CS202546B2 - Chromates containing waste water treatment - Google Patents

Abstract

For the purification of effluents containing heavy metal compounds, these waters are contacted with an element selected from those more electropositive than hydrogen. The reduction reaction must be carried out at a pH less than or equal to 3. The water to be treated is caused to flow through one or more columns filled with the more electropositive element in the form of granules, beads, strips, shavings or any form compatible with the dimensions of said column. If, for example, the waters to be treated contain chromates, the element used is iron. In this case, the exit pH from the reduction system is kept below the value corresponding to that at which the iron is precipitated as hydroxide. In this way, purification of the waters is achieved with a reduced consumption of more electropositive elements, in this example, iron.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a process for the purification of chromate-containing effluents with minimum iron consumption, which process comprises contacting these waters with metallic iron.

There is known a method of reducing hexavalent chromium Cr @ ⁺ to trivalent chromium Cr @ ⁺ using metallic iron. However, this process has not been widely used in industry, particularly for high iron consumption, thereby producing a large amount of sludge to be removed _r and pro. high consumption of a precipitating agent (calcium hydroxide Ca (OH) g or sodium hydroxide NaOH), which is higher than the consumption of reagents used in conventional processes (sulfur dioxide SO 2 and acid sulfite).

However, if this reaction were to take place with an iron consumption equal to the theoretical consumption, this method would be economically advantageous.

It has now been found that if the pH of the solution to be treated and its linear flow rate in the reduction zone are suitably controlled, this process can be carried out using iron which is very close to the theoretical value, without simultaneous passivation phenomena on the metal used. caused by the formation of a passivation layer on the metal surface, which gradually deactivates the metal. If the pH in the reduction zone is kept lower than that at which iron is excreted in the form of hydroxide, these phenomena do not occur at all.

Accordingly, the present invention provides a process for treating chromate-containing effluents with a minimum iron consumption, characterized in that they are contacted with metallic iron in a reduction zone at a pH in the range of 1 to 3, wherein the wastewater flow Q through the reduction zone is defined. relationship '202546 2

Q = 1.6, ΙΟ “ ⁴ . (10 ^and -1). d ² . h ² , where.

Q means the waste water flow,. d denotes the diameter of the reduction zone in cm, h denotes the height of the reduction zone in cm, and denotes the index, assigned to the pH value by + + 1.687) ² · ²⁷ .lg (C + 1) where

C means the concentration of hexavalent chromium ions, expressed in mg / liter.

Preferably, the process according to the invention is carried out by allowing the waste water to be treated to be passed through at least one reduction zone filled with metallic iron having a shape appropriate to the size of the reduction zone, in particular the shape of granules, balls, rollers or shavings.

The reaction that occurs in the water flow is to oxidize the zero-valence element that forms the packed column and simultaneously to reduce the polluting material to a lower oxidation state or even to a metal state. For example, when a metal ion with several valencies is part of an anionic group, the method of the invention produces a lower valency salt of the metal which is readily removed by known methods. Conversely, if the water to be treated contains a noble metal salt, the metal according to the invention is formed. ·

For example, chromenes are reduced to trivalent chromium salts and copper or mercury salts are reduced to metal or mercury. The reactions taking place in some of the above cases can be illustrated by the following equations in which iron is used as an electropositive element:.

Fe + CrO ₇ ⁼ + 14 H% = ¹³ Fe ^{2 -} + 2 Cr ³⁺ + 7 HgO

Fe ²⁺ + 1/2 Cr 2 O 7 + 7 H 2 = 3 Fe ³⁺ + Cr ³⁺ + 7/2 H2O

These equations show the reduction of chromates, while the reduction of the noble metal is described by the equation 94n Fe + 2 Me ^{n +} n Fe + 2 Me, where

Me means a noble metal, for example the above-mentioned meO or mercury, and n denotes its oxidation degree.

The reaction proceeds without external energy supply at a pH of less than or equal to 3 "

The cleaning process according to the invention can be carried out at high flow; a particular and advantageous feature of the process according to the invention lies precisely in the fact that work is carried out at the highest flow rates depending on the pH and the concentration of the polluting material.

The invention is illustrated in more detail by the following non-limiting examples.

He did

Effect of pH

A 2.7 cm diameter column is packed with Iron Gags (4 mm diameter and 6 mm height) and allowed to flow through solutions containing hexavalent Cr @ ^{+ +} , at different pH values.

. 3 All experiments are kept constant 202546 speed. The results are shown in Table I Table AND - KoosenSracn 'Krnncntrace ^p H Fe consumption Cr ⁺⁶ in inflow Cr ^{+ 6} in ^d stream (kg Fe / kg reduced 'Cr') (mg / 1) >  (mg / 1) 30 . <0.02 0.3 10. 30 <0,02- 1 . 7 30 <0,02- 2 2 30 <0.02 2.5 , f .1-1 Example 2 Size effect flow

Further tests were carried out in the same column used. . v. in the previous example, using the same filling volume, but with the pH keeps constant but the flow rate changes. The results are shown in Table II.

Table 11

Cr ⁺⁶ concentration in inflow (mg / l) KooscnSгacn Cr ⁺⁶ in odto ^to u (mg / 1)  Q pH (l / h) Fe consumption. (kg Fe / kg reduced Cr) 30 <.0.02 1. 6 2.2 30 <0.02 1 8 2,0 ... 30 <0.02 1. 10 . '18 30 <0.02 1 · 16 .126 '

Example 3

Influence of linear velocity

Three columns of different diameters are successively filled with 320 cm @ 3 of iron gags. Each of ^you wanted Olon ^ok the NEC ^{hook p} rowse solution encompass ion ^Y of chromium estimocné ^{H p F} Rice konstastsí maintained pH value and in all three cases · .stejný hourly flow rate so as to achieve equally long periods of contact with the filling solution at different linear speeds. The results are shown in Table III. .

Table III. KooscnSrace Kooscnsrace pH ^j q Column diameter Cr ^{+ b} in the inflow Cr ^{+ 6} in ^d stream (l / h) (cm) (mg / 1) (mg / 1) 168  3.2 1 32 2 168 17-, 5 1 32 2.7 168 42.5 1 32 4.1

Example 4

Influence of column diameter

Columns of different diameters are used. The results in Table IV show that, at the same flow rate, as the column diameter decreases, the total amount of iron required to completely reduce Cr ° to Cr ^J decreases.

Table IV Concentration Q · Iron consumption (G) CR ⁺ 6 in inflow pH (liter / h) column diameter (mg / 1) 4.1 cm 9.7 cm 32 2.03 40 1 050 1 500 32 2.03 80 1 550 2 050 32 ' 2.03 120 1 800 2 600 32 2.03 160 1 950 3 050

SUBJECT OF THE INVENTION

Claims (2)

Method for the purification of chromate-containing effluents, with minimum iron consumption, characterized in that these waters are contacted with metallic iron in the reduction zone at a pH in the range of 1 to 3, wherein the wastewater flow rate Q through the reduction zone is defined by Q = 1.6. 10 ' ⁴ . (10 ^and -1). d ² . h ² , where ¹ Q is the waste water flow, d is the diameter of the reduction zone in cm, h is the height of the reduction zone in cm, and is the index assigned to the pH by (4η- * 1.687) ² · ²⁷ ,. lg (C + 1) where C means the concentration of hexavalent chromium ions, expressed in mg / liter. Method according to claim 1, characterized in that the waste water to be treated is passed through at least one reduction zone filled with metal iron having a shape appropriate to the size of the reduction zone, in particular the shape of granules, balls, rollers and shavings.