ACID WASTEWATER TREATMENT

Abstract

The invention relates to a process for the selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation by neutralization as well as the use of an aqueous slurry of at least one calcium carbonate source for the selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation by neutralization.

Claims

1. Process for the selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation by neutralization, the process comprising the steps of: a) providing at least one calcium carbonate source having a weight median particle size d.sub.50 from 0.1 to 500.0 m in the form of a powder or an aqueous slurry, b) providing an aqueous solution having a pH value from 0.0 to 7.0, c) providing an acidic preparation comprising at least one conjugate base and/or at least one metal cation, d) contacting the aqueous solution of step b) with the at least one calcium carbonate source of step a) for obtaining an aqueous slurry in which at least a part of the at least one calcium carbonate source is dissolved in the water phase of the aqueous slurry as calcium hydrogen carbonate, e) contacting the acidic preparation of step c) with the aqueous slurry obtained in step d) for adjusting the pH of the resulting reaction mixture to a pH value being higher than the pH value of the acidic preparation of process step c) for precipitating the at least one conjugate base and/or the at least one metal cation from the acidic preparation as water insoluble salt/salts.

2. Process according to claim 1, wherein the at least one calcium carbonate source of process step a) is a natural ground calcium carbonate and/or precipitated calcium carbonate and/or surface-modified calcium carbonate, preferably natural ground calcium carbonate.

3. Process according to claim 2, wherein the source of natural ground calcium carbonate (GCC) is selected from marble, chalk, dolomite, limestone and mixtures thereof and/or the precipitated calcium carbonate (PCC) is selected from one or more of the aragonitic, vateritic and calcitic mineralogical crystal forms.

4. Process according to claim 1, wherein the at least one calcium carbonate source a) has a weight median particle size d.sub.50 from 0.1 to 150.0 m preferably from 0.1 to 100.0 m and more preferably from 0.5 to 60.0 m, and/or b) contains calcium carbonate in an amount of 90.0 wt.-%, preferably 95.0 wt.-%, and most preferably from 97.0 to 99.9 wt.-%, based on the total weight of the at least one calcium carbonate source.

5. Process according to claim 1, wherein the acidic preparation of process step c) a) is selected from industrial waste water, urban waste water, waste water or process water from breweries or other beverage industries, waste water or process water in the paper industry, battery industry or battery recycling industry, colour, paints or coatings industries, galvanizing industry, mining industry, agricultural waste water, leather industry waste water and leather tanning industry, and/or b) has a pH value being below the pH value of the aqueous slurry obtained in process step d), preferably from 1.0 to 7.0, more preferably from 0.0 to 5.0, and most preferably from 0.2 to 3.0.

6. Process according to claim 1, wherein the aqueous slurry obtained in process step d) a) has solids content of from 0.01 to 50.0 wt.-%, preferably from 0.1 to 40.0 wt.-% and most preferably from 0.2 to 30.0 wt.-%, based on the total weight of the aqueous slurry, and/or b) has a pH value from 3.0 to 9.0 and preferably from 5.0 to 8.0.

7. Process according to claim 1, wherein contacting step e) is carried out such that the obtained reaction mixture has a pH value being higher than the pH value of the acidic preparation of step c).

8. Process according to claim 1, wherein process step d) and step e) are carried out in separate but successive reactors.

9. Process according to claim 1, wherein the process further comprises process step f) of stepwise increasing the pH value of the supernatant of the reaction mixture obtained in process step e), preferably by adding the aqueous slurry obtained in process step d) and/or at least one water soluble base such as sodium hydroxide, potassium hydroxide, calcium hydroxide and the like in form of an aqueous solution or slurry and/or at least one reactive component, such as a flocculent, coagulent and the like, suitable for precipitating of a further at least one conjugate base and/or at least one metal cation from the supernatant of the reaction mixture as water insoluble salt/salts.

10. Process according to claim 9, wherein process step e) and process step f) are carried out in separate but successive reactors, like three separate and successive reactors, preferably process step e) is carried out in a separate first reactor and in an optional separate but successive second reactor and process step f) is carried out in each separate but successive reactor following the separate first reactor or, if present, the separate but successive second reactor.

11. Process according to claim 10, wherein the pH in the separate first reactor and in the optional separate but successive second reactor is adjusted to a pH value being higher than the pH value of the acidic preparation of process step c), and the pH in each separate but successive reactor following the separate first reactor or, if present, the separate but successive second reactor is adjusted to a pH value being higher than the pH value of the reaction mixture in a previous reactor.

12. Process according to claim 1, wherein contacting step d) is carried out in that the pH value of the aqueous solution provided in process step b) is adjusted to a targeted pH by a) the addition of the at least one calcium carbonate source provided in process step a), and/or b) the addition of the supernatant of the reaction mixture obtained in process step e) and/or process step f).

13. Process according to claim 1, wherein the process further comprises process step g) of separating the water insoluble salt/salts of the at least one conjugate base and/or the at least one metal cation from the reaction mixture obtained in process step e) and/or process step f).

14. Process according to claim 1, wherein the process is a continuous process, preferably a continuous process in which the supernatant of the reaction mixture obtained after separating the water insoluble salt/salts of the at least one conjugate base and/or the at least one metal cation from the reaction mixture is used as the aqueous solution of process step b).

15. Use of an aqueous slurry of at least one calcium carbonate source for the selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation by neutralization, wherein the at least one calcium carbonate source has a weight median particle size d.sub.50 from 0.1 to 500.0 m.

16. The use according to claim 15, wherein the aqueous slurry of at least one calcium carbonate source a) has solids content of from 0.01 to 50.0 wt.-%, preferably from 0.1 to 40.0 wt.-% and most preferably from 0.2 to 30.0 wt.-%, based on the total weight of the aqueous slurry, and/or b) has a pH value from 3.0 to 9.0 and preferably from 5.0 to 8.0.

17. The use according to claim 15, wherein the at least one calcium carbonate source a) is a natural ground calcium carbonate and/or precipitated calcium carbonate and/or surface-modified calcium carbonate, preferably natural ground calcium carbonate, and/or b) has a weight median particle size d.sub.50 from 0.1 to 150.0 m, preferably from 0.1 to 100.0 m and more preferably from 0.5 to 60.0 m, and/or c) contains calcium carbonate in an amount of 90.0 wt.-%, preferably 95.0 wt.-% and most preferably from 97.0 to 99.9 wt.-%, based on the total weight of the at least one calcium carbonate source.

Description

EXAMPLES

Measurement Methods

[0193] The following measurement methods are used to evaluate the parameters given in the examples and claims.

BET Specific Surface Area of a Material

[0194] The BET specific surface area was measured via the BET process according to ISO 9277 using nitrogen.

Particle Size Distribution (Mass % Particles with a Diameter <X) and Weight Median Diameter (d.sub.50) of a Particulate Material

[0195] Weight median grain diameter and grain diameter mass distribution of a particulate material were determined via the sedimentation process, i.e. an analysis of sedimentation behaviour in a gravitational field, or via laser diffraction, i.e. the particle size is determined by measuring the intensity of light scattered as a laser beam passes through a dispersed particulate sample. The measurement was made with a Sedigraph 5100 of Micromeritics Instrument Corporation or a Mastersizer 2000 of Malvern Instruments Ltd, United Kingdom, using the Fraunhofer light scattering model.

[0196] The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement via laser diffraction is carried out in an aqueous solution of 0.1 wt.-% Na.sub.4P.sub.2O.sub.7. The samples are dispersed using a high speed stirrer and supersonics.

pH Measurement

[0197] The pH of the aqueous samples can be measured by all types of pH measurement devices ensuring a measuring error of 0.05 at a pH between 0 and 2. The pH of the aqueous samples was measured by using a standard pH-meter at approximately 22 C. (1 C.).

Solids Content

[0198] The solids content was measured using a Moisture Analyzer of Mettler-Toledo HP43. The method and the instrument are known to the skilled person.

Conductivity

[0199] The conductivity can be measured by all types of conductivity measurement devices ensuring a measuring error of 5% between pH of 0 and 2. For example, the conductivity can be measured by using a Multimeter WTW 3420 or a Sonde Tetra Con 925.

Whiteness R457

[0200] The whiteness R457 was measured by using a spectrophotometer (Datacolor Elrepho 3000) equipped with Datacolor software in accordance with the ISO 2469 Standard. The measurement was carried out at a standard illumination of D65 and a standard observer of 10. Furthermore, BaSO.sub.4 for brightness standard DIN 5033 from Merck KGaA, Germany was used for calibration. Calibration was repeated every six hours and checked by measuring a reference sample.

Example 1

[0201] The following example illustrates the selective precipitation of defined conjugate bases and metal cations from an acidic preparation by neutralization by using the process of the instant invention on lab scale basis.

[0202] The trials were performed under room temperature, i.e. at a temperature of from 15 to 25 C. and ambient pressure.

[0203] 6 liters of acidic wastewater from Circuit Printed Boards production (CPB/PCB) with heavy metal contents (assembly of rinsing and acid waters from layout-exposing-etching-registration-contacting-galvanization) was used as acidic preparation.

[0204] The following Table 1 summarizes the different calcium carbonate sources and their properties used in the process for selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation by neutralization.

TABLE-US-00001 TABLE 1 calcium carbonate sources Calcium carbonate d.sub.50 CaCO.sub.3 HCl insoluble Samples.sup.[1] source [m] [wt.-%] [wt.-%] A Limestone 3.0 99.5 0.1 B Chalk 2.5 99.6 0.1 C Marble 5.5 98.0 2.0 .sup.[1]All calcium carbonate sources used in the present invention are commercially available from Omya International AG, Switzerland.

[0205] The calcium carbonate sources as outlined in Table 1 were provided in powder form. From the calcium carbonate sources and water aqueous calcium carbonate slurries of differing solids content were prepared.

[0206] The solids content and pH of the respective aqueous slurries were adapted to the required needs such as pumpability. For example, an aqueous calcium carbonate slurry of sample C was prepared having solids content of 35.0 wt.-%, based on the total weight of the aqueous slurry, a pH of 8.45 and conductivity of 320 mS/cm.

[0207] The dosage of the calcium carbonate slurries (slurry 1 comprising limestone or slurry 2 comprising marble) into the acidic preparation is based on reaction kinetics and thus the acidic wastewater used as acidic preparation was analyzed before carrying out the instant process by IC for anions and ICP-OES for cations. In the acidic wastewater used as acidic preparation, the acid content was basically generated by SO.sub.4.sup.2 ionsthe composition of the wastewater is shown in Table 2. Further specification data are shown in Table 3.

TABLE-US-00002 TABLE 2 Composition of wastewater [ppm] Cl.sup. SO.sub.4.sup.2 PO.sub.4.sup.3 NO.sub.3.sup. Al Cu Fe Ni Zn 1 100 8 100 40 10 1 950 220 3 2

TABLE-US-00003 TABLE 3 Specification and quality of wastewater Solid content Conductivity pH [mg/L] [mS/cm] Acidic wastewater 0.5-1.3 0.01-0.3 30-40

[0208] The process was carried out in that the respective calcium carbonate slurry (i.e. slurry 1 or slurry 2) was dosed in the acidic preparation under agitation such that the calcium carbonate is present in the respective stoichiometric ratio required for pH neutralization. The precipitation in this example was carried out without the addition of further additives such as flocculants or coagulents.

[0209] The precipitation was controlled by standard conductivity and pH measurements. Such conductivity measurement is also possible if the original wastewater is not exactly analyzed beforehand. The results of the selective precipitation can be gathered from FIG. 1, showing points of precipitating for e.g. white gypsum (point 2), Fe salts (point 3) and Cu salts (point 4). The light grey curve was obtained by dosing a limestone CaCO.sub.3 slurry, and the dark grey curve was obtained by dosing a marble CaCO.sub.3 slurry.

[0210] Precipitation of white gypsum was obtained by dosing 13.29 g/L of CaCO.sub.3 (at pH=2) in the industrial acid waste as shown in Table 7. Precipitation of iron hydroxides was performed by dosing 1.5 g/L of CaCO3 (at pH=2.95) on the supernatant obtained after the removal of the white gypsum. Precipitation of copper hydroxides was performed by dosing 11.5 g/L of CaCO3 (at pH=4.9) on the supernatant obtained after the removal of the iron gypsum. This three-step precipitation was the same for slurry 1 as well as for slurry 2.

[0211] Compared to the classical treatment with milk of lime, also shown in FIG. 1 (upper broken line), the instant process allows the selective precipitation of conjugate bases as well as metal cations which can thus be reused as raw materials for other applications. The acidic wastewater used as acidic preparation can be declared as raw material for white gypsum production with high degree of whiteness (>80%). In contrast thereto, the precipitate obtained from the classical lime treatment has to be disposed as multivalent waste.

[0212] The composition of the precipitates obtained by using slurry 1 (comprising limestone) or slurry 2 (comprising marble) is shown in Table 4. The hydrates of Fe and Cu are embedded in precipitated gypsum and calcite structures.

TABLE-US-00004 TABLE 4 Composition of precipitates Precipitation 2 Precipitation 3 Fe.sup.[3] Precipitation 4 Cu.sup.[4] Gypsum/yield Gyps./Calcite/yield Gyps./Calcite/yield [g/l] [g/l] [g/l] Marble 100 wt.-%/8.5 85 wt.-%/15 wt.-%/3.5 45 wt.-%/55 wt.-%/2 Lime- 100 wt.-%/9.5 60 wt.-%/40 wt.-%/2 20 wt.-%/80 wt.-%/0.5 stone .sup.[3]Fe content in sludge - 100 wt.-% of origin amount in wastewater .sup.[4]Cu content in sludge - 99.2 wt.-% of origin amount in wastewater.

[0213] With regard to the selective precipitation of the Fe and Cu cations from the liquid phase of the acidic wastewater by using slurry 1 (comprising limestone) in the instant process, the data obtained by conductivity and pH measurements are outlined in Table 5. From the data, it can be gathered that it was possible to discharge 3 charges of water insoluble salts.

TABLE-US-00005 TABLE 5 Precipitation of Fe and Cu cations Conductivity Sample pH (mS/cm) Fe (ppm) Cu (ppm) origin 1.12 35 220 950 1 1.19 32 215 950 2 1.29 30 220 970 3 1.39 24 215 900 4 1.71 17 200 930 5 2.00 14 220 990 6 2.60 11 190 910 7 3.40 9 0.09 805 8 4.00 9 <0.01 620 9 4.50 9 <0.01 110 10 4.90 8 <0.01 55 11 5.05 7.5 <0.01 40 12 5.23 6.8 <0.01 10 13 5.35 6.5 <0.01 14 5.65 6 <0.01 1.8 15 6.00 5.6 <0.01 1.3 16 6.15 5.3 <0.01 0.8 17 6.30 4.4 <0.01 0.5

[0214] Qualitative precipitation with quantitative amounts of water insoluble salts can be controlled by retention time, dosing sequence of respective agents, temperature and sludge discharging.

[0215] Main goal of the instant process is to enrich solids through precipitation. The instant process increases crystal growth and thus allows precipitation. The solid content of the obtained solution after separating the precipitates by centrifuging or settling was about 0. The specific surface area of the precipitated sludge decreases by using the instant process and is from 0.1 to 10.0 m.sup.2/g, measured using nitrogen and the BET method. In contrast thereto, the classical milk of lime treatment results in a sludge having a specific surface area of <20.0 m.sup.2/g, measured using nitrogen and the BET method.

Example 2

[0216] The following example illustrates the selective precipitation of at least one conjugate base and/or at least one metal cation from an acidic preparation of unknown composition by neutralization using the process of the instant invention on industrial scale basis.

[0217] The trials were performed under room temperature, i.e. at a temperature of from 15 to 25 C.

[0218] An acidic wastewater from Circuit Printed Boards production (CPB/PCB) with heavy metal contents (assembly of rinsing and acid waters from layout-exposing-etching-registration-contacting-galvanizing) was used as acidic preparation.

[0219] The test was performed with 1 000 liters of mixed CPB wastewater in batch mode (steps). The exact composition of conjugate bases and metal cations in the wastewater was unknown when delivered. The analysis of the original wastewater was performed before or after the test.

[0220] The basic data of the acidic wastewater are shown in Table 6.

TABLE-US-00006 TABLE 6 Basic data of the acidic wastewater El. conductivity Fe Cu Ag Ni Zn SO.sub.4.sup.2 Cl.sup. PO.sub.4.sup.3 NO.sub.3.sup. Step pH (mS/cm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) origin 0.45 61.9 high high low low low high high low low

[0221] For neutralization and precipitation, a calcium carbonate slurry comprising marble as described in Table 1 has been prepared. The calcium carbonate slurry had solids content of 50.0 wt.-%, based on the total weight of the slurry.

[0222] The process was carried out in that the calcium carbonate slurry was dosed in the acidic preparation under agitation such that the calcium carbonate is present in the respective stoichiometric ratio required for pH neutralization. Each step of dosing was followed by a hold-up time for complete precipitation and subsequent filtration of suspension (clarification by centrifuge and filter press of high solid phase).

[0223] The precipitation was carried out stepwise controlling pH, conductivity and character of precipitates. The further selective precipitation was performed by dosing a 2.sup.nd agent for further increasing the pH and precipitation of metals with higher pH solubility product such as Ni and Zn. Ca(OH).sub.2 was used as the 2.sup.nd agent. The single dosing steps were carried out as outlined in Table 7.

TABLE-US-00007 TABLE 7 Dosing steps Dosage Dosage Dosing solid slurry Step agent [g/l] [ml/l] Comment origin Procedure similar to lab trials 1 Marble 13.29 18.4 Slurry with 0.5 kg/kg - gypsum 2 Marble 1.07 1.5 red gypsum 3 Marble 15.37 21.3 blue calcite 4 Ca(OH).sub.2 3.5 9 pH up to 10.75

[0224] The total dosage of marble was 29.73 g/l based on completion of stoichiometric equilibrium, i.e. assuming that 90% of anions contain SO.sub.4.sup.2, such that the molar concentration amounted to 0.35 mol/l of H.sup.+, which corresponds to a total dosage of 66.8 g/l marble for neutralization.

[0225] Thus, the total amount of marble was 45.0 wt.-%, based on the total dosage of marble. This was confirmed as pH did not change anymore. Furthermore, the dosing time for the Cu precipitation was 40 min, starting from the addition of the calcium carbonate slurry, and the Cu precipitation was completed after 40 min of reaction time.

[0226] The solids content of the resulting acidic wastewater, obtained after each precipitation step of the respective high solid phases, after centrifugation (by using the centrifuge Alfa Laval Clara 20 of Alfa Laval, Sweden) and of the respective filtercakes was measured. The filtercakes were obtained by using a mobile batch pressure filterpress of 10 liters capacity of Clear Creek Systems, USA, at a maximum pressure of 7 bars. The obtained filtrates and original acidic wastewater were examined for anions and cations. The data are summarized in Tables 8 and 9.

TABLE-US-00008 TABLE 8 Analysis of the original wastewater and filtrates El. conductivity Fe Cu Ag Ni Zn SO.sub.4.sup.2 Cl.sup. PO.sub.4.sup.3 NO.sub.3.sup. Step pH (mS/cm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) (ppm) origin 0.45 61.91 230 1 090 0.24 7.4 0.6 16 900 950 30 15 1 2.65 15.45 230 1 090 7.4 0.6 3 529 950 15 13 2 4.75 11.22 0.2 1 090 7.4 0.6 2 590 910 13 3 5.86 8.82 18 7.2 0.6 2 010 905 11 4 10.75 8.02 0.07 0.05 1 960 895 10

TABLE-US-00009 TABLE 9 Analysis of the filtrates, high solid phase and filtercakes solution centrifuge filtercake Gypsum Fe Cu Zn, Ni, Brightness Step s.c. [%] s.c. [%] s.c. [%] s.c. [%] [g/kg] [g/kg] [g/kg] L* [%] Orig. 1 2.51 36.30 59.90 100.00 93.35 2 1.53 10.73 54.90 36.40 31.00 3 2.30 35.00 69.00 2.00 43.25 4 0.87 11.35 19.97 11.30 1.70

[0227] The obtained precipitates were further analyzed with regard to the specific (BET) surface area as well as to its particle size distribution. The data obtained are outlined in Table 10.

TABLE-US-00010 TABLE 10 Pigment data of precipitates SSA d.sub.10 d.sub.50 d.sub.99 Step [m.sup.2/g] [%] [%] [%] Orig. 1 1.91 1.50 10.93 60.04 2 13.01 0.90 2.50 23.75 3 6.80 1.20 5.30 28.11 4 5.05 2.30 5.33 32.10

Example 3

[0228] The following example presents an alternative way of neutralization of acidic wastewater by selective precipitation of heavy metals from wastewater using a continuous process. The neutralization agent is using CaCO.sub.3 for the two first precipitation steps instead of the classical treatment with milk of lime or sodium hydroxide. The last precipitation step is using a milk of lime.

Acid Wastewater:

[0229] As example a synthetic acidic wastewater was prepared that has similar composition than acid wastewater from Circuit Printed Boards production (CPB) including sulfate anion, iron and copper cations to be removed selectively during a three-step precipitation process, run in a semi-continuous way.

[0230] The acidic wastewater was obtained by mixing together: [0231] 1,770 L H.sub.2O [0232] 25.3 kg H.sub.2SO.sub.4 (96%): pH down to =0.8, to reach a concentration close to 14,000 ppm sulfate [0233] 1.9 kg Fe.sub.2(SO.sub.4).sub.3.7H.sub.2O: Fe(III) sulphate-heptahydrate for 210 ppm Fe [0234] 3.4 kg CuSO.sub.4.5H.sub.2O: Cu(II) sulphate-pentahydrate for 500 ppm Cu

[0235] The resulting solution was mixed until complete dilution of the added salts during 10 hours, providing 1, 800 L of acidic wastewater. The synthetic wastewater stored in pre-product tank has the following estimated concentration:

TABLE-US-00011 SO.sub.4.sup.2 14000 ppm Fe.sup.3+ 210 ppm Cu.sup.2+ 500 ppm

[0236] Measured parameters of synthetically produced wastewater:

pH=0.84
Conductivity=48.6 mS/cm

CaCO.SUB.3 .Slurry:

[0237] The following Table 11 summarizes the calcium carbonate product used during this neutralization pilot trial.

TABLE-US-00012 TABLE 11 micronized calcium carbonate Calcium d.sub.50 CaCO.sub.3 HCl insoluble Samples.sup.[1] carbonate rock [m] [wt.-%] [wt.-%] D Marble 13 98.0 2 .sup.[1]All calcium carbonates used in the present invention are commercially available from Omya International AG, Switzerland.

[0238] The micronized CaCO.sub.3 powder (Sample D) was mixed with water to prepare a 30 wt % suspension, mentioned thereafter as CaCO.sub.3 slurry.

1.SUP.st .Precipitation Step: White Gypsum

[0239] An initial 1,000-liter batch for the precipitation of calcium sulfate, as white gypsum, is prepared by transferring 1,000 L of the synthetic acidic wastewater into the pilot reactor. The CaCO.sub.3 slurry is dosed slowly to the synthetic acidic wastewater until pH=2.2. In total 24 L of 30 wt % CaCO.sub.3 slurry were added. The resulting suspension is then stirred and recirculated during 3 hours as batch for homogenization, before starting the 1.sup.st precipitation step started as a continuous process. The continuous precipitation process takes place by dosing in parallel 100 L/h of the synthetic acidic wastewater and 2.4 L/h of the 30 wt % CaCO.sub.3 slurry during 8 hours. This setting results in the continuous precipitation of white gypsum that can be removed selectively from the aqueous phase by extracting at 150 L/h the white precipitate from the reactor for continuous dewatering and washing on a belt filter. The produced 1,600 L of filtrate (first filtrate) is stored in the pre-product tank for the following precipitation step (second step).

2.SUP.nd .Precipitation Step: Red Gypsum

[0240] An initial 1,000-liter batch for the precipitation of iron within the calcium sulfate, as red gypsum, is prepared by transferring 1,000 L of the filtrate from the first precipitation step into the pilot reactor. The CaCO.sub.3 slurry is dosed slowly to the first filtrate until pH=4.6. The resulting suspension is then stirred and recirculated during 3 hour as batch for homogenization, before starting the 2.sup.nd precipitation step started as a continuous process. In total 3 L of 30 wt % CaCO.sub.3 slurry were added. The continuous precipitation process takes place by dosing in parallel 150 L/h of the first filtrate and 1.35 L/h of the 30 wt % CaCO.sub.3 slurry during 4 hours. This setting results in the continuous precipitation of the iron ions within the gypsum (red gypsum) that can be removed selectively from the aqueous phase by extracting the red precipitate at 150 L/h from the reactor for continuous dewatering and washing on a belt filter. The 1500 L of filtrate (second filtrate) is stored in the pre-product tank for the following precipitation step (third step).

3.SUP.rd .Precipitation Step: Copper

[0241] An initial 1,000-liter batch for the precipitation of copper within the calcium sulfate, as green gypsum, is prepared by transferring 1,000 L of the filtrate from the second precipitation step into the pilot reactor. A 10 wt % Ca(OH).sub.2 slurry is dosed slowly to the second filtrate until pH=9.9. The resulting suspension is then stirred and recirculated during 1 hour as batch for homogenization, before starting the 3.sup.rd precipitation step started as a continuous process. In total 24 L of 10 wt % Ca(OH).sub.2 slurry (0.6 kg of calcium hydroxide) were added. The continuous precipitation process takes place by dosing in parallel 150 L/h of the second filtrate and 1.6 L/h of the 10 wt % Ca(OH).sub.2 slurry during 3.5 hours. This setting results in the continuous precipitation of the copper ions within the gypsum (green gypsum) that can be removed selectively from the aqueous phase by extracting the green precipitate at 150 L/h from the reactor for continuous dewatering and washing on a belt filter. The third filtrate is stored in the pre-product tank.

Results

[0242] The synthetic acidic wastewater and the filtrates obtained after each precipitation steps were analyzed. The following Table 12 summarizes the analytical results.

TABLE-US-00013 TABLE 12 Composition of synthetic acidic wastewater and the filtrates each after selective precipitation steps SO.sub.4.sup.2 Fe Cu pH Conductivity Units ppm ppm ppm mS/cm Synthetic acidic 13180 210 495 0.84 48.6 wastewater Cake after 1.sup.st 3300 370 545 2.5 6.9 filtration Cake after 2.sup.nd 2880 150 500 5.0 4.9 filtration Cake after 3.sup.rd 2160 3.9 7.2 8.7 4.3 filtration

[0243] In addition the cakes from each precipitation steps were analyzed. The following Table 13 summarizes the analytical results.

TABLE-US-00014 TABLE 13 Composition of cakes after each selective precipitation steps Gypsum Fe Cu Solid content Units % % % % 1.sup.st filtrate 99.8 0 0 64.0 2.sup.nd filtrate 56.6 23.1 0 56.8 3.sup.rd filtrate 59.8 9.3 28.5 48.2