WO1990002709A1

WO1990002709A1 - Buffered electrolysis

Info

Publication number: WO1990002709A1
Application number: PCT/NO1989/000091
Authority: WO
Inventors: Thomas Thomassen
Original assignee: Cheminor A.S.
Priority date: 1988-09-12
Filing date: 1989-09-11
Publication date: 1990-03-22
Also published as: NO884048L; NO884048D0

Abstract

Electrolysis of a diluted aqueous solution which contains metal ions and hydroxides of metals, such as copper, zinc, ferri/ferro iron, cadmium and suspended colloidal and organic matter, is effected controlling the pH of the solution to between 3 and 4 for the selective removal of copper and the ferri-iron as an easily filterable hydroxide, or by carrying out the electrolysis at a pH-value of 6-8 units to completely remove the suspended matter and said metals as easily filterable hydroxides.

Description

BUFFERED ELECTROLYSIS

The present invention concerns a process for the complete removal of heavy metal ions, iron and suspended organic or colloidal matter from aqueous solutions in the form of easily filterable solids, by electrolysing said solution and simultaneously buffering said solution to a pH-value where said metal ions precipitate aε hydroxides which catch the insoluble suspended collodial matter.

It is known to persons skilled in the field of electro- winning of copper that red/ox pairs aε well as Fe+++/Fe++ ions lower the current efficiency with respect to the copper electrowinning and that the electrolysis itself becomes ineconomic. Therefore the iron along with other impurities are removed prior to the electrowinning, as deεcribed in Norwegian patent No. 139.096.

Electrowinning of copper from diluted solutions are well documentated, however, there has been no direct mentioning of electrowinning of copper in solutions which contain larger amounts of iron, which is one of the objects of the present application.

We tried to carry out electrowinning of copper in a solution which contained only 200 mg/1 copper and 2300 mg/1 iron. Applying a current which theoretically εhould be large enough to remove the copper, did not work at all. Absolute¬ ly no copper deposited on the cathode.

To our great surprise, however, by introducing a medium, such as seasand (CaC0₃) , or sodium hydroxide (NaOH) to the solution inside the electrolysing device until the solution had a pH of 3-4 units, the ferri-ions precipitated as a hydroxide and the copper electrowinning proceeded as expected. Surprisingly, by carrying out the electrolysis at even higher pH-values of the solution, such aε 6-8 unitε using CaO, all the etall ions precipitated as eaεily filterable hydroxides, even the ferro-ionε, and this at only 10-20"C.

The essence of the present invention is the technique of eletrolysing an aqeouε solution, in which the pH of the εolution iε adjusted to a value of 3-4 for the selective removal of copper, or to a pH-value of 6-8 for the total removal of metal ions in the form of an easily filterable hydroxide.

It must be mentined that we have here uεed calcium car- bonate, sodium hydroxide and calcium oxide to precipitate the metal ions and remove the insoluble suspended colloidal matter in the solution. It is within the scope of this invention to use other media which increases the pH-value of the solution, such as potassium hydroxide and metal carbo- nates, metal hydroxides, for example those made from the same metals as those contained in the solution.

In our opinion the present invention may be performed not only on acid mine drainages, drainage from waste dumps or domestic sewer, but of course also on any εolution where easily filterable hydroxides are wanted in a batch or as separated and purified hydroxide by controlling the pH value carefully to just above the value where said metal preci¬ pitates as a hydroxide.

The present invention iε described in detail in the examples below.

Example 1 Acid mine drainage was electrolysed in an electrolyser like the one described in the Norwegian patent application No. 89.2388. Current = 13 Amp, cell voltage = 6,3 V, temperature = 20 ' . Flow = 12 litres/hour , pH was adjusted using εhell εand (εea sand) ( 72% CaC0₃ )

Analysis rog/1

— ₌ Cu Fe Zn pH

AMD inlet 200 2300 140 2,36 Outlet:

Without pH adjustment 220 2550 149 1,97 With pH adjustment 29 1770 136 3,56

AMD = acid mine drainage

The example shows that the addition of εhell εand to the electrolyεing device until the outlet had a pH-value of

3,56, made it possible to reduce the copper contentε of this AMD-water from 200 mg/1 to 29 mg/1.

This example also showε that without the pH-adjustmend it waε not posεible to remove the copper.

Example 2 The object of this example was to prove that a pH range between the values 3 and 4 exists, where practically all the ferri-ions (Fe+++) are precipitated, whereas no copper is precipitated as Cu(OH)2, and that an electrolysis within this pH range really is an electrolysis of copper.

2 litres of an acid mine drainage (AMD) was titrated using a 1 molar NaOH-solution. The pH-value of the AMD was increased from a value of 1,95 up to pH 5,08. Samples were taken at intervals and the filtrate analysed for residual metal contents.

The example εhowε that at pH-valueε > 4 the copper iε precipitated, possibly aε a Cu(OH) ₂-hydroxide. At pH-values > 5 units, the Fe(0H) and Zn(0H)2 type hydroxides begin to precipitate.

This is known to those skilled in thiε field of chemistry.

Example 3

The object of this example was to show that if there is no interest in saving the copper separately, thiε technique is effective also at a pH value higher than 4 if all metal elements are to be removed.

An acid mine drainage was fed into an electrolysing device like the one described in Norwegian patent application

No. 87.2388. The AMD was pumped into the unit for about one hour with no current applied to the unit and the pH of the outlet was adjusted to a pH-value of 5,5-5,8.

Thus pH-values in practical work is just below the pH-value at which the Fe(OH)₂ and the Zn(OOH)₂ hydroxideε precipita¬ te, but above the pH-values which precipitate the CufOH)₂ and the Fe(0H)₃ hydroxides. We intended to show that by supplying D.C. current to the unit, iron and zinc would precipitate, εomething v/hich would not happen otherwiεe.

Sea sand (CaCθ3) waε uεesd to adjust the pH.

Analysiε mg/1

Cu Fe Zn pH ADM inlet 240 2400 155 2,83 Outlet: Current off 2 1015 133 5,77 Current on 2 352 42 5,51

This example shows that the current produces ferri-ions and that these are precipitated when the current is on.

If desired, all the heavy metals, as shown above, can be removed out of environmental considerationε.

Example 4

This example was carried out to prove further that all the heavy metals Cu, Fe, Zn could be removed from a diluted solution, repeating example 5, only at a higher pH-value. Here NaOH waε uεed to adjuεt the pH.

Analysis mg/1

Cu Fe Zn pH

AMD inlet 140 660 230 2,53 Outlet:

Current on 20 6,20

This example shows that by effecting the electrolysis at a pH-value of 6,20, there was practically no heavy metal, like copper, iron and sine left in the filtrate.

Example 5

This example was carried out to prove further the tota removal of heavy metal ions from acid mine drainage, with the emphasiε on the filterability of the produced hydroxideε.

An electrolysing device like the one described in Norwegian patent application No. 87.2388 was used. Thiε unit was, however, 15 times largeε than the one deεcribed in εaid application and capable of carrying 3000 amperes if warrented. The unit was operated continuouεly over a longer period. The hydroxides produced were removed in a filter- press (area 1 m², volume 20 litres) . Cake thicknesε waε 40 mm. A current of only 50 ampereε waε used and CaO was used to increase the pH to values of 6-8 during theεe teεts.

Cd pH

0,5 2,68

6,50

0,02 8,05

This example confirms that the acid mine drainage (AMD) could be cleaned to heavy metal contentε of very low valueε. The hydroxideε produced at 18-20°C were easily filtered in the filterpresε. When the preεε waε completely full, the pressure on the inlet pump was only 5-6 bar. The filtration was performed on a constant volume basis.

Theεe filtration results indicate that the hydroxideε produced behaved "as a cryεtalline matter" and not as a slime. The filter cakeε were 40 mm thick and were easily removed from the filter frame. The cakeε alεo fell off the filtercloth eaεily. The moiεture content was 60-65% and the volume weight was 35 kg/20 1 = 1,75 kg/1. The hydroxyde was of a green colour, indicating Fe++ hydroxyde. This green hydroxyde easily oxidized to a brown solid in air. Example 6

This example was carried out on domestic sewer, or the "gray" water after the course sewer filtration of > 25 mm solidε. Such sewer water contains a variety of solids and dissolved matter. The object is to remove all the sus¬ pended solids, the nutrients and, if any, the metal ions.

The test was carried out in the same electrolysing device as mentioned in the test of example 5 above.

200 1/hr of "gray" sewer water waε pumped into the unit. Shellsand (72% CaC0₃) was uεed to increase the pH of the outlet water to about 8. The outlet from the unit waε filtered in a filter preεε. The current used waε only 20 amperes.

Analysis

COD = chemical oxygen demand

The sewer water contained < 5 mg/1 of heavy metal ionε (Cu, Zn, Mn) and 60 mg/1 Fe. Thiε waε not further analyεed. For test 1 εewer waε uεed which waε very diluted with rain due to faulty drainage pipeε. Teεt 2 waε carried out uεing normal undiluted εewer. The outlet water filtered very well, just as mentioned in example 5.

The inlet usually had a bad odeur, but the filtrate was odour free and was discharged directly to the resipien^*c.

Claims

C l a i m s

1. Process of electrolysiε of diluted aqueouε solutions containing copper, iron, zinc, cadmium and other metals, as well as suspended organic matter, c h a r a c t e r i z e d i n that a medium is added to the aqueous solution which at leaεt partly precipitateε the metals as hydroxydes and that the electrolysis is carried out in the presence of said precipitated metal hydroxides.

2. Process according to claim 1, c h a r a c t e r i z e d i n that the reagents added are soluble hydroxides or metal hydroxide precursors, such as carbonates.

3. Process according to claim 1, c h a r a c t e r i z e d i n that the reagent iε added to the solution prior to being introduced to the cell or directly to the cell, and that liquid and the εolidε formed are separated after the electrolysis.

4. Process according to claim 1, c h a r a c t e r i z e d i n that the pH-value of the solution during the electrolysis is between 3 and 4 for the production of copper on to the cathode.

5. Process according to claim 1, c h a r a c t e r i z e d i n that the pH value of the solution during the electrolysis is higher than 4 for the complete removal of the metals copper, iron, zinc, cadmium and suspended collodial and organic matter.