CH617642A5 - Process for purifying effluents containing heavy metal compounds - Google Patents

Description

The present invention relates to a process for purifying waste water in which compounds of chromium, mercury or other heavy metals are dissolved.

The process of reducing Cr * -6 to Cr * 3 which uses metallic Fe is known in the literature. However, the process has not found a large industrial application mainly due to the high consumption of Fe, which in turn is responsible for obtaining large quantities of sludge to be disposed of and consumption of the precipitating agent [Ca (OH) 2 or NaOH]. higher than traditional processes (S02 and bisulfite).

On the other hand, as it is easy to verify, if the reaction took place with a consumption of Fe equal to the theoretical one, the process would be economically advantageous.

We have now found that by appropriately adjusting the pH of the solution to be treated and the linear speed of the same in the reduction system, it is possible to carry out the process with consumption of Fe very close to the theoretical, without simultaneously having passivation phenomena of the metal over time; these phenomena are in fact due to the formation of a passivated film on the metal surface, which in the long run leads to the complete deactivation of the same; if the pH leaving! reduction system is kept below the value at which there is precipitation of Fe as hydroxide, these phenomena are canceled.

The process for purifying waste water containing heavy metal compounds of the invention is defined in claim 1.

More particularly, the process of the invention is carried out by making the water to be treated flow through one or more columns filled with the less noble element than hydrogen in the form of granules, balls, bars, shavings or any shape compatible with the dimensions of the column itself.

The reaction that occurs when the water passes involves oxidation of the element in the zero state and the simultaneous reduction of the pollutant in lower oxidation stages or even in the metallic state.

For example, if an ion of a multivalent metal belongs to an anionic group, the process gives rise to a salt of the same metal in a lower state of valence, which can be easily eliminated with known techniques. On the contrary, if the water contains the salt of a noble metal, the process gives rise to the corresponding metal.

Thus the chromates are reduced to salts of trivalent chromium, and the salts of copper or mercury to copper or metallic mercury respectively.

The reactions that occur in some of the cases mentioned can be summarized as follows, assuming iron is used as an electropositive element 3 Fe + Cr207 = + 14 H + 3 Fe2 + + 2 Cr3 + + 7 H20

1 7

3 Fe2 + + - Cr207 = + 7H + # 3 Fe3 + + Cr3 + - H20

2 2

in the case of chromates, while in the case of noble metals it is possible to schematize n Fe + 2 Men + n Fe2 + + 2 Me

Being the noble metal, for example, as mentioned, copper or mercury, and its oxidation state.

The reaction starts without external energy supply, at pH lower than or equal to 3.

The purification process according to the present invention can be carried out at high feed rates, indeed it is a particularly advantageous aspect of the process to work at the highest possible flows according to the pH and concentration of the pollutant: in fact, with the same quantity of pollutant reduced, as the flow rate increases, the consumption of the electropositive element approaches the theoretical significantly.

The invention will be more clearly understandable by examining the following illustrative examples, which, however, are not intended to limit their purposes.

Example 1 Influence of PH

A column with a diameter of 2.7 cm is filled with cylindrical granules of Fe (0 = 4 mm, h = 6 mm); through this column solutions containing Cr + 6 are made to flow, at different pH.

All tests were conducted at a constant flow rate; the results obtained are shown in table 1.

Example 2 Influence of flow rate

In the same column as in the previous example and with the same filling volume, some tests were conducted at constant pH by varying the flow rate.

The results are summarized in table 2.

Example 3 Influence of linear speed

320 cm3 of Fe were subsequently placed in three columns of different diameters, through which a solution containing Cr * 6 was made to flow, keeping the flow rate and pH constant, so as to achieve the same contact time between solution and filling, at three different linear speeds.

The results are summarized in table 3.

TABLE 1

Cr + 6 mg / 1 input

Cr * 6 mg / 1 output

PH

Reduced consumption of Fe Kg Fe / Kg Cr

30

<0.02

0.3

10

30

<0.02

1

7

30

<0.02

2

2

30

<0.02

2.5

1.1

5

10

15

20

25

30

35

40

45

50

55

60

65

3

617642

TABLE 2

". Q Cr «exit" Co "sucm ° Fe:

Cr + and input ,, pH KgFe / Kg

1 / h mg / 1 Cr reduced

300

6

<0.02

1

2.2

300

8

<0.02

1

2.0

300

10

<0.02

1

1.8

300

16

<0.02

1

1.26

TABLE 3

Cr1-6 inlet "Q 0 column Cr + 6 outlet mg / 1 p 1 / h cm mg / 1

168 1 32 2 3.2

168 1 32 2.7 17.5

168 1 32 4.1 42.5

Example 4 Influence of the column diameter

Operating on columns of different diameters, it has been observed, 5 as shown in table 4, that, for the same flow rate, the amount of Fe necessary for the total reduction of Cr0 + to Cr3 + is much smaller the smaller the diameter of the column itself.

i TABLE 4

Cr + 6 mg / 1 input

PH

Q

1 / h

Fe (g)

9.7 cm

4.1 cm

15 32

2.03

40

1050

1500

32

2.03

80

1550

2050

32

2.03

120

1800

2600

20 32

2.03

160

1950

3050

25

V

Claims (3)

617642 1. A process for the purification of waste water containing heavy metal compounds consisting of putting these waters in contact with an element chosen from among those less noble than hydrogen, characterized in that the water to be treated is made to flow through one or more columns filled with the less noble element in the form of granules, spheres, bars, shavings or any shape compatible with the dimensions of the column itself and that the reaction takes place at pH lower than or equal to 3. 2. Process according to claim 1, characterized in that the waters to be treated contain chromates and the element used is iron. 2 3. Process according to claim 2, characterized in that the pH output from the reduction system is kept below the value at which the precipitation of iron as hydroxide occurs.