Process for recovering heavy metal using insoluble starch-xanthates

A precoat layer comprising a mixture of insoluble starch-xanthate and a filter-aid is deposited on a filter leaf through which a solution containing a heavy metal flows in a continuous stream and insoluble starch-xanthate and filter-aid are continuously added upstream of the precoat layer.

The present invention relates in general to the art of reacting insoluble 
starch-xanthates with heavy metal ions in an aqueous solution to recover 
the heavy metal, and it relates in particular to a new and improved 
process wherein insoluble starch-xanthate and filter-aid are continuously 
added to a continuously flowing stream of water containing heavy metal 
ions upstream of a filter which is coated with a mixture of insoluble 
stach-xanthate and filter-aid. 
BACKGROUND OF THE INVENTION 
In the treatment of water to purify it for discharge into sewage systems, 
streams and the like, it is necessary or desirable to remove contaminants 
even when the contaminants are contained in very small concentrations such 
as a few hundred parts per million or less. In many situations industrial 
waste waters may contain small quantities of heavy metals such as copper, 
iron, lead, zinc, nickel, silver or gold with the concentration thereof 
varying from time to time over a substantial range. For example, the 
concentration of copper in an aqueous bath exiting a chrome plating system 
may vary from ten parts per million to thirty parts per million. Since, 
however, the concentration will peak at thirty parts per million only for 
brief periods, the prior art metal removal processes have not performed in 
an efficient manner when designed to function at the maximum concentration 
levels. Nevertheless, it is important that whatever process is used does 
not permit the heavy metals to be discharged even during the brief periods 
of maximum concentration. 
It is known in the prior art that heavy metals can be precipitated out of 
aqueous solutions by placing the solution in a settling tank and adding 
insoluble starch-xanthate (ISX) to the solution. The ISX reacts with the 
heavy metal ions in solution to cause the heavy metals and the ISX to 
co-precipitate out of solution. The ISX may be prepared in any suitable 
manner such as by any of the methods described in U.S. Pat. Nos. 3,979,286 
and 4,051,316. Japanese patent 46-39544 describes such a heavy metal 
removal process wherein the aqueous solution is treated with ISX. A prime 
disadvantage of the prior art processes of this type has been the need for 
holding tanks, settling basins or the like. 
In my copending application, Ser. No. 961,238 filed Nov. 16, 1978, now 
abandoned there is disclosed a heavy metal removal process wherein a 
mixture of ISX and a filter-aid, such as diatomaceous earth, is coated on 
a filter element to form a filter cake through which the aqueous solution 
containing the heavy metals is passed. As the solution flows through the 
cake the heavy metal ions become attached to the active sites on the 
starch-xanthate and are thus removed from the solution. This process is 
better than the other prior art metal removal processes using ISX in that 
it may be used with a continuously flowing stream since no holding tanks, 
settling basins or the like are required. However, where the concentration 
of heavy metals varies, the amount of ISX deposited on the filter elements 
must be sufficient to react with all of the metal contained in the 
solution during the periods of maximum concentration. Consequently, this 
process necessitates the use of a greater amount of ISX than is required 
with the batch type processes of the prior art which operate on the 
average concentration of heavy metals in the solution being treated. 
It is also known in the prior art that reactant material such as 
ion-exchange resins can be precoated on a filter element so as to react 
with a liquid solution passed through the resin precoat as the liquid 
flows through the filter element. U.S. Pat. No. 3,250,704 describes one 
such system which used a resin precoat in a water purfication process. 
The precipitation processes known in the prior art and discussed above are 
not affected by momentary increases in concentration, peaks, as they are 
called, inasmuch as they are batch processes operating on the average 
concentration of the batch. However, where the water is purified as it 
continuously flows through the system as in the process described in my 
said copending application, it is absolutely necessary that the process 
functions to remove substantially all of the heavy metal during the peak 
concentration periods if the minimum discharge requirements are to be met. 
This can, of course, be achieved by the use of sufficient precoat volume 
to provide the necessary reaction during peak concentrations, but a 
considerably greater quantity of the insoluble starch-xanthate and 
filter-aid and a considerably greater filter area must be used than would 
be required to remove all of the heavy metal during the average or usual 
periods of concentration. 
SUMMARY OF THE INVENTION 
Briefly, there is provided in accordance with the present invention a new 
and improved process which utilizes insoluble starch-xanthate to remove 
heavy metals from an aqueous solution by flowing the solution through a 
filter coated with a mixture of ISX and filter-aid by continuously adding 
ISX and filter-aid to the aqueous solution upstream of the filter. The 
amount of ISX added upstream of the filter is at least sufficient to 
remove all of the heavy metal ions from the solution during periods of 
minimum concentration but preferably during periods of about average 
concentration. During those periods when the heavy metal concentration is 
in excess of the capacity of the amount of ISX being added to the solution 
to capture all of the heavy metal, some free metal ions flow through the 
cake of ISX and filter-aid on the filter and are thereby removed from the 
solution. As will be apparent to those skilled in the art as this 
description of the invention proceeds, this process results in a 
substantial savings in the cost of the ISX used to remove a given amount 
of heavy metal from solution and to subsequently dispose of the spent ISX 
and filter-aid. 
Inasmuch as the ISX in the filter cake functions to remove the heavy metal 
ions only during the brief periods of peak concentration, the precoat cake 
may be relatively thick. Also, since the metal removal process occurs 
primarily while the ISX is freely floating in the solution, the effective 
capabity of a given amount of ISX to remove heavy metal ions is 
appreciably greater than would be the case if all the ISX were contained 
in the filter cake where many of its active sites would be occluded. 
The process of the present invention thus enables the removal of heavy 
metals from a continuous stream of an aqueous solution in an economically 
efficient manner. This metal removal process may be carried out on a 
continuous basis in conjunction with other continuous treating processes. 
DETAILED DESCRIPTION OF THE INVENTION 
For simplicity of description my process will be described particularly in 
connection with the removal of copper from a continuously flowing stream 
of water containing copper in concentrations which vary from about 10 
p.p.m. to 30 p.p.m. and which averages about 20 p.p.m. It will be 
understood by those skilled in the art, however, that the process of the 
present invention may be used to remove other heavy metals such as zinc, 
cadmium, lead, chromium, nickel, iron, silver and gold, wither alone or in 
combination, from aqueous solutions as well as to remove heavy metals 
contained in the water in other small concentrations. 
Insoluble starch-xanthate, or ISX as it is commonly known, and hereinafter 
referred to, is a crosslinked-starch-xanthate which is insoluble in water 
and which contains reactive sites for attachment to heavy metal ions. U.S. 
Pat. Nos. 3,947,534, 3,979,286 and 4,051,316 describe several such 
insoluble starch-xanthate and methods of manufacturing them. The process 
of the present invention may be carried out by using any of these 
particular starch-xanthate, but the invention is not so limited. 
The filter used in the process of my invention may be of any suitable type 
having a pressure tank in which tubular filter elements, pressure leaf 
filter elements or other precoatable filter elements are mounted, and 
through which the liquid to be filtered flows. A mixture of insoluble 
starch-xanthate and filter-aid, such as diatomaceous earth, is initially 
circulated through the filter to cause a substantially homogeneous mixture 
of the insoluble starch-xanthate and the filter-aid to be deposited on the 
filter elements in the form of a porous cake. Thereafter, any subsequent 
flow of liquid through the filter necessarily flow through the precoat 
cake. I have found that the precoat cake should preferably have a 
thickness of at least one centimeter to provide a stable cake which does 
not crack or shift on the filter element under normal operating 
conditions. However, cake thicknesses of two and one-half centimeters or 
more may be used depending on the system parameters such as flow rates and 
range of heavy metal concentrations. 
After the filter elements have been precoated with the mixture of 
filter-aid and insoluble starch-xanthate the filter is connected on stream 
so that the water containing the heavy metal continuously flows through 
the precoat cake. In addition, a mixture of filter-aid and insoluble 
starch-xanthate is continuously added to the stream of water upstream of 
the precoat to remove heavy metal ions from the water before they reach 
the precoat cake. The amount of insoluble starch-xanthate which is thus 
body fed to the stream of water is at least sufficient to react with all 
of the heavy metal in the water during periods of minimum concentration. 
Even greater economies of operation can be achieved by body feeding into 
the stream an amount of insoluble starch-xanthate sufficient to react with 
all of the heavy metal contained in the water during periods of average 
concentration. In this process the body-fed insoluble starch-xanthate 
forms a complex with the heavy metal ions and thus captures all of the 
heavy metal ions from the water in the stream except during the brief 
periods of maximum concentration, and the insoluble starch-xanthate and 
the attached heavy metal ions together with the filter-aid are deposited 
over the precoat cake in the form of a porous cake. During the periods of 
peak concentration some heavy metal ions remain in solution until they 
enter the precoat layer where they become attached to the exposed active 
sites of the insoluble starch-xanthate in the precoat layer. In this 
manner, the water exiting the filter has been purified by the removal of 
substantially all heavy metal ions from the water irrespective of the 
substantial variation in the heavy metal concentration. 
The efficiency of this process is substantially improved by mixing 
filter-aid with the insoluble starch-xanthate applied to the filter 
element to form a precoat. The filter-aid performs two important functions 
in the precoat cake; one, it increases the porosity of the cake to permit 
better water flow through the cake, and two, it exposes more active sites 
on the insoluble starch-xanthate. The latter function thus enables the use 
of a lesser amount of insoluble starch-xanthate than would otherwise be 
required. When the filter-aid is not used, many of the active sites on the 
insoluble starch-xanthate are occluded. Nevertheless, a still greater 
efficiency of use occurs when the reaction takes place upstream of the 
filter cake. I believe the reason for this is that no active sites on the 
insoluble starch-xanthate are occluded while the insoluble starch-xanthate 
is floating in the water. However, filter-aid should nevertheless be used 
in the body feed to maintain the filter cake sufficiently porous so that 
it does not appreciably restrict the flow of water therethrough. It may be 
seen, therefore, that a more complete use of the insoluble starch-xanthate 
is achieved when the amount thereof being fed body fed reacts with all of 
the heavy metal in solution during periods of average concentration since, 
except during brief periods of maximum concentration, all of the heavy 
metal ions have reacted with the body-fed insoluble starch-xanthate before 
reaching the filter cake. During periods of minimum concentration, some 
unspent insoluble starch-xanthate is, of course, deposited on the filter 
cake, but being interspersed with the filter-aid it is available for 
complexing with heavy metal ions during subsequent periods of maximum 
concentration. 
I have found that the prefilt cake should preferably contain a filter-aid 
such as diatomaceous earth. The ratio of diatomaceous earth to ISX should 
be in the range of 1.75 to 3.0 but preferably between 1.75 and 2.0. I have 
also found that filter-aid should be added along with the ISX upstream of 
the filter in this same proportion. 
When calculating the optimum rate at which the ISX should be fed into the 
stream and the optimum amount of ISX to be coated on the filter, it will 
be desirable in some cases to take into account the durations of the 
minimum and peak concentrations. For convenience, however, calculations 
based on the average concentration will give results which are superior to 
simply body feeding the requisite amount of ISX to remove all of the heavy 
metal at all times or to use only a filter cake containing the requisite 
amount of ISX to remove all of the heavy metal at all times. 
If all of the requisite insoluble starch-xanthate were deposited as a 
precoat cake or if lesser amounts thereof were periodically added to the 
stream to form successive active layers of filter cake for reacting with 
the heavy metal in solution, a greater amount of the insoluble 
starch-xanthate would be required because of the fact that some of the 
active sites will necessarily be occluded within the cake. I have found 
that the capacity of the insoluble starch-xanthate to remove heavy metals 
from an aqueous solution is about fifteen to twenty percent greater when 
the starch-xanthate particles are suspended in the aqueous solution being 
treated as compared to the particles being mixed with filter-aid at a 
ratio of one part insoluble starch-xanthate to two parts filter-aid and 
deposited as a filter cake through which solution flows. The capacity of 
the precoat to remove heavy metals is much lower if the filter-aid is not 
used. 
When the filter cake is formed by a mixture of over three parts filter-aid 
to one part insoluble starch-xanthate, the effective capacity of the 
insoluble starch-xanthate to remove heavy metals from solution is also 
lower, and the use of the extra amount of filter-aid is uneconomical. 
The following examples are intended to supplement the disclosure of the 
invention and are not to be construed as limiting the scope of the 
invention as defined by the appended claims.

EXAMPLE 1 
The following test was conducted to determine the relative amounts of ISX 
and filter-aid to be contained in the precoat layer for most efficient 
operation. 
Mixtures of one part ISX to one and one-half parts diatomite (diatomaceous 
earth) were deposited on a one-half square foot filter and found to 
provide unstable cakes which cracked after a short time of use. While it 
might be possible to deposit a layer of filter-aid over or under the 
precoat to avoid the problems associated with cake cracking, it was found 
that a mixture of 1.75 parts of diatomite to one part ISX provided a 
stable, satisfactory cake. Tests of Mixtures of two parts diatomite to one 
part ISX and three parts diatomite to one part ISX also provided stable, 
satisfactory cakes. However, as the ratio of diatomite to ISX is increased 
the problem of cake disposal also increases. Consequently, it appears that 
mixtures in the range of 1.75 to two parts filter-aid to one part ISX are 
preferable. 
EXAMPLE 2 
This test was conducted to determine the results achieved by using washed 
and unwashed ISX in the process of the present invention. Washed ISX was 
deposited on a bench filter while the flow rate through the filter was 
monitored. It was found that the flow rate was reduced by about eight 
percent when the cake thickness was one inch. Repeating the same test with 
unwashed ISX resulted in a reduction in flow rate of twenty-eight percent. 
However, tests using washed and unwashed ISX mixed with diatomite in the 
ratio of one part ISX to two parts diatomite, showed no significant 
difference in flow rates between washed and unwashed ISX. It therefore 
appears preferable to use unwashed ISX in carrying out the process of the 
present invention because the washing step can be eliminated. 
EXAMPLE 3 
The capacity or effectiveness of the ISX to remove heavy metals from 
solution when used in a precoat as compared to adding the ISX to the 
solution was determined in the following manner. 
One gram of ISX was added to 930 mililiters of an aqueous solution having a 
concentration of copper ions of 142 parts per million and the amount of 
copper remaining in solution about one minute thereafter was measured and 
found to be 24 parts per million. When the same amount of ISX mixed with 
diatomite was deposited as a one-inch thick precoat on a bench filter and 
930 milileters of the same aqueous solution was passed through the 
precoat, it was found that the capacity of the ISX to remove copper was 
about fifteen to twenty percent less, i.e., the copper concentration in 
the effluent reached 24 parts per million when between 745 milileters and 
840 milileters had passed through the cake. It will be understood that 
these tests were repeated several times. 
EXAMPLE 4 
Tests were conducted to determine the capacity of ISX to remove heavy 
metals from aqueous solutions. 
An aqueous solution containing 115 ppm of nickel was passed through a 
precoat cake one inch thick containing a mixture of 4.0 gms. of diatomite 
and 2.0 gms. of ISX at a flow rate of 1.5 gallons per minute. The 
concentration of nickel in the effluent was monitored. The capacity of ISX 
to absorb nickel was calculated to be 29.0 mg. in per gm. of ISX. 
An aqueous solution containing 115 ppm of zinc was passed through a precoat 
cake one inck thick containing a mixture of 4.0 gms. of diatomite and 2.0 
gms. of ISX at a flow rate of 1.5 gallons per minute. The concentration of 
zinc in the effluent was monitored. The capacity of ISX to absorb zinc was 
found to be 29.0 mg. of zinc per gm. of ISX. 
Aqueous solutions containing 117 ppm of copper were passed through a 
precoat one inch thick containing a mixture of 4.0 gms. of diatomite and 
2.0 gms. of ISX at flow rates of 2.0 gallons per minute, 2.5 and 3.0 
gallons per minute. The concentration of copper in the effluent was 
monitored, and the capacity of ISX to absorb copper was found to be about 
106 gms. of copper per gm. of ISX. 
The results of these tests indicate that ISX is effective in removing all 
heavy metals from aqueous solutions but has a greater capacity for 
absorbing copper than it does for absorbing zinc and nickel. 
While the present invention has been described in connection with 
particular embodiments thereof, it will be understood by those skilled in 
the art that many changes and modifications may be made without departing 
from the true spirit and scope of the present invention. Therefore, it is 
intended by the appended claims to cover all such changes and 
modifications which come within the true spirit and scope of this 
invention.