation of iron, it is important to control the solution pH. The tin precipitate can be separated by filtration. Lead can be recovered from the filtrate by cementation, accomplished by adding iron powder into the solution. Because of the oxidizing nature of nitric acid, cementation of metal ions is very difficult to achieve from the nitric acid solution. In order to suppress the oxidizing properties of nitric acid, sodium pyrosulphate (Na2 S2 O5 ) can be added into the solution, and a reduction reaction of nitric acid to nitrogen dioxide will occur. The solution pH should also be adjusted to 2 by sodium hydroxide because the reduction potential of hydrogen ion is higher than that of lead ion, which means that the cementation percentage of lead increases as the concentration of hydrogen ion decreases. According to Lee et al. (2003), lead can be recovered by this method with 99% purity. The method has major advantages because all the metals and also nitric acid can be recovered. The disadvantage is the high chemical consumption especially in the solvent extraction stage. Additional chemicals are also needed in the precipitation and cementation stages. These reasons might lower the profitability expectations of the recovery process. 3.3. Sustainable technologies for the regeneration of tin stripping waste Kerr (2004) discusses in his article ‘sustainable technologies for the regeneration of acidic tin stripping solutions used in PCB fabrication’ fully sustainable regeneration of tin stripping waste by using ion exchange membrane technology in combination with other techniques. The process can be divided into three operations: (1) acid reclamation, (2) metal recovery (electrowinning) and (3) filtration. Acid recovery would be performed by diffusion dialysis. Diffusion dialysis is a process that requires a two-compartment cell. One compartment would contain water and the other acidic tin stripping waste. The compartments should be separated by an ion exchange membrane, which blocks the transport of metal ions while allowing the passage of hydrogen and nitrate ions, as shown in Fig. 7. Diffusion dialysis is based on a concentration gradient between the two compartments, which makes the overall efficiency of the process directly dependent on the surface area of the ion exchange membrane. On an industrial scale, several two-cell compartments are stacked to enable large volumes of solution to be treated. One major advantage of this process is that energy is required only to drive the circulatory pump. The recovered acid could be reused in tin stripping after the addition of small quantities of additives into the solution to optimize the performance of the new tin stripping chemistry. Once the acid has been recovered, Kerr (2004) suggests membrane filtration for the removal of stannic oxide. Membrane filtration is the separation of components of a pressurized fluid by a polymer or inorganic membrane. The class of the filter is defined by its pore size. Since stannic oxide particles are very fine (20 nm–1 ␮m), ultrafiltration (UF, pore size 20–50 nm) and microfiltration (MF, pore size 1 ␮m) would be suitable for removing tin oxide. It is known that filters are easily blocked when filtering solutions containing stannic oxide particles are used. Therefore, the filtering requires some modifications, which could be achieved by using cross-flow filtration (Kerr, 2004). That would prevent the build-up of particles at the pore site. The separated stannic oxide can be sold, which would make it

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Fig. 7. A two-compartment diffusion dialysis cell (Kerr, 2004).

an important marketable commodity rather than a waste product. Stannic oxide has several applications. It is used, for example, for polishing cabochons, in the manufacturing of lead crystal glass and as an opacifier in ceramic glazes. To make the process fully sustainable, the third step would be to recover the metals other than tin dissolved in the stripper. This could be done by electrowinning. The copper electrowinning technology is well established. The removal of ferrous/ferric ions by electrowinning is technically possible, but unlikely because the level of iron is very low in a waste tin-strip solution (Kerr and Coultard, 2004). However, the above technology is still relatively expensive, but in the future, the rising cost of landfills and the increasingly stringent legislation may force PCB manufacturers to adopt regeneration technologies out of economic necessity. 3.4. Summary of the methods for the regeneration of nitric acid-based tin stripping waste Similarly to the regeneration of cupric chloride etchant waste, the advantages and disadvantages of the methods applied to spent tin stripping solution are summarized in a tabular form. The advantages and disadvantages are shown in Table 2. With regard to tin stripping, it seems even more obvious than in the case of cupric chloride, that there is a need to develop an efficient regeneration process.

Regeneration method

Advantages

Electrolytic regeneration

+ Recovery of metallic tin, copper and lead

Method introduced by Scott et al. (1997)

Method patented by RD Chemical Company (McKesson et al., 2001)

Recovery of metallic tin and copper

Method patented by Amia Co. and Persee Chemical Co. (Chen et al., 2004)

Recovery of metallic tin, copper and lead

Disadvantages + Alternatively precipitating tin as SnO2 and lead as PbSO4 , which are sent to furnace refineries. Copper can be recovered as metallic copper

The filtrate from the filtration step can be used to prepare fresh tin stripping solution

+ The tin stripping solution can be regenerated for reuse

− Electrowinning cannot be done directly from used tin stripping solutions because tin is not truly soluble in the solution. Before electrowinning, some further treatment of the spent tin stripper must be done

− Filtered tin oxide precipitate is solubilised in hyrdrochloric acid. However, filtration is difficult due to the very small particle size of tin oxide

Sodium hydroxide is consumed when solubilising the tin. Complexing agents are also consumed when keeping the other metal ions in a soluble state

Tin stripping solution in itself cannot be regenerated; after metal recovery, the bath is pH-adjusted and can be discharged into most effluent systems

Copper ions are electrolytically reduced directly from used tin stripping solution, which is very difficult

Electrowinning of tin and lead is difficult

− Electrowinning is difficult from the solution containing hydrochloric acid in such a way that chlorine gas is not generated

High energy consumption

− High energy consumption in tin electrowinning and difficulties in lead electrowinning

− Can be applied only to more dilute tin stripping solutions than commercial solutions, which makes the method impractical

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Table 2 Summary of the advantages and disadvantages of the methods for the regeneration of nitric acid-based tin stripping waste

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Regeneration method Regeneration based on solvent extraction

Regeneration based on sustainable technologies

Advantages

Disadvantages

+ Copper can be recovered electrolytically as metallic copper, tin precipitated and lead recovered by cementation as metallic lead powder

+

+

Copper can be recovered as metallic copper and tin as metallic tin or as tin oxide precipitate

Acid recovery

Avoiding waste formation

− Abundant chemical consumption in, for example, the solvent extraction and cementation steps

− Low profitability expectations

−

Acid recovery by diffusion dialysis and membrane filtration of tin oxide are too expensive, while neutralization is still possible

Membrane fouling

−

−

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Table 2 (Continued )

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4. Discussion Since the metal concentration is relatively high in spent cupric chloride etching solutions and tin stripping solutions, electrolytic regeneration seems a reasonable alternative for the regeneration of those wastes. For cupric chloride etchant waste, there are inventions that utilize a regeneration process which reverses reaction (1) in such a way that the evolution of chlorine and hydrogen gas is avoided. Those methods can be operated in a closed-loop cycle with the etching process. However, the methods suffer from several drawbacks. Since electrolytic recovery from cupric chloride etchant waste solutions containing chloride ions by avoiding chlorine gas evolution is difficult, there are also electrolytic inventions that involve recovery of copper and use of the generated Cl2 in the chemical regeneration of the etchant. These methods, however, are impractical because the use of Cl2 is forbidden in many areas. For the regeneration of nitric acid-based tin stripping solutions, there are also inventions that are based on electrolytic regeneration. However, some further treatment of the spent tin stripper must be done before electrowinning because tin is not truly soluble in nitric acid-based tin stripping solutions. In addition, electrowinning of tin and lead has proven to be complicated. For the regeneration of cupric chloride etchant waste, there are also some methods based on cementation. Copper can be recovered as metallic copper, whereas metal chloride other than copper is formed as a by-product. However, metal chloride by-products do not necessarily have a sufficient market. Some methods based on precipitation of copper oxide have also been developed. A lot of caustic solution is consumed in the neutralization reaction. Additionally, a lot of saline solution that must be discharged into waste water treatment is formed. These reasons make the methods uneconomic. There are also inventions based on solvent extraction for the regeneration of both acidic cupric chloride etching and nitric acid-based tin stripping solutions. Those inventions involve usually a quite complicated sequence of steps involving repeated extraction and stripping stages. They are also very chemical consuming. For spent tin stripping solutions, also ion exchange membrane technology in combination with other techniques has been studied. For this purpose, however, ion membrane technology is still too expensive.

5. Conclusions Both spent cupric chloride and tin stripping solutions are classified as hazardous wastes. They are usually shipped off-site for reclamation, which is becoming increasingly more expensive. These wastes are a significant problem for PCB industry. Usually, spent tin stripping solutions, and often also spent cupric chloride solutions, are treated by neutralization, resulting in sludge which is discarded in special landfills. There have been several attempts to develop technically and economically feasible processes for the regeneration of spent cupric chloride and tin stripping solutions. The developed methods are based on such techniques as electrowinning, cementation, solvent extraction, precipitation and membrane technology. Some of the methods suffer from technical difficulties and complexity. Some, on the other hand, are technically very appealing but not so economical, while neutralization is still possible. In the future, however, the increasingly stringent legislation and rising land-

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fill costs might force PCB industry to adopt such technologies out of economic necessity. Meanwhile, it would be very important to develop technically and economically satisfactory solutions for the treatment of spent acidic cupric chloride and tin stripping solutions.

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