Method of removing inorganic metallic compounds from solutions

Inorganic metallic compounds, particularly salts, oxides and hydroxides, are removed from solutions by removing at least a part of the solution liquid and stirring and cooling this part while in the metastable state until a crystallization of the metallic compounds is rendered possible. Then energy is fed in bursts to the cooled down part in order to initiate the crystallization. The crystallized metallic compounds are separated, leaving a residual liquor which then is transferred back to the solution.

FIELD OF THE INVENTION 
The present invention refers to a method of removing inorganic metallic 
compounds from solutions by cooling the solution. Particularly, the 
invention refers to a method of removing salts, oxides and hydroxides from 
solutions by cooling. 
BACKGROUND OF THE INVENTION 
Up to now, it appeared not to be possible to remove disturbing 
decomposition products from electrolytic baths and from waste water 
produced generally by the galvanic industry, particularly by the hot dip 
galvanizing industry, by the hardening industry and by numerous other 
industries without an interruption of the production. 
OBJECT OF THE INVENTION 
It is an object of the invention to provide a method of removing inorganic 
metallic compounds from solutions by cooling the solution without the need 
to interrupt the production. 
SUMMARY OF THE INVENTION 
To meet this and other objects, the invention provides a method of removing 
inorganic metallic compounds, particularly salts, oxides and hydroxides, 
from solutions by cooling the solution.

DETAILED DESCRIPTION OF THE INVENTION 
According to the invention, in a first step, at least part 10 of the 
solution 14 liquid is removed from the solution containing the inorganic 
metallic compounds. In a second step, the removed part 10 is stirred 20 
and cooled 24 in a cooling container 30 while in the metastable state 
until a crystallization of the metallic compounds is rendered possible in 
this part 10. 
Thereafter, energy 34 is fed in bursts to the cooled down part 10 in order 
to initiate the crystallization. Finally, the crystallized metallic 
compounds 38 are filtered 40 from the part 10, leaving a residual liquor 
44, and the residual liquor 44 is transferred back to the solution 14. 
The above mentioned burst feeding of energy 34 can be accomplished by a 
single energy burst or by a plurality of individual energy bursts fed one 
after the other one. 
The forms of energy which appear to be most suitable for the burst feeding 
of energy 34 are: 
ultrasonic energy (four times 15 seconds, 600 W); 
direct current energy (four times 15 s, 5 V/5 A=25 W); 
alternating current energy (four times 15 s, 42 V / 1.2 A=50.4 W); 
micro wave energy; 
magnetic energy; 
laser radiation energy; 
infrared radiation energy; and 
ultraviolet radiation energy. 
Preferably, the stirring 20 and cooling 24 of the part 10 of removed 
solution is continued until a first temperature is reached at which an 
exothermic reaction starts. This exothermic reaction is caused by the 
liberation of the heat of crystallization. Subsequently, the stirred and 
cooled part 10 of removed solution is kept at a second temperature which 
is 0.5.degree. to 1.degree. C. below the above mentioned first temperature 
during 2 to 4 minutes prior to separating the inorganic metallic compounds 
38 from the part 10 of removed solution. 
It has proven advantageous to apply a cooling rate in the cooling container 
30 of -0.3.degree. to -0.7.degree. C./minute, preferably of -0.5.degree. 
C./minute. 
The method of the invention is particularly suitable for the treatment of 
the following media: 
cyanide containing zinc electrolytes; 
cyanide containing copper and copper plating electrolytes; 
cyanide containing brass electrolytes; 
cyanide containing cadmium electrolytes; 
cyanide containing bronze electrolytes; 
cyanide containing silver and silver plating electrolytes; 
cyanide containing decoction and degreasing electrolytes; 
cyanide containing tin electrolytes. 
The method according to the invention can be used for removing: 
iron dichloride in hydrochloric mordanting baths; 
iron dichloride in sulfuric mordanting baths; 
Al(OH).sub.3 ; 
AlSO.sub.4 ; 
potassium carbonate; and 
waste water of hardening plants. 
Preferably, the stirring apparatus used is a slow-speed cone stirrer (cf. 
Swiss Patent No. 675,215) sold by Viscojet AG, Basle (Switzerland), under 
the trade name "Viscojet". A preferred stirrer of this kind has a 
container diameter of 50 cm, a cone diameter of 32 cm, and is working at 
60 to 100 r.p.m., preferably at 80 r.p.m. Under these conditions, no big 
crystals, and particularly no needle shaped crystals, can be formed. 
The method according to the present invention provides essential technical, 
ecological and economical advantages, the most important of them being as 
follows: 
The method avoids production interrupts; thus, it is ensured that the 
medium, i.e. the solution 14, has an optimal and constant composition and 
a longer useful life, resulting in an improved and constant product 
quality. 
The method reduces the abduction of the desired components of the solution 
and, thus, a reduced amount of waste water accumulates. 
The method reduces electric power consumption and requires less chemicals. 
There are no deposits on the container walls and on the stirrer 20. The 
separated inorganic metallic compounds 38 can very easily be re-dissolved 
in cold water. So far, re-dissolution of crystallized inorganic metallic 
compounds , even in hot water, was very hard, if possible at all. As a 
result of this ease of re-dissolution, smaller quantities of waste water 
are produced, which in turn involve smaller labor and waste disposal 
costs. 
In executing such waste disposal, the precipitated inorganic metallic 
compounds are preferably re-dissolved, and the small quantities of 
co-precipitated plating metal are electrolyticly separated from the 
obtained solution. Thereby, about 80% of the co-precipitated cyanides are 
oxidized to cyanates. At the same time, the metal ions are reduced at the 
cathode to metal. 
EXAMPLE 
From a bath of hydrochloric mordanting agent with an iron content of 196 
g/l, an amount of 55 to 60 l bath liquid is removed and plumped into a 
cooling vessel having a diameter of 50 cm and a height of 35 cm. Then, the 
stirring apparatus of the type "Vioscojet 55 ST/v" incorporating a stirrer 
with a diameter of 32 cm is operated at a speed of 80 rpm. 
Simultaneously, the cooling is switched on and controlled such that the 
liquid in the vessel is cooled down at a rate of -0.5.degree. C./min, 
starting from an initial temperature of 30.degree. C. As soon as a 
temperature of -4.1.degree. C. is reached, a total of four 600 W 
ultrasonic energy bursts are fed, each during 15 s. Thus, the 
crystallization starts. Simultaneously, due to the liberation of the heat 
of crystallization of the so formed fine grain crystal suspension, an 
exothermic reaction, a so-called "temperature transition", occurs which 
causes a rise in temperature to -2.4.degree. C. After 18 to 20 min, the 
temperature sank to -2.8.degree. C. again and the crystallization was 
terminated. 
Thereafter, the so obtained crystal suspension, in which the crystals had a 
size of appr. 0.3 to 0.4 mm, has been drained under continuous stirring 
and, finally, filtered with a filter having a mesh size of 0.2 mm. About 
20 kg FeCl.sub.2 are obtained. 
Now, the treated bath liquid is pumped back to the bath of hydrochloric 
mordanting agent and the apparatus, which did not show any deposits on its 
walls and stirrer, was cleaned. It was then ready for the treatment of 
another charge. 
Preferably all essential parameters are automatically controlled by level 
control, temperature sensing and timer means.