WO2008139021A1

WO2008139021A1 - Metthod of regeneration method of acid cupper(ii)chloride etching waste

Info

Publication number: WO2008139021A1
Application number: PCT/FI2007/050282
Authority: WO
Inventors: Juha Tanskanen; Toivo Kuokkanen; Tiina Keskitalo; Pekka Piltonen
Original assignee: Pp Recycling Ltd.
Priority date: 2007-05-16
Filing date: 2007-05-16
Publication date: 2008-11-20

Abstract

The present invention relates to regeneration of acidic copper(ll)chloride etching wastes (CuCI2) formed in printed circuit board industry. The regeneration process yields high quality liquid hydrochloric acid and pure copper sulfate or copper sulfate containing sodium sulfate as impurity. In the present method, acidic CuCI2 etching waste together with sulfuric acid is fed to a distillation column, thereby yielding, from the top of the column, concentrated, liquid, 30-35% hydrochloric acid and, as secondary flow, more dilute, liquid 20% sulfuric acid having azeotropic composition. Metal sulfates are crystallized from the metal-containing sulfuric acid flow obtained from the bottom of the column. Sulfuric acid flow, from which metal sulfates have been separated, may be recycled back to the column as sulfuric acid feed.

Description

Method of regeneration for acidic copper(ll)chloride etching waste

Description

The present invention relates to regeneration of acidic copper(ll)ch!oride etching waste formed in printed circuit board industry.

One of the most important steps of manufacturing printed circuit boards is etching the copper outside the printed circuit pattern, i.e. oxidizing and dissolving it to a liquid solvent. There are different kinds of etching liquids and the choice of the liquid is dependent on various factors, i.e. the structure of the printed circuit board and the availability of etching liquid providers. Several methods of etching have disappeared over the years and been replaced by new methods of etching. Currently etching with copper(ll)chloride is a general method for etching. Cop- per(ll)chloride etching solution is the most preferred etching solution when an organic etching serves as etching resist on the printed circuit pattern, i.e. prevents etching. Organic etching resists are widely used in internal layers of printed circuit boards. CuCb etching solution is an acidic solution containing hydrochloric acid as acid. Usage of hydrochloric acid provides the solution with chloride ions that are required for the copper to maintain soluble instead of precipitating as hydroxide. Etching as such is a redox reaction, wherein hydrochloric acid is not actually required. Additionally, if the metal is covered by an oxide layer, the metal will not be oxidized unless the oxide layer is first dissolved by an acid or by a base. Etching with copper(ll)chloride is not applicable when the etching resist on the printed circuit pattern is a metallic resist, for the acidic solution of CuCb would corrode it, too. A metallic resist serves as etching resist when manufacturing outer layers of printed circuit boards.

When etching with CuCb, bivalent copper ion oxidizes metallic copper, thereby forming monovalent copper chloride according to the following reaction formula:

Cu⁰ + Cu⁺²CI₂ → 2Cu⁺¹CI (1)

During the etching the copper accumulates to the solution, whereas the number of the ions effecting the etching of copper, Cu²⁺ ions, decreases. If etching rate is to be maintained Cu⁺ has to be oxidized back to the form of Cu²⁺ in order to etch more metallic copper. This may be carried out by chemical regeneration, which enables continuous etching processing. In chemistry the simplest way to oxidize Cu⁺ ion back to Cu²⁺ ion is to add gaseous chlorine (Cb) to the etching process, thereby effecting the following reaction: 2Cu⁺¹C! + Cl₂ → 2Cu⁺²CI₂ (2)

However, several practical problems are related to the use of gaseous chlorine. In addition to security issues, usage of gaseous chlorine causes environmental problems, which has been the reason for tightening legislation, and the usage of gase- ous CI₂ has been prohibited in several areas. Generally, oxidizing chemicals such as hydrogen peroxide (H₂O₂) or sodium chlorate (NaCIOa) are used for chemically regenerating CuCI₂ etching solutions, whereby Cu⁺ ions may be oxidized back to Cu²⁺ ions as presented by reaction formulas (3) and (4). If sodium chlorate is used in chemical regeneration, also sodium chloride is formed as side product of the re- generation reaction, the sodium chloride thus being also an impurity in the etching solution used. This brings about additional challenges in handling of etching waste.

2Cu⁺¹CI + H₂O₂ + 2HCI -> 2Cu⁺²CI₂ + 2H₂O (3)

6Cu⁺¹CI + NaCIO₃ + 6HCI ->6Cu⁺²CI₂ + NaCI + 3H₂O (4)

In order to allow continuous processing, in addition to chemical regeneration, the accumulating copper has to be removed from the etching process. The etching solution is analyzed continuously and the concentration of copper is kept constant by removing etching solution and replacing the removed portion with new etching solution. In general, excess etching solution has to be disposed as waste. Etching solution is classified as hazardous waste. Generally, the used etching waste is stored in containers and, finally, transported outside the plant for treatment. Used solutions are frequently treated by neutralizing, thereby forming metal-containing slurry that is classified as hazardous waste. As a method of waste treatment neutralization is not economical since neutralization uses base and the valuable com- ponents contained in the waste can not be recovered by it. Therefore treatment of CuCI₂ etching solution is a significant problem in printed circuit board industry.

The used acidic CuCI₂ etching solution formed in printed circuit board industry mainly contains copper(ll)chloride, hydrochloric acid and water, and sodium, in case sodium chlorate has been used as oxidizing agent in the etching process. If sodium chlorate is used as oxidizing agent in the etching process, the copper concentration of the etching solution may be maintained on higher level than in case hydrogen peroxide is used as oxidizing agent. If hydrogen peroxide has been used as oxidizing agent, copper contents of the etching waste is about 100 g/l and, if sodium chlorate has been used as oxidizing agent, copper contents of the etching waste is about 200 g/l. The copper in the etching waste is found mainly as bivalent copper (Cu²⁺); the etching waste barely contains Cu⁺ ions. If sodium chlorate has been used as an oxidizing agent in the etching process, sodium (Na⁺) contents of the etching waste is about 50 g/l. If hydrogen peroxide has been used in the chemical regeneration of the etching process, the contents of free acids in the etching waste is higher than if sodium chlorate has been the oxidizing agent, because in that case the amount of the free acid may be maintained close to zero.

Several methods based on i.a., electrolysis, cementation, liquid-liquid extraction and precipitation, have been developed and patented for treatment of cop- per(ll)chloride etching waste. Since the concentration of copper in etching waste is relatively high, electrolytic regeneration would seem a logical alternative. Several electrolytic methods of regeneration for acidic CuCI₂ etching waste have been developed. If the electrolysis is carried out directly from the etching acid solution, it is difficult to avoid formation of gaseous chlorine. The formed gaseous chlorine should be recovered and, for example, it could be used in chemical regeneration of the etching process. One such method has been described in U.S. patent 5,393,387, published February 28, 1995. However, the use of gaseous chlorine has been prohibited in several areas, which makes such electrolytic methods somewhat impractical.

On the other hand, several electrolytic methods of regeneration have also been developed, whereby the etching reaction (1) is electrochemically converted in order to avoid formation of gaseous chlorine on the anode and formation of gaseous hydrogen on the cathode. In this types of methods of regeneration Cu⁺ ions are oxidized to Cu²⁺ ions on the anode and copper ions are reduced to metallic copper on the cathode. Formation of gaseous chlorine may be avoided by arranging current density that is much higher on the cathode than on the anode. In case the current densities were the same, the monovalent copper would be oxidized into bivalent copper on the anode and bivalent copper would be reduced into monovalent copper on the cathode, without any kind of regeneration as a net reaction. Appar- ently the first attempts to develop commercial electrolytic acidic methods of regeneration for copper chloride etching wastes are the methods described in U.S. patent 2,964,453, published December 13, 1960, and in U.S. patent 3,784,455, published January 8, 1974. In said methods the current density on the cathode has been arranged to be higher by using smaller cathodes than anodes. However, us- ing anodes and cathodes of different sizes leads to uneven distribution of current, which in turn leads to uneven distribution of voltage, especially on the anode. Hence, in practice it is impossible to avoid the undesired formation of Cl₂. The electrolytic method of regeneration described in U.S. patent 4,468,305, published August 28, 1984, comprises separate flow cycles for anolyte and catholyte, in which the electrolytes have different compositions. The anolyte comprises about 10 times more copper than the catholyte. Thus the usage of anodes and cathodes of different sizes may be avoided, since the flow of limit of reduction of Cu²⁺ ions to Cu⁺ ions can be exceeded because of differences in concentrations. However, the above methods have certain practical problems. For example, the high operational voltages increase waste heat, which requires using heat exchangers, thereby in- creasing costs. U.S. patent 5,421,966, published June 6, 1995, describes the current density on the cathode arranged higher by using porous flow-through anodes instead of flow-by anodes, thereby increasing the anode surface area internally. As the cathodes still remain planar, the cross sectional surface areas of the electrodes may be maintained the same, thus allowing more accurate control of cur- rent or voltage. An electrolytic method of regeneration has also been described in U.S. patent 5,705,048, published January 6, 1998. In electrolytic regeneration, wherein the etching reaction is converted electrochemically, the control of the process has to be very accurate, especially in order to avoid formation of gaseous chlorine on the anode. Additionally all the electrolytic methods described above have been designed to operate in closed cycle with the etching process. Thus chemical regeneration in the etching process itself is aimed to be avoided. Currently oxidizing chemicals are almost always used in copper(ll)chloride etching in order to maintain etching efficiency for prolonged period of time. Thus the above described electrolytic methods of regeneration become even more complicated.

Methods of regeneration for acidic CuCI₂ etching waste based on cementation have also been developed. Cementation is an electro-chemical exchange reaction, wherein the more noble metal in the solution is precipitated and the less noble metal in solid state is dissolved. In cementation based methods of regeneration for CuCI₂ etching waste the copper ions in the etching waste are exchanged to other metal ions, whereby, in addition to metallic copper, valuable metal chloride containing other metals than copper is obtained. For example, in JP patent 63014883, published January 22, 1988, copper is replaced with aluminum. U.S. patent 5,279,641 , published January 18, 1994, suggests several other alternatives for the substituting metal. The metal chloride obtained as side product finds different uses in industry. The solution may be employed either as such for a specified purpose or, alternatively, it may be concentrated by evaporating from water. Metal chloride can also be crystallized. However, the market is not necessarily big enough; for example demand for ferric chloride and aluminum chloride, that are being used as flocculants in waste water treatment, will probably not grow anymore in the future since organic polymers have become more common in waste water treatment. Additionally, in regeneration methods based on cementation, it has not been taken into account that the etching waste may also have relatively high contents of sodium in case NaCIO₃ has been used as oxidizing agent in the etching process. Since sodium is a base metal, it does not oxidize but remains as an impurity in the chloride solution obtained. This would further decrease possibilities to use the chloride solution.

Recovery of copper by liquid-liquid extraction has developed remarkably and is presently used in various applications of use. For regeneration of acidic CuCb etching waste, also regeneration methods based on liquid-liquid extraction have been developed, whereby copper is finally recovered either by precipitating it as copper sulfate or by electrolyzing it as metallic copper. For example, in U.S. patent 4,272,492, published June 9, 1981 , and EP patent 0,301 ,783, published February 1 , 1989, liquid-liquid extraction has been applied to regeneration of acidic copper chloride etching waste. Especially the former patent describes quite complex combination of steps of repeated extraction and stripping in different conditions, which makes the method economically unfeasible. U.S. patent 6,375,713, published April 23, 2002, describes a method which intends to use copper chloride etching waste as additional raw material for hydrometallic production of metallic copper in liquid- liquid extraction phase. Thus, the yield of copper is increased without need to increase excavation of ore. To the already existing process, only one or more steps of washing with water before electrolysis would have to be added in order to inhibit or in order to minimize passage of chlorides into the electrolysis. The primary objective of the method, however, is not to find methods of regeneration for etching waste, which makes it impossible for the method to solve the problem of regeneration of acidic copper(ll)chloride etching wastes in larger scale.

Methods of regeneration for CuCI₂ etching waste based on precipitation of copper oxide have also been developed. For example, in U.S. patent 5,560,838, published October 1 , 1996, and in U.S. patent 6,649,131 , published November 18, 2003, a method of regeneration based on neutralization has been described, thereby obtaining copper oxide and water containing sodium chloride, which may be poured into the drain. In said methods only copper may be regenerated as copper oxide which does not necessarily have enough market. The acid in the etching waste can not be regenerated - instead, high amounts of water containing sodium chloride is formed in neutralization. The methods also use high amounts of base, which makes said methods economically unappealing. However, the positive side of this kind of methods of regeneration is that the sodium chloride that is possibly present as an impurity in the etching waste, is not detrimental since so- dium chloride is formed in the neutralization process in any case.

The method according to the present invention differs from the above methods and provides benefits that are obvious from the below description of the present invention. It is an objective of the present invention to develop a technically and economically feasible method for regeneration of acidic CuCI₂ etching wastes formed in printed circuit board industry. Especially, it is an objective to recover high quality liquid hydrochloric acid and non-chlorous copper sulfate precipitate from the etching waste with high efficiency of the process. Also sodium-containing etching waste, mainly in the form of sodium chloride, i.e., in case NaCIO₃ is used as oxidizing agent in chemical regeneration, has to be suitable as acidic CuCIa etching waste to be treated. According to the present invention these objectives may be met by a method of regeneration based on reactive distillation wherein, in addition to acidic CuCI₂ etching waste, also sulfuric acid (H₂SO₄) is fed into the distillation column. H₂SO₄ donates both its hydrogen ions to the Cl^" ions present in the etching waste, thereby forming hydrochloric acid that may be distilled in addi- tion to the free hydrochloric acid and water present in the CuCI₂ etching waste. The top of the distillation column gives concentrated, liquid, 30-35% hydrochloric acid, whereby the hydrochloric acid is also condensed from steam to liquid. Additionally, more dilute 20% hydrochloric acid comprised of hydrochloric acid and water and having maximal azeotropic composition is taken as side stream from the distillation column. Side stream allows obtaining concentrated hydrochloric acid as the contents of water of the feed varies. Water is added to the column to replace reflux flow in case the feed is so rich in chloride ions that the strength of top product is above condensation limit. Since demand for 20 % hydrochloric acid is much smaller than demand for concentrated, 30-35% hydrochloric acid, it is desired to minimize the amount of secondary flow. If there are two types of acidic CuCI₂ etching wastes to be processed, depending on the oxidizing agents used in the etching process, the amount of secondary flow to be taken in distillation may be minimized by adjusting the mixing rates of the fed etching wastes. The proportions of the feeds may be adjusted for the CuCI₂ etching waste feed to contain chloride ions just enough for 30-35% hydrochloric acid to be nearly the only product formed, whereby there will be no need for adding water to replace the reflux flow, and, in practice, no secondary flow will be formed. The content of Cl^" ions of CuCI₂ etching waste is generally higher in the etching waste when NaCIO₃ has been used as oxidizing agent in the chemical regeneration of the etching process. In case the processed etching waste contains Cl^" ions in such an amount that during the distillation also plenty of secondary flow will be formed, the amount of Cl^" ions in the feed and thus the amount of 30-35% hydrochloric acid formed may be increased by adding metal chlorides into the etching waste, most preferably sodium chloride or cop- per(ll)chloride. In distillation, from the bottom of the column, a metal-containing stream of sulfuric acid is obtained, from which the metals may be crystallized as sulfates in one or two steps. Thus, the method of regeneration according to the present invention also provides reactive continuous distillation and crystallization, from which, in addition to concentrated and dilute hydrochloric acid, a precipitate of copper/sodium sulfate is obtained as commercial product. If desired, also metallic copper may be electrolyzed in a further process from the low chloride ion containing precipitate obtained. The sulfuric acid stream with very low metal content, separated from the sulfate precipitates after crystallization, may be recycled into the process.

More precisely, the characterization of the method of regeneration for acidic cop- per(ll)chloride etching waste will become apparent from the claims.

The method according to the present invention will be described in detail with ref- erence to the accompanying drawings. Figure 1 shows flow chart of a method of regeneration according to the present invention, wherein the precipitation of metallic sulfate after the reactive distillation is performed in one step. Figure 2 shows the principle of the two-step crystallization that may be preformed instead of one-step crystallization after the reactive distillation.

The principles of the reactive distillation related to the present method of regeneration are apparent from Figure 1. The used copper chloride etching waste 101 is fed into the distillation column 100 together with sulfuric acid 102. Hydrogen ions of the sulfuric acid together with chloride ions of copper(ll)chloride and sodium chloride in the etching waste form hydrochloric acid which may be distillated to- gether with hydrochloric acid and water already present in the CuCI₂ etching waste. Reaction formula (5) shows the reaction related to the process in case there is no sodium chloride present in the etching waste. If the etching waste also contains sodium chloride, the reaction formula related to the process is similar to formula (6). The basis for the method of regeneration has been keeping the tem- perature on the bottom of the distillation column high enough for practically no chloride ions to remain on the bottom in order to avoid residual chlorides in copper or/and sodium sulfate precipitates; if metallic copper is to be electrolyzed as the final product, no Cl^" ions are allowed in precipitates. The contents of sulfuric acid on the bottom is dependent on the conditions of the column since metallic sulfuric acid at its boiling point is present on the bottom. High temperature and corroding, acidic metal-containing solutions set high demands on the material of the distillation column. The material of the distillation column is preferably boro silicate glass. The bottom temperature needed for distillation is relatively high but can be somewhat lowered by performing the distillation under reduced pressure. Preferably, the bottom temperature is 160-200⁰C and the pressure of distillation is 200- 1000 mbar. Thus, practically no chloride ions will remain on the bottom and the investment costs for, for example, the distillation column, pumps and heat exchangers, will be reasonable.

CuCI₂ + nHCI + H₂SO₄ → CuSO₄ + (n+2)HCI (5)

CuCI₂ + 2NaCI + nHCI + 2H₂SO₄ → CuSO₄ + Na₂SO₄ + (n+4)HCI (6)

The etching waste flow 101 and sulfuric acid flow 102 in the distillation column are fed, most preferably, to the same tray, thereby sparing the cost of an extra feeding means. The location of sulfuric acid inlet between the tray and the inlet does not affect purity of the products. Alternatively, the sulfuric acid flow 102 can therefore be fed also to a lower tray or directly to the bottom well. The top of the distillation column gives concentrated, liquid, 30-35%, most preferably 33% hydrochloric acid 104 suitable for commercial use, whereby hydrochloric acid is also condensed from steam to liquid. As water contents of the feed varies, obtaining of concentrated hydrochloric acid is allowed by 20% hydrochloric acid 105 having maximal azeotropic composition, taken as side stream. Steam-liquid equilibrium curves of water and hydrochloric acid differ markedly, and therefore secondary flow reaches almost immediately the azeotropic concentration that cannot be overridden by increasing the number of trays or reflux ratio, and thus adjusting the strength of the top flow 104 is based merely on varying the amount of azeotropic flow 105. Total of top flow and azeotropic flow is determined by conditions in the column so that the temperature on the bottom is the desired temperature. Azeotropic flow 105 is taken above the feed since it is not desired for any metals from the feed flows to pass into it. Top flow and the secondary flow are also not allowed to contain sulfuric acid, which may be achieved without special attention since the difference between boiling temperatures of sulfuric acid and hydrochloric acid is remarkable. If concentration of chloride ions in the feed is high enough for the strength of top product to be above condensation limit, water 103 is added to replace reflux flow thereby avoiding separate step of absorbing gaseous hydrochloric acid. The water added does not really increase consumption of energy since it replaces the flow returned from the distillate. The height of distillation column is not required to be extensive, since differences of evaporability of the components in the present method are high. The diameter of the column is dependent on the amount of the treated CuCI₂ etching waste.

The flow rate of sulfuric acid feed needed is dependent on solubility of copper under the conditions on the bottom of the column. It is not desired for the copper sulfate to considerably form precipitate in the bottom well of the distillation column and, therefore, the flow rate of sulfuric acid is adjusted according to the amount of copper in the feed for the concentration of copper on the bottom of the column not to considerably exceed the solubility limit of copper under said conditions. On the other hand, it is not desired to feed sulfuric acid in excess for it is desired to operate near the limit of solubility in order to facilitate performing crystallization process after distillation simply by cooling the liquid. Solubility of sodium in high temperatures in concentrated sulfuric acid is much higher than solubility of copper and, therefore, the limiting factor is solubility of copper.

From the bottom flow 106 of the column, i.e., from the metal-containing sulfuric acid flow, formation of precipitate, i.e. crystallization step, may be carried out either in one or in two steps. The principle of crystallization in one step is shown in figure 1 and the flow chart for alternative two-step crystallization is shown in figure 2. The single step of crystallization 107 in one-step crystallization or the latter step of crystallization 115 in two-step crystallization is carried out by cooling the bottom flow 106 in the column or the flow 114 of two-step crystallization to lowest possible temperature that may be reached by cooling water in order to minimum possible amount of metals to remain in sulfuric acid flow 109 or 117 to be separated. Separation of formed precipitate is carried out by any known method for solid-liquid separation, such as, for example, by ceηtrifugation or by filtration. Concentrations of copper and sodium in concentrated sulfuric acid flow at room temperature are minimum (1-4 g/l), and therefore sulfuric acid flow 109/117 after crystallization step(s) may be recycled to the distillation column. The minimum concentrations of metals in sulfuric acid flow 109/117 only have some effect in increasing the amount of fresh sulfuric acid flow 119 to be added. Thus, the sulfuric acid used in the process is replaced by fresh, concentrated, most preferably 93.5% sulfuric acid 117 in order to avoid feeding the column water that would unnecessarily decrease the proportion of top product, i.e. concentrated sulfuric acid, and increase con- sumption of energy. Amount of impurities accumulated in sulfuric acid cycle may be controlled by removing small proportion 110 from the flow 109/117.

If precipitation is carried out in one step, as it is shown in figure 1 , the formed precipitate 108 contains both copper sulfate and sodium sulfate in case the etching waste contains also sodium. However, the precipitate contains much more copper sulfate; the proportion of metal sulfates is dependent on the amount of copper and sodium in the etching waste. If precipitation is carried out in two steps, as it is shown in figure 2, the precipitate 113 formed in first crystallization step 112 is purely copper sulfate and the precipitate 116 formed in second crystallization step 115 is a mixture of copper sulfate and sodium sulfate. In the first step of two-step crystallization, the bottom flow of the column is cooled most preferably 60-80⁰C whereby sodium will still remain in soluble form in the concentrated sulfuric acid and thus the formed precipitate will be pure copper sulfate. Most preferably, separation of the precipitate is carried out at 80⁰C since the limit of solubility of copper is not considerably higher at 80⁰C than the limit of solubility thereof at 60⁰C in concentrated sulfuric acid, whereby yield of copper sulfate will not considerably decrease if the formed precipitate is separated at 8O⁰C. In contrast, the limit of solubility of sodium in concentrated sulfuric acid still changes considerably between 60-80⁰C, and therefore it is certain that at 80⁰C sodium sulfate has not yet crystal- lized. Copper sulfate obtained in the first step of two-step crystallization is pure copper sulfate.

The method according to the present invention will be illustrated in more detail in the following example.

Example:

Pilot study for regeneration of acidic CuCI? etching waste with the method of the present invention

A batch of acidic copper(ll)chloride etching waste was subjected to experiment according to present invention in pilot scale. The acidic CuCI₂ etching waste used in this experiment contained 224 g/l copper, 49.52 g/l sodium and 312 g/l chlorine. Concentration of free acids in the etching waste was 0.005 mol/l and the density thereof in room temperature was 1418 g/l. In addition to copper and sodium, only few metals were present in the etching waste. The distillation column was a tray column manufactured of glass. The temperature on the bottom of the distillation column was maintained at 190°C since at said temperature almost all the chloride ions are distillated and practically no chloride ions remain on the bottom. The concentration of sulfuric acid fed into the distillation column was 76.2% since sulfuric acid of such composition boils at 19O⁰C. The proportions of the feeds, i.e. proportion of CuCb etching waste and proportion of 76.2% sulfuric acid, were calculated based on material balances assuming all the chloride ions and water in the etching waste to be distilled as liquid hydrochloric acid, and based on experiments on laboratory scale it was known that in 76.2% sulfuric acid at 190°C at atmospheric pressure the solubility of copper is of order 40 g/l. It would have been most preferable to feed 6.36-fold sulfuric acid in regard to CuCb etching waste for the copper sulfate not to crystallize already in the bottom well of the column. The solubility of sodium in said conditions is remarkable and, therefore, there will be no need to separately take care of crystallization of sodium sulfate on the bottom of the column. The distillation column was fed with CuCI₂ solution at app. 6 g/min. When the experiment had reached the state of continuous process, the bottom product was let to a vessel provided with a mixer. During the experiment the sulfuric acid feed was 5.24-fold in regard to the feed of CuCI₂ solution, i.e. slightly smaller than calculated. Copper sulfate in the bottom well remained soluble, but started crystallizing immediately when letting out the bottom product was started. The solution was let to cool to room temperature with stirring, i.e. crystallization was carried out in one step. The formed precipitate was filtered by vacuum filtration. The bottom filtrate contained 1.046 g/l of copper and 2.781 g/l of sodium in room temperature, i.e. very little of both. Sulfuric acid was not recycled in the pilot experiment, but it could have been recycled, thereby only slightly increasing the proportion of sulfuric acid feed to CuCI₂ etching waste feed based on the amount of copper in the circu- lating sulfuric acid.

Thirty-three-per-cent hydrochloric acid and secondary flow of azeotropic composition were not separately recovered in the experiment. However, the hydrochloric acid obtained was very pure. In the experiment, distillate was formed at rate of 5 g/min and bottom flow was obtained at 32.4 g/min. From the bottom flow, the fil- trate was obtained at 24.6 g/min and the filtrated precipitate at 7.8 g/min, but the precipitate contained more moisture than would be obtained, for example, by cen- trifuging in real life industrial process. Sulfate precipitate was dried at 400⁰C in oven in order to evaporate sulfuric acid and hygroscopic water. The dried precipitate contained 284 g/kg of copper and 119 g/kg of sodium. The precipitate con- tained only minimum amounts of other metals; for example 2.53 mg/kg of nickel, 0.05 mg/kg of lead, 0.65 mg/kg of arsenic and 0.71 mg/kg of chromium. The precipitate contained less than 0.24 mg/g of chloride ions, i.e., the content was very low. There was neither organics nor nitrogen present in the precipitates. The precipitate was very high quality mixture of copper sulfate and sodium sulfate.

Within the scope of the present invention, other solutions differing from the above mentioned can also be contemplated.

Claims

1. A method for regeneration of acidic copper(ll)chloride etching waste, characterized in that the method comprises the following steps:

- copper(ll)chloride etching waste together with sulfuric acid is fed into a distillation column of continuous process in order to distillate high quality liquid hydrochloric acid

- metallic sulfates are crystallized from the bottom flow of the distillation column by cooling the bottom flow

- from the precipitate, sulfuric acid containing some metals is sepa- rated and recycled as feed to the distillation column.

2. The method according to claim 1 , characterized in that the acidic copper chloride(ll) etching waste to be regenerated may also contain sodium (mainly as sodium chloride)

3. The method according to claim 1, characterized in that high quality, com- mercial grade, concentrated, liquid, 30-35% hydrochloric acid is obtained from the top of the distillation column, which is allowed by more dilute, liquid, high quality 20% hydrochloric acid of azeotropic composition taken as secondary flow as the concentration of water in the feed varies.

4. The method according to claim 3, characterized in that the azeotropic flow is taken above the feed in order to inhibit metals from feed flows passing into it and it is easier to take in liquid state.

5. The method according to claim 3, characterized in that water or dilute hydrochloric acid is fed to the top of the distillation column in order to replace the reflux flow from the distillate in case the chloride ion concentration of the CuCb etching waste feed is high enough for the strength of top product to be above condensation limit, thereby avoiding separate step of absorbing gaseous hydrochloric acid.

6. The method according to claim 3, characterized in that it is desired to minimize the amount of secondary flow taken in the distillation since 20 % hydrochloric acid finds less uses than concentrated, 30-35% hydrochloric acid obtained from the top.

7. The method according to claim 6, characterized in that the amount of secondary flow formed in the distillation may be minimized by adjusting mixing rate of the acidic CuCI₂ etching wastes of different compositions to be treated so that the CuCI₂ etching waste feed will contain Cl^" ions just enough for concentrated, 30- 35% hydrochloric acid to be nearly the only product formed in distillation, thereby avoiding the need of adding water to replace reflux flow and, in practice, no secondary flow will be formed.

8. The method according to claim 6, characterized in that in case the etching waste to be regenerated contains only few Cl^" ions and therefore also plenty of secondary flow will be formed, the amount of Cl^" ions in the feed, and thus the amount of top flow formed in the distillation, may be increased by adding metal chlorides into the etching waste to be treated.

9. The method according to claim 8, characterized in that the metal chloride to be added to the etching waste is most preferably sodium or copper(ll)chloride.

10. The method according to claim 1 , characterized in that the acidic cop- per(ll)chloride etching waste and the sulfuric acid stream are fed to the same tray.

11. The method according to claim 1 , characterized in that the sulfuric acid stream is fed to a tray below the tray of acidic copper(ll)chloride etching waste.

12. The method according to claim 1, characterized in that the metal sulfate pre- cipitate is not allowed to crystallize considerably in the distillation column.

13. The method according to claims 1 and 12, characterized in that the amount of sulfuric acid feed to be fed is dependent on content of copper in the feed flows so that the concentration of copper on the bottom of the column will not considerably exceed the solubility limit of copper in said conditions.

14. The method according to claim 1 , characterized in that the content of chloride ions in the commercial grade metal sulfate precipitate obtained as product is very low.

15. The method according to claim 1 , characterized in that the metal sulfate crystallized from the bottom flow is copper sulfate having sodium sulfate as impu- rity.

16. The method according to claim 1 , characterized in that the metal sulfate crystallized from the bottom flow is pure copper sulfate.

17. The method according to claim 14, characterized in that on the bottom of the distillation column the temperature is high, preferably from 160 to 200⁰C, and distillation is carried out at atmospheric pressure or under slightly reduced pressure, the pressure being preferably 200-1000 mbar.

18. The method according to claim 15 or 16, characterized in that the crystallization of metal sulfates is carried out in one step by cooling the metal-containing sulfuric acid flow as low as it is possible by means of cooling water.

19. The method according to claim 15 or 16, characterized in that the crystallization of metal sulfates is carried out in two steps so that pure copper sulfate precipi- tate is obtained in the first step of crystallization by separating the formed precipitate advantageously at 60-80⁰C and, in the second step, yielding mixture of copper sulfate and sodium sulfate by cooling the metal-containing sulfuric acid flow as low as it is possible by means of cooling water.

20. The method according to claim 1 , characterized in that the formed metal sul- fate precipitate is separated from the sulfuric acid flow by any known liquid-solid separation method.

21. The method according to claim 1 , characterized in that the impurities accumulated in sulfuric acid cycle may be controlled by removing small proportion of the recycled sulfuric acid flow.

22. The method according to claim 1, characterized in that the sulfuric acid consumed in the regeneration process is replaced by fresh, concentrated sulfuric acid.