Method for coalescing mercury particles

Mercury particles which are difficult to strip from sewage waters of industries employing mercury compounds, due to their extremely tiny size, can be coalesced into larger mercury drops by applying a strong magnetic field to the liquor which contains such tiny particles.

This invention relates to a method for coalescing particles of metallic 
mercury in media which contain such particles, especially in fluid media, 
so as to encourage the formation of particles having an increased size 
with respect to the starting size. 
More particularly, an object of the present invention is to provide a 
method for coalescing particles of metalic mercury in fluid media 
containing them, by using the action of a magnetic field so as to form 
particles having an increased size and to permit their withdrawal from the 
medium and, possibly, their recovery. 
Methods are known which permit to remove metallic mercury from media 
containing it. The particles having the larger sizes are mechanically 
collected, for example, by decantation or deposit in specially provided 
vessels. 
When particles contained in liquid media are involved, it is possible to 
carry out the flocculation of hydroxides of metals contained in the liquor 
by entraining the mercury particles with the flocculates and separating 
such particles thereafter with conventional means. 
Sometime, however, metallic mercury is in the form of so tiny particles 
that they cannot be decanted or they cannot be deposited within 
commercially acceptable a time, or they cannot be encased in the metal 
hydroxide. 
This is the case of the purification of the industrial sewage waters which 
contain mercury in the form of organic or inorganic compounds, such 
purification being carried out by chemical or electrochemical reduction of 
the mercury compounds to elemental mercury. 
Electrochemical reduction can be carried out, for example, by the action of 
two elements of a different nature immersed in the solution to be purified 
under appropriate pH conditions, said elements being electrically 
connected in the interior of the liquid phase or externally of it, such as 
disclosed in the Italian Patent Specification No. 926 615 by SNAMPROGETTI 
S.p.A., which corresponds to U.S. Pat. No. 4,035,269. 
According to the method of said patent, to which reference is made 
hereinafter but without being anyway restricted thereto, the quantitative 
conversion of mercury compounds, both organic and inorganic, can be 
achieved by properly selecting the elements to be used for the reduction 
(a metal less noble than hydrogen and carbon, or another metal nobler than 
hydrogen). 
Metals of the type of aluminum, iron, zinc and others, in addition to 
effecting the reduction of the mercury contained in the inorganic 
compounds, displace mercury also from its organic compounds and originate, 
in addition to elemental mercury, also other metallic organic compounds 
which can easily be hydrolyzed. 
These latter compounds, when hydrolyzed under appropriate pH conditions, 
give rise to bulky precipitates which, during flocculation, can 
electrostatically encase the mercury particles of more minute size and 
entrain them therewith. 
Mercury is then separated from the metal hydroxide according to 
conventional procedures. 
In spite of this, metallic mercury particles can be formed having such a 
size as to not permit their decantation within a commercially acceptable 
time interval, or their encasing in the metal hydroxide mass. 
In such cases the purification would be both incomplete and unsatisfactory. 
It has now been surprisingly ascertained that by subjecting the sewage 
waters after the reduction of the mercury compounds to elemental mercury 
to a magnetic field, coalescence of the mercury particles is experienced, 
and such particles thus form drops of an increased size which can be 
separated with the conventional mechanical methods. 
The applied magnetic field must have an appropriate strength, which is 
advantageously equal to 6,000 Gauss or over. 
This method can be applied to any system in which there is production of 
particulated mercury in the form of tiny particles, both in a liquid and 
in a gaseous medium, and also to the purification of sewage waters by 
reduction to metallic mercury, from the roasting fumes of 
mercury-containing materials, from the distillation of mercury and so on. 
The examples reported herein are referred, however, but without limitation, 
to the separation from liquid media as indicated in the foregoing.

EXAMPLE 1 
The installations for the production of acetaldehyde from acetylene use a 
catalyst which is composed by ferrous sulfate, mercury and nitric acid, 
which, during progress of its catalytic action, undergoes several 
reactions and intermediate combinations (for example with acetylene) so 
that mercury is eventually found in the dumped waters, partly in the form 
of inorganic salts, and partly as methylmercury sulfate and 
dimethylmercury sulfate. 
The contents of mercury (expressed as a metal) in the solutions dumped from 
the catalytic sections of installations for the production of acetaldehyde 
is in the range of 10 milligrams per liter and from 5% to 25% of this is 
in the form of organic compounds, consistently with the recycle ratio of 
the waters from the distillation to the abatment of aldehyde vapors. 
A sewage solution of this kind has been acidified to a pH of about 2 and 
fed to a reduction tower filled with iron and carbon with a superficial 
ratio of the anode to the cathode of from about 1:1 to about 2:1 at a 
velocity in excess of 15 meters an hour. The velocity has been selected so 
high to limit the reaction between iron and the acid and the evolution of 
hydrogen. The solution coming from the reduction tower had a turbid 
appearance (probably due to the evolved metallic mercury dragged by the 
water stream) has been transferred to a decanting vessel wherein mercury 
was decanted. The reduction tower is described in Italian Pat. No. 926,615 
which corresponds to U.S. Pat. No. 4,035,269. 
Prior to being transferred to the decatation vessel, the solution had been 
subjected to the action of a magnetic field, as obtained by a Polar 
water-conditioner equipped with a magnetic filter, the field having an 
intensity in the neighborhood of 8,000 Gauss. 
The solution emerging from the decantation vessel after 30 minutes of stay 
therein, contained about 0.24 milligrams of elemental mercury per liter. 
By a subsequent hydrolysis of the iron salts contained in the solution and 
consequent flocculation of the iron hydroxides which had been formed, 
another fraction of the mercury residues was removed by encasing and 
entraining in the flocculate, the latter being subsequently separated from 
the sewage waters by filtration. The solution emerging from the filter 
contained less than 1 microgram (1 thousandth of a milligram) of mercury 
per liter. 
EXAMPLE 2 
For comparison purpose, the treatment of the sewage solution of the 
previous example has been repeated without subjecting the solution to the 
action of the magnetic field after the treatment in the reduction tower. 
The solution emerging from the decantation tube after a 30-minute stay 
therein still contained 3.5 milligrams of mercury per liter. 
After a 12-hour stay in the decantation vessel, the solution still 
contained 0.9 milligrams of mercury per liter.