REMOVAL OF CHROMATES FROM
PICKLING LIQUORS BY ADSORPTION
OF HYDROUS FERRIC OXIDE
D.A. WILMS, F.F. NOTEBAERT and A.A. VAN HAUTE
Institute of Industrial Chemistry, Water Pollution Division,
University of Leuven, de Croylaan 2, B—3030, Heverles, Belgium
INTRODUCTION
Chromates from pickling liquors are usually removed either by ion-exchange or by
reduction of hexavalent chromium to the trivalent state in strong acid medium (pH = 2.5)
with S0 2 or with sodium sulfite, -bisulfite or -metabisulfite, followed by a neutralisation
to pH « 8, whereafter the precipitated Cr(OH) 3 can be separated by decantation.O)
In a number of cases chromate wastes are very alkaline, for instance in the rinse waters
of molten salt baths (consisting mainly of NaN0 3 and NaOH), used for dissolving
annealing scales of high alloy steels. During the regeneration of the strong alkaline anion
exchange resins which are used to remove chromates and other anions from the rinsing
waters, alkaline chromate wastes are produced too. In such cases the classic reduction by
means of S0 2 is less attractive, as large amounts of acids have to be added and then the
pH has to be raised to 8 to precipitate Cr(OH) 3 . But hexavalent chromium can be
reduced as well in an alkaline as in an acid medium by iron(II)salts.(2,3) i n pickling steel
considerable amounts of iron dissolve in the acid pickle liquors. These iron salts are
frequently disposed of as a sludge. The regeneration of the cation exchange resins can also
contain important amounts of iron. It is therefore desirable to find the conditions for an
optimum removal of the chromates by mixing acid iron- containing effluents with the
alkaline chromate-containing effluents.
In a previous research it has been found that a not inconsiderable amount of
hexavalent chromium can be adsorbed on floes of Fe(OH) 3 . The aim of the first part of
this study is to examine which factors play a major part in this adsorption, and under
which conditions optimum adsorption can be assured. Since the concentration of
chromate cannot be reduced sufficiently by adsorption only (the admissible CrVI
concentration in the effluent is only a few mg/1), the residual amount must be rendered
harmless by reduction with Fell. Consequently, in the second part of this study the best
way to link the reduction process to the previous adsorption process is determined.
A. Adsorption of CrVI o n Fe(OH) 3
The adsorption of CrVI o n fl ocs 0 f Fe(OH) 3 depends on a great number of factors, of
which the most important are: initial concentration of CrVI; amount of Fe(OH) 3 ; pH of
the mixture; presence of other ions in the solution; temperature; way in which Fe(OH) 3
is precipitated; time of contact between the floes and the chromate solution.
From previous experiments the last three factors appeared to have only a secondary
influence on the adsorption equilibrium; when Fe(OH) 3 is precipitated directly in the
CrVI-solution, the adsorption is somewhat better than if one prepares the Fe(OH) 3 -floes
separately and mixes them with the CrVI-solution afterwards. The residual chromate
concentration appears to be slightly lower when using Ca(OH) 2 instead of NaOH for the
615
PICKLING LIQUORS BY ADSORPTION
OF HYDROUS FERRIC OXIDE
D.A. WILMS, F.F. NOTEBAERT and A.A. VAN HAUTE
Institute of Industrial Chemistry, Water Pollution Division,
University of Leuven, de Croylaan 2, B—3030, Heverles, Belgium
INTRODUCTION
Chromates from pickling liquors are usually removed either by ion-exchange or by
reduction of hexavalent chromium to the trivalent state in strong acid medium (pH = 2.5)
with S0 2 or with sodium sulfite, -bisulfite or -metabisulfite, followed by a neutralisation
to pH « 8, whereafter the precipitated Cr(OH) 3 can be separated by decantation.O)
In a number of cases chromate wastes are very alkaline, for instance in the rinse waters
of molten salt baths (consisting mainly of NaN0 3 and NaOH), used for dissolving
annealing scales of high alloy steels. During the regeneration of the strong alkaline anion
exchange resins which are used to remove chromates and other anions from the rinsing
waters, alkaline chromate wastes are produced too. In such cases the classic reduction by
means of S0 2 is less attractive, as large amounts of acids have to be added and then the
pH has to be raised to 8 to precipitate Cr(OH) 3 . But hexavalent chromium can be
reduced as well in an alkaline as in an acid medium by iron(II)salts.(2,3) i n pickling steel
considerable amounts of iron dissolve in the acid pickle liquors. These iron salts are
frequently disposed of as a sludge. The regeneration of the cation exchange resins can also
contain important amounts of iron. It is therefore desirable to find the conditions for an
optimum removal of the chromates by mixing acid iron- containing effluents with the
alkaline chromate-containing effluents.
In a previous research it has been found that a not inconsiderable amount of
hexavalent chromium can be adsorbed on floes of Fe(OH) 3 . The aim of the first part of
this study is to examine which factors play a major part in this adsorption, and under
which conditions optimum adsorption can be assured. Since the concentration of
chromate cannot be reduced sufficiently by adsorption only (the admissible CrVI
concentration in the effluent is only a few mg/1), the residual amount must be rendered
harmless by reduction with Fell. Consequently, in the second part of this study the best
way to link the reduction process to the previous adsorption process is determined.
A. Adsorption of CrVI o n Fe(OH) 3
The adsorption of CrVI o n fl ocs 0 f Fe(OH) 3 depends on a great number of factors, of
which the most important are: initial concentration of CrVI; amount of Fe(OH) 3 ; pH of
the mixture; presence of other ions in the solution; temperature; way in which Fe(OH) 3
is precipitated; time of contact between the floes and the chromate solution.
From previous experiments the last three factors appeared to have only a secondary
influence on the adsorption equilibrium; when Fe(OH) 3 is precipitated directly in the
CrVI-solution, the adsorption is somewhat better than if one prepares the Fe(OH) 3 -floes
separately and mixes them with the CrVI-solution afterwards. The residual chromate
concentration appears to be slightly lower when using Ca(OH) 2 instead of NaOH for the
615