Process for removing of heavy metal from water in particular from waste water

Process for removing Sr and heavy metals from water in particular from waste water by throughly mixing it with a solution of an alkali metal carbonate and/or hydrogen carbonate in a reactor containing a fluidized bed of suitable bed material. The obtained heavy metal carbonate crystals crystallize onto said bed material and the obtained heavy metal carbonate crystals in granular form are removed from the reactor from time to time. The heavy metals which may be removed are Ni, Zn, Cu, Fe, Ag, Pb, Cd or Hg.

A similar process is known from the Netherlands published patent 
application No. 7108132. Said patent application generally relates to 
waste water derived from pickling devices. 
Calcium carbonate and/or optionally carbon dioxide are added to waste water 
to be treated. Then lime milk is added to the pretreated waste water, 
whereupon the waste water is subject to decarbonization. After 
decarbonization the waste water is filtered in a gravel filter, whereas 
furthermore prior to the filtration a part of the waste water is passed 
through a cation exchanger. 
According to the prior art method the heavy metals precipitate in the form 
of carbonates together with their hydroxides. The thus formed granulates 
are composed both of heavy metal carbonates and heavy metal hydroxides. 
Apparently the obtained granulates include a substantial amount of water, 
because said granulates have to be de-watered by means of filter units 
according to the above-mentioned patent application. After de-watering the 
same they still have a considerable residual water content of about 10%. 
Furthermore this patent application shows that the water from which the 
heavy metals are precipitated should further be purified by means of a 
gravel filter, from which the contaminents should be removed prior to 
passing it to the sewer. 
The prior art method has a number of serious draw backs to wit that said 
method is complicated and it requires a number of steps, the obtained 
granulates contain a substantial amount of water, presumably because said 
granulates besides the heavy metal carbonates contain heavy metal 
hydroxides which means that a further de-watering is required and despite 
this the de-watered granulates still contain about 10% of water. 
Furthermore the known method has the disadvantage of carry-over of lime 
milk, etc. 
The invention aims now to provide a process, wherein the above-mentioned 
drawbacks are efficiently removed. 
Furthermore it is found that the process of the invention is useful for the 
production for relative pure heavy metal carbonates from their aqueous 
solutions. 
In order to achieve this the present process is characterized in that said 
heavy metal containing water is throughly mixed with a solution of an 
alkali metal carbonate and/or hydrogen carbonate in a reactor containing a 
fluidized bed of a suitable bed material, whereupon the obtained heavy 
metal carbonate crystals crystallize and the obtained heavy metal 
carbonate in granular form is recovered from time to time. 
The process according to the invention is simple, wherein in a reactor the 
heavy metal carbonates crystallize on the bed material, which is in a 
fluidized state during the process. 
The process according to the invention has a number important advantages: 
the purification may occur within a few minutes, there is no carry-over 
and a further purification is not required. 
Besides that, a very important advantage of the present application is the 
fact that a granular product is obtained which has a very low water 
content of about 0.5%. The obtained granular material may be used for 
various industrial purposes without further de-watering, among other for 
the recovery of the heavy metals. 
Because of this low water content it appears further more that the volume 
of the produced granules is reduced by a factor 50 compared with the 
conventional prior art. 
The present process is for instance very suitable for waste water, derived 
from galvanization processes, to which however the present process is not 
limited at all. 
According to the invention it is essential that the process is carried out 
in a fluidized bed of the bed c.q. seed material, which ensures an 
exclusive crystallization of the heavy metal carbonate. The carbonates are 
built into the crystal lattice of the seed material. In certain cases, 
depending on the pH, a heavy metal hydroxy carbonate may crystallize 
instead of the metal carbonate. 
Because of the growth of the metal carbonate crystals it is necessary to 
remove the obtained granules having a size of 1-3 mm from the reactor from 
time to time. From these granules the heavy metals may be recovered 
according to any conventional technique and the same may be added again to 
the galvanization bath. 
It is preferred according to the invention to use sodium carbonate and/or 
hydrogen carbonate as alkali metal carbonate or -hydrogen carbonate, 
because the easy availability of these materials. Furthermore potassium 
carbonate or -hydrogen carbonate may be used too. 
As suitable seed material preferably use is made of metal carbonate 
crystals of the heavy metal to be removed. When for instance as heavy 
metal zinc is removed, one uses zinc carbonate crystals as seed material. 
In starting up the process one may use sand or an other suitable material 
as bed material. The sand granules in fluidized state may serve as nucleus 
for the precipitation of the heavy metal carbonate crystals. 
The present invention may advantageously be used for removing Sr and the 
heavy metals such as Ni, Zn, Cu, Fe, Ag, Pb, Cd or Hg. 
It should be understood that the conditions during the crystallization of 
the metal carbonates should be such, that the solubility product of the 
metal carbonate of interest is exceeded and that metal hydroxides don't 
precipitate. These conditions are different from heavy metal to heavy 
metal, since their solubility product is different. The avoidance of heavy 
metal hydroxy carbonate precipitates is attained through varying the pH 
depending on the solubility product of the heavy metal carbonates of the 
heavy metal to be removed. As can be seen in the examples delineated 
below, in the case of zinc the pH should be 7.5-8.0, whereas in the case 
of nickel the pH should be 9 and in the case of lead the pH should be 7.5. 
It is self-explanatory that when the difference between the solubility of 
the carbonate salt and the hydroxide salt is sufficiently high, the 
reaction conditions may be chosen in such a way that the carbonate salt 
crystallizes but no crystallization occurs of the hydroxide. The 
difference in solubility is greater at a higher total carbonate 
concentration (C.sub.T). 
It is to be noted that the present method is also particularly suited for 
removing and recovering metals from aqueous concentrates. Since the metal 
concentrations in that case may be very high one may work at a lower pH. 
In practice the favourable process conditions may be obtained by 
recirculating a solution having a high carbonate concentration (C.sub.T) 
in the reactor containing a fluidized bed. In the reactor the solution to 
be treated may be injected, wherein the metal carbonates crystallize on 
the seed material.

Schematic diagram 1 shows the recovery of heavy metals from a galvanic bath 
and a drag-out bath of a galvanization process, whereas schematic diagram 
2 illustrates the removing of heavy metals by means of crystallization 
from rinsing water of a galvanizing process. 
According to schematic diagram 1 the galvanizing bath 1 is through a valve 
4, a conduct 7 and a valve 5 connected with a drag-out bath 2. Further a 
rinse bath 3 is connected with an effluent conduct 12 through a valve 6 
and a conduct 8. Said drag-out bath 2 is connected with the reactor 10 
through a supply conduct 9, which reactor 10 contains the fluidized bed 11 
of the seed material. A tank 14 containing a solution of carbonate and/or 
hydrogen carbonate is connected with a reactor 10 through a supply conduct 
13, whereas the galvanizing bath 1 is in contact with the reactor 10 
through the conduct 15. The product to be galvanized is treated in the 
galvanizing bath 1. Then it is transported in said drag-out bath 2 and 
finally it is post-rinsed in the rinse bath 3. 
In order to remove the heavy metals from the galvanizing bath and from the 
drag-out bath the liquid present therein is passed to the underside of the 
reactor 10 through the conducts 7 and 9 respectively. Then the solution of 
carbonate and/or hydrogen carbonate from the tank 14 is introduced into 
the reactor 10 through supply conduct 13. The rate of supply of the 
liquids to be purified and the solution of carbonate and/or hydrogen 
carbonate should be such, that a fluidized state of the bed c.q. seed 
material is ensured. The liquids containing the heavy metal and the 
solution of carbonate and/or hydrogen carbonate are throughly mixed in the 
reactor 10, wherein the formed heavy metal carbonates crystallize on the 
bed e.q. seed material. From time to time the formed metal carbonate 
granules are removed from reactor 10 through the conduct 15 and the heavy 
metals which are recovered from the granules in a conventional way are 
added again to the galvanizing bath 1. 
The purified galvanization bath liquid and the dragout bath liquid are then 
discharged for further treatment, if desired, or to the sewer, together 
with the rinse water derived from the rinse bath 3, valve 6 and the 
conduct 8. 
The schematic diagram 2 is similar to schematic diagram 1, provided that 
here the rinse water is purified, by passing it through the valve 6 and 
the supply conduct 8 to the reactor 10 and mixing it throughly with a 
solution of carbonate and/or hydrogen carbonate, which is introduced in 
the reactor from the tank 14 through supply conduct 13. The purified rinse 
water leaves the reactor through the effluent conduct 12 for instance to a 
sewer. 
The invention will be further illustrated in light of the following 
examples, to which the process of the invention is not limited at all. 
EXAMPLE I 
As heavy metal containing waste water a zinc containing water having a zinc 
concentration of 45 mg/l was used. This waste water was introduced into a 
fluidized bed reactor with a superficial rate of 40 m/h at a height of the 
bed material in resting state of 2 m. 
In the reactor the waste water was treated at a pH of 7.5-8 with a solution 
of sodium carbonate having a total concentration of 2 mmol/l. Initially a 
reduction of the zinc contents of 5-10% was obtained. After increasing the 
total carbonate concentration up to 10 mmol/l 70-80% of the heavy metal 
zinc was removed. After further increasing the carbonate concentration to 
100 mmol/l, 90-95% of the zinc was removed. 
EXAMPLE II 
The procedure of example I was repeated provided that as heavy metal 
containing waste water nickel containing waste water having a 
concentration of 50 mg/l was used. The pH was 9 and the total carbonate 
concentration was 50 mmol/l. Herein the nickel concentration in the waste 
water was reduced to 5 mg/l, which corresponds with a 90% removal. 
EXAMPLE III 
The procedure of example I was repeated provided that as heavy metal 
containing waste water lead containing waste water was used with a 
concentration of 50 mg/l; the pH was 7.5 and the total carbonate 
concentration was 20 mmol/l. The lead concentration of the waste water was 
reduced to 1 mg/l, which corresponds with a 98% removal.