Removal of Complexed Zinc-Cyanide From Wastewater With Improved Sludge Stability

1. Field of the Invention

This invention is directed to a method for treating wastewater, such as blast furnace blowdown water, for removal of zinc and cyanide.

2. Description of Prior Art

In steel plant blast furnace technology, water is utilized to cool and cleanse gases coming out of the furnace. Such water is generally recycled, but a certain portion of this water is removed from the system for discharge into the environment. Such discharge water contains soluble zinc and cyanide which are objectionable. Such discharge water containing soluble zinc and free cyanide is referred to herein as blast furnace blowdown.

Environmental standards governing zinc and cyanide contents are stated in terms of total amounts of each element permitted in the discharged wastewater. Presently, the Environmental Protection Agency mandated maximum concentration for total zinc is 0.45 ppm for blast furnace blowdown. As one skilled in the art knows, a total amount of zinc is composed of that portion of zinc still in solution as a soluble compound (herein referred to as soluble zinc) plus that portion of zinc which is in solid precipitated form but still in suspension (referred to herein as suspended zinc). There are conventional means for effectively removing most but not all suspended zinc, such as high-rate filtration. Therefore, it becomes apparent that for a given inlet soluble zinc concentration, in order to reach the mandated total zinc standard, one must first reduce soluble zinc far enough below the total zinc standard so that any unremoved suspended zinc will not raise the total zinc to above the limit. Of course, the further below the total zinc standard a removal process lowers soluble zinc, the greater amount of suspended zinc permitted before the total zinc standard is reached. Therefore, there is a need for a process which removes soluble zinc at least to a level below total zinc standards (0.45 ppm) sufficient to permit the soluble zinc plus suspended zinc to be less than the total zinc standard.

A similar set of circumstances applies to cyanide. Presently, the Environmental Protection Agency mandated maximum concentration for total cyanide is 3 ppm for blast furnace blowdown. Cyanide which is soluble and available to react with zinc is referred to herein as free cyanide. Total soluble cyanide includes free cyanide (as defined herein) plus a soluble ferrocyanide and/or a soluble ferricyanide which, unlike free cyanide, are not available to react with zinc. Free cyanide will react with soluble zinc to form a zinc-cyanide complex, which is an environmentally harmful complex that is not removed from solution by traditional pH adjustment of the water.

One method for removing soluble zinc from acid steel plant wastewater consists of treating wastewater with a mixture of alkali-water slurry and recycled sludge formed later in the method. This mix neutralizes the acidity of the wastewater and precipitates a substantial portion of the zinc value as solids which settle out as a sludge, a portion of which is recycled as described above. Such process is described in commonly assigned U.S. patent application Ser. No. 291,439 filed Aug. 10, 1981, by Stewart T. Herman et al. The process described therein is referred to as a high density sludge process. Unfortunately, this process is not effective for treating zinc-cyanide complexes.

A method for removing cyanide which has been complexed with zinc in a wastewater is disclosed in U.S. Pat. No. 312,760 issued Jan. 26, 1982 to Roy G. Neville. The method disclosed therein requires the addition of ferrous sulfate in the presence of bisulfite ions. Neville discloses using a ferrous ion concentration of 200 to 300 ppm. Neville also requires the presence of bisulfite ions and the presence of an oil layer on top of the water to be treated to avoid oxidation of ferrous ion to ferric ion. Ferric ion will not react with complexed zinc-cyanide. It is, therefore, desirable to develop a process which does not require the expense and added step of bisulfite addition and an oil covering.

This invention can lower soluble zinc and free cyanide levels of blast furnace blowdown without the disadvantages of the prior art. The invention includes a multiple stage process wherein the first stage includes addition of ferrous ions to a critical minimum concentration of 250 to 400 ppm, while the pH of the water is held below 7.0 by means of addition of a mineral acid, e.g. sulfuric or hydrochloric acid. Such acid may already be present with the source of ferrous iron as in the case of waste pickling liquors from steel mills.

The wastewater in a second reactor next has its pH adjusted to between 9.5 to 10.0 to cause zinc to precipitate as zinc hydroxide and to cause the cyanide to precipitate as prussian blue. As an alternative, sludge from a settling tank is recycled to a third reactor and mixed with an alkaline material, whereupon the mixture is processed back to the second reactor, such that the amount of dry solids recycled per unit time is equal to between 20 and 100 times the amount precipitated per unit time in the second reactor, and the pH is controlled to between 8.8 and 9.4. Simultaneously with the pH adjustment and sludge recycling, the wastewater is stirred to prevent settling of precipitates. Thereafter, the wastewater is sent to an aeration reactor where sufficient air is provided to oxidize ferrous hydroxide to ferric hydroxide. Thereafter, the wastewater is sent to a settling tank from which clean effluent overflows, and the sludge is sent to landfill, with a small portion recycled as described above.

FIG. 1 is a graphical depiction of the unexpected relationship between free cyanide concentration (ppm) and ferrous ion concentration (ppm) in blast furnace blowdown.

FIG. 2 is a graphical depiction of the unexpected relationship between free cyanide concentration (ppm) and ferrous ion concentration (ppm) for starting cyanide concentrations of 6, 15 and 30 ppm in blast furnace blowdown.

FIG. 3 is a graphical depiction of the unexpected relationship between soluble zinc concentration and free cyanide concentration in blast furnace blowdown.

FIG. 4 is a diagrammatic flow sheet showing the invention without sludge recycling, and without aeration.

FIG. 5 is a diagrammatic flow sheet showing the invention having sludge recycling, without aeration.

FIG. 6 is a diagrammatic flow sheet showing the invention with sludge recycling and with aeration.

Referring now to the drawings, FIG. 1 shows the unexpected relationship between free cyanide (defined herein as cyanide available to react with soluble zinc) and the concentration of ferrous ion in a mixture of ferrous ions and blast furnace blowdown. As the curve in FIG. 1 suggests, free cyanide decreases to an unexpected low plateau (between 1-2 ppm) when ferrous ion concentration is in the range of 250 to 400 ppm. Such relationship was experimentally observed in blast furnace blowdown waters having a cyanide inlet concentration over a range of 6 ppm (curve A), 15 ppm (curve B) and 30 ppm (curve C) as shown in FIG. 2.

As shown in FIG. 3, as the amount of free cyanide in the blast furnace blowdown is reduced, the concentration of soluble zinc decreases due to the destruction of the zinc-cyanide complex. By reactions which will be described hereinafter, the zinc-cyanide complex is destroyed, and zinc is converted to zinc ions which can be precipitated as zinc hydroxide by proper pH control. Likewise, the cyanide is released from the zinc-cyanide complex to form soluble ferrocyanide which can be precipitated as prussian blue by proper pH control. Thus, it can be understood that by a combination of lowering of free cyanide through excess presence of ferrous ions and removal of soluble zinc by proper pH control, a dual mechanism is disclosed for lowering both cyanide and soluble zinc to acceptable levels.

FlG. 4 discloses a flow diagram of a process of the invention. Incoming blast furnace blowdown to be treated is processed to a first reactor 1.

As used herein, the term "processed" includes flow by gravity or movement by means of suitable pumping and valving devices. Sufficient soluble ferrous ion in the form of aqueous ferrous sulfate, aqueous ferrous chloride or other aqueous soluble ferrous salts is added to provide a concentration of ferrous ion of at least 250 to 400 ppm. It is important that the pH in reactor 1 be maintained below 7 in order to keep all the ferrous ions in solution and available to break the zinc-cyanide complex. The pH is controlled by means of the addition of a mineral acid, e.g. sulfuric or hydrochloric acid. Such acid may already be present with the source of ferrous ion as in the case of waste pickling liquors from steel mills.

In reactor 1, the zinc-cyanide complex is broken by the affinity of ferrous ion for cyanide according to the following reaction:

Thus, zinc-cyanide complex is converted to free zinc ion (which can be precipitated at a later stage by pH control as described below) plus soluble ferrocyanide which precipitates as prussian blue in the presence of excess ferrous ion at a later stage by pH control.

The mixture is processed as overflow from first reactor 1 to a second reactor 3, where pH is adjusted to between 9.5 and 10.0 by means of addition of an alkaline material, e.g., aqueous lime, sodium hydroxide or sodium carbonate. The pH control is accomplished automatically by conventional pH measuring and controlling device 4. At this pH range, zinc ion precipitates as zinc hydroxide according to the following reaction:

Also at this pH, the iron cyanide complex (ferrocyanide) in the presence of excess ferrous ion precipitates as prussian blue according to the following reaction:

Also at this pH, any excess ferrous ion precipitates as ferrous hydroxide according to the following reaction:

The contents of reactors 1 and 3 are continuously stirred by conventional stirring devices (not shown), in order to keep solids suspended therein.

Ferrous hydroxide constitutes the majority of the solids produced by this process. Although the slurry has a high pH as produced, if it is disposed of in a landfill which is exposed to rainfall (rainfall has a pH of 4.0 to 5.5, depending upon the "acid rain" constituents), the pH of the sludge may be lowered and a portion of the iron and other metals may dissolve and contaminate the water. Ferric hydroxide is much less soluble in water than ferrous hydroxide. (It is essentially insoluble at pH levels above 3.5.) Thus, it would be better to produce a ferric hydroxide sludge for disposal according to the process shown in FIG. 6.

An aerator 6 of any conventional design is added in series after reactor 3. Air is introduced into aerator 6 to oxidize the precipitated ferrous hydroxide to ferric hydroxide prior to the solids/liquid separation step in basin 5. The oxidation reaction is shown in the following equation:

This produces a more stable sludge for disposal, and, as long as the oxidation is carried out after the precipitation and before the settling out of the ferrous iron, there are no detrimental effects on the removal of zinc and cyanide. Sufficient air should be provided to oxidize the precipitated ferrous hydroxide to ferric hydroxide. The rate of aeration depends upon the size of aerator 4 and the amount of materials therein. A range of 65 to 100 ft.sup.3 of air per pound of ferrous iron treated would be sufficient for most applications.

The contents of aerator 6 are next processed to settling basin 5, to permit the solid matter (zinc hydroxide, prussian blue and ferric hydroxide) to settle out. Overflow from settling basin 5 now only contains soluble zinc in an amount less than 0.45 ppm, and a soluble cyanide in an amount of less than 3 ppm. Such overflow can now be discharged, or sent to further treatment to remove solids, if necessary in order to meet the standards for total zinc and/or cyanide. The precipitates being settled in settling basin 5 settle out slowly, and in order to enhance this settling process, the effluent can also be passed to a conventional filter process (not shown) in order to lower the total suspended solids, and thereby lower total zinc and total cyanide content before such effluent is discharged. Sludge underflow can be removed from settling basin 5 for dumping.

In order to densify the sludge, to make disposal of the sludge easier, we prefer to treat and recycle the settled sludge according to the steps disclosed in commonly assigned U.S. patent application Ser. No. 291,439 filed Aug. 10, 1981 by Stewart T. Herman et al.

Generally, these steps are shown in FIG. 5. Settled sludge from settling basin 5 is recycled to a third reactor 7 such that the amount of solids recycled to the third reactor 7 per unit time is equal to between 20 and 100 times the amount of solids precipitated per unit time in the second reactor 3. The recycled sludge is mixed with an alkaline material, e.g. a lime solution, in the third reactor 7 under control of pH measuring and controlling device 4.

After sufficient time in reactor 7, overflow mixture is processed into second reactor 3, where it combines with overflow from first reactor 1. In this instance, pH is preferably kept at about 8.8 and 9.4 for optimum zinc removal. These steps are performed in conjunction with aeration as described above.