Metal scavengers and processes for the production thereof

The present invention is intended to provide a metal scavenger that is free from the defects of metal scavengers formed by introducing dithioic acid groups to polyalkylenepolyamines or the like, is high in settling speed of flock when used in waste water treatment, and can carry out treatment of waste water efficiently and also to provide a process of producing the same efficiently. The present metal scavenger has a structure that is formed by replacing the hydrogen atoms linked to the nitrogen atoms of a polyamine having aminomethyl groups by dithioic acid groups or their salts and that is represented by the following general formula: ##STR1## wherein R represents a dithioic acid group, an alkali metal dithioate group, an alkaline earth metal dithioate group, or an ammonium dithioate group and R' represents a hydrogen atom, a dithioic acid group, an alkali metal dithioate group, an alkaline earth metal dithioate group, or an ammonium dithioate group. The present metal scavenger can be obtained by causing carbon disulfide to act on a polyamine having aminomethyl groups or by polymerizing a monomer having the following formula: ##STR2## wherein R represents a dithioic acid group, an alkali metal dithioate group, an alkaline earth metal dithioate group, or an ammonium dithioate group and R' represents a hydrogen atom, a dithioic acid group, an alkali metal dithioate group, an alkaline earth metal dithioate group, or an ammonium dithioate group.

BACKGROUND OF THE INVENTION 
1) Field of the Invention 
The present invention relates to metal scavengers and processes for the 
production thereof. 
2) Description of the Related Art 
There are strict regulations concerning metals in waste water, particularly 
heavy metals harmful to the human body, such as mercury, cadmium, zinc, 
lead, copper, and chromium in waste water various techniques of removing 
metals contained in waste water and the like have been studied and used. 
The neutralization coagulation settling technique wherein an alkali 
neutralizing agent such as slaked lime and sodium hydroxide is added to 
waste water to convert metals in the waste water into the hydroxides and 
then the hydroxides are coagulated and settled with a polymer coagulant is 
widely employed for treating waste water. 
However, this technique results in a large amount of sludge of metal 
hydroxides and it is quite difficult by this technique to remove metals in 
waste water to or below the extent standardized by this country. Further, 
there is a problem that since the above sludge of metal hydroxides is 
dehydrated poorly and is large in volume, the handlability when the sludge 
is handled (e.g., transported) is poor. Further, there is also a problem 
that sometimes the way of discarding the sludge causes the metals to 
redissolve in rivers and the sea, resulting in secondary pollution. 
Besides the above neutralization coagulation settling technique, for 
example, the ion flotation method, the ion exchange method, the 
electrolysis flotation method, the electrodialysis method, the adsorption 
method, and the reverse osmosis method are also known for the treatment of 
heavy metals, actually they are used only for some special cases of 
treatment of waste water in view, for example, of the removal rates of 
heavy metals, the operability, and the running cost. 
Therefore, recently, in place of these methods, a method of collecting and 
removing metals with a metal scavenger is being adopted and the present 
applicant made various suggestions wherein use is made of a metal 
scavenger having a structure formed by linking at least one dithioic acid 
group as a functional group to the nitrogen atoms of a 
polyalkylenepolyamine or a polyethyleneimine or a method of treating water 
water or the like with that metal scavenger (e.g., Japanese Patent 
Application Laid-Open Nos. 17128/1985 and 81478/1987). 
The metal collecting and removing method previously suggested by the 
present applicant, in comparison with other methods wherein use is made of 
other metal scavengers, has an advantage capable of efficient waste water 
treatment since the settling speed of the flock formed by collecting 
metals is high and the process from the addition of the metal scavenger to 
the separation and removal of the flock can be carried out in a short 
period of time. However, in practical treatment of waste water, since it 
is required to improve the efficiency of separation of flock and to 
decrease efficiently and positively metals remaining in the waste water 
separated from the flock, it is demanded to settle the flock in a short 
period of time as far as possible, but that demand cannot be satisfied 
completely when any of the conventional metal scavengers is added alone to 
the waste water. 
Therefore, conventionally, a metal scavenger is added together with a 
polymer coagulant to waste water or a method is used wherein a metal 
scavenger is crosslinked with a crosslinking agent such as epichlorohydrin 
so that the metal scavenger may have a higher molecular weight to increase 
the settling speed of flock (e.g., Japanese Patent Application Laid-Open 
No. 249590/1986). 
However, there is a problem that when a polymer coagulant is used, the 
amount of the formed flock increases, leading to troublesome 
post-treatment of the flock. Further, the process of producing the metal 
scavenger crosslinked with epichlorohydrin or the like is complicated and 
the production efficiency and the yield are likely lowered. 
Further, in the case of the conventional metal scavenger formed by 
introducing dithioic acid groups or their salts as functional groups to 
nitrogen atoms of polyamines such as a polyalkylenepolyamine and a 
polyethyleneimine, there is also a problem that the number of the 
functional groups is small in comparison with the number of the nitrogen 
atoms of the polyamines and therefore a relatively large amount of the 
metal scavenger must be used to collect and remove positively metals in 
waste water. 
The present invention has been made taking the above points into 
consideration and an object of the present invention is to provide a metal 
scavenger having excellent metal collecting performance that, for example, 
makes the settling speed of flock high and allows easy and positive waste 
water treatment without increasing the molecular weight by crosslinking 
with epichlorohydrin or the like or without using a polymer coagulant 
additionally and to provide a process of producing the same. 
The metal scavenger of the present invention has a structure that is formed 
by replacing the hydrogen atoms linked to the nitrogen atoms of a 
polyamine having aminomethyl groups by dithioic acid groups or their salts 
and that is represented by the following general formula (1): 
##STR3## 
wherein R represents a dithioic acid group, an alkali metal dithioate 
group, an alkaline earth metal dithioate group, or an ammonium dithioate 
group and R' represents a hydrogen atom, a dithioic acid group, an alkali 
metal dithioate group, an alkaline earth metal dithioate group, or an 
ammonium dithioate group. 
The present metal scavenger has as functional groups dithioic acid groups 
or dithioate groups and, as the dithioate groups, dithioate groups of 
alkali metals such as lithium, sodium, and potassium, alkaline earth 
metals such as beryllium, magnesium, and calcium, and ammonia can be 
mentioned. 
Although the molecular weight of the present metal scavenger may widely 
range from about 300 to 1,000,000, preferably the molecular weight is 
1,000 to 500,000. 
The backbone of the polyamine is a straight carbon chain consisting only of 
carbon-carbon linkages as shown in the formula (1). In the formula, "n" is 
a whole number having a value such that the aforementioned molecular 
weights are obtained. 
The present metal scavenger can be prepared by causing carbon disulfide to 
act on a polyamine with aminomethyl groups or by causing carbon disulfide 
to act on allylamine to obtain a monomer (mono- or bis-(dithioic) 
allylamine or its salt) formed by replacing the hydrogen atom(s) linked to 
the nitrogen atom of the allylamine by a dithioic acid(s) or its salt and 
having the following formula (2): 
##STR4## 
wherein R represents a dithioic acid group, an alkali metal dithioate 
group, an alkaline earth metal dithioate group, or an ammonium dithioate 
group and R' represents a hydrogen atom, a dithioic acid group, an alkali 
metal dithioate group, an alkaline earth metal dithioate group, or an 
ammonium dithioate group, 
and polymerizing the monomer. 
The polyamine with aminomethyl groups used in the first process of 
producing the present metal scavenger by causing carbon disulfide to act 
on a polyamine with aminomethyl groups can be produced by the following 
processes. 
The first method of producing a polyamine with aminomethyl groups is a 
process wherein a polyacrylonitrile, a polyacrylamide, or a 
polymethacrylamide is reduced. A polyacrylonitrile can be reduced by 
hydrogenation in the presence of a hydrogenation catalyst, and 
specifically a process can be mentioned wherein a polyacrylonitrile is 
dissolved in a solvent such as dimethylformamide or dimethylsulfoxide and 
is hydrogenated under a high pressure (about 3.5 kg/cm.sup.2) by using 
Raney nickel as a catalyst. For the reduction of a polyacrylamide or a 
polymethacrylamide, a process can be used wherein a polyacrylamide or a 
polymethacrylamide is dissolved in a solvent such as tetrahydrofuran, 
dimethylformamide, and dimethylsulfoxide and is reduced with a metal 
alcoholate such as sodium methylate or lithium aluminum hydride. 
The second method of producing a polyamine with aminomethyl groups is a 
process of polymerizing allylamine. The process of polymerizing allylamine 
comprises converting allylamine, for example, to its hydrochloride and 
polymerizing it by using a redox type initiator or a radical 
polymerization initiator. In this case, allylamine may be polymerized 
singly or may be copolymerized with other monomer having a copolymerizable 
unsaturated bond such as acrylic acid, methacrylic acid, styrenesulfonic 
acid, their salts, dimethylacrylamide, styrene, methyl methacrylate, 
methyl acrylate, acrylamide, and methacrylamide. However, in the case of 
copolymerization, preferably the allylamine is present in an amount of 5 
mol % or over, particularly 15 mol % or more. 
To introduce a dithioic acid group which will replace the hydrogen atom 
linked to the nitrogen atom of each aminomethyl group of the polyamine 
with aminomethyl groups, the polyamine with aminomethyl groups is 
dissolved in a solvent such as water or an alcohol, carbon disulfide is 
added thereto, and they are reacted at 10.degree. to 100.degree. C. for 
about 1 to 20 hours. 
In this way, a metal scavenger having a structure wherein one or two 
hydrogen atoms linked to the nitrogen atom of each aminomethyl group are 
replaced by dithioic acid groups can be obtained. The reaction ratio of 
the polyamine with aminomethyl groups to the carbon disulfide varies 
depending on the number of dithioic acid groups to be introduced into the 
molecule and in the case wherein the amount of carbon disulfide used is 
one equivalent weight or less to the nitrogen atoms in the molecule of the 
polyamine with aminomethyl groups, one or less dithioic acid group on 
average is introduced to each of the nitrogen atoms of the aminomethyl 
groups of the polyamine. 
On the other hand, in the case wherein the amount of carbon disulfide used 
is one equivalent weight or more to the nitrogen atoms in the molecule of 
the polyamine with aminomethyl groups, one or more dithioic acid groups on 
average are introduced to each of the nitrogen atoms of the aminomethyl 
groups of the polyamine. Even in the case wherein one or more dithioic 
acid groups on average are introduced to each of the nitrogen atoms of the 
aminomethyl groups of the polyamine, preferably the amount of carbon 
disulfide to be used is 2 equivalent weights or less to the nitrogen atoms 
in the polyamine molecule. 
Dithioic acid groups as substituent groups are introduced to the nitrogen 
atoms of the aminomethyl groups of a polyamine as mentioned above, and 
optionally after the reaction, the resulting polyamine is treated with an 
alkali such as an alkali metal hydroxide, for example, sodium hydroxide or 
potassium hydroxide, or an alkaline earth metal hydroxide, for example, 
calcium hydroxide or magnesium hydroxide, or ammonia, so that the dithioic 
acid groups may be converted, for example, to the alkali metal salts, 
alkali earth metal salts, or ammonium salts. Alternatively, the above 
reaction between the polyamine and carbon disulfide may be carried out in 
the presence of the above alkali to introduce the alkali metal salts, 
alkali earth metal salts, or ammonium salts of dithioic acid groups. 
In the present second process wherein allylamine is interacted with carbon 
disulfide so that the hydrogen atoms linked to the nitrogen atom of the 
allylamine may be replaced by dithioic acid groups or their salts and the 
resulting product is polymerized to obtain a polymer, allylamine and 
carbon disulfide are reacted in the same way as in the above-described 
reaction between a polyamine and carbon disulfide, thereby obtaining a 
monomer (mono- or bis-(dithioic) allylamine) represented by the above 
formula (2). 
In this case, also, after the reaction, the product may be treated with 
such an alkali as mentioned above, for example, with an alkali metal 
hydroxide, an alkali earth metal hydroxide, or ammonia, or the reaction 
with carbon disulfide may be carried out in the presence of such an 
alkali, so that the dithioic acid group can be converted to its alkali 
metal salt, alkali earth metal salt, or ammonium salt. 
Then, the monomer (mono- or bis-(dithioic) allylamine) formed by replacing 
the hydrogen atom(s) linked to the nitrogen atom of allylamine by a 
dithioic acid group(s) or a dithionate group(s) is homopolymerized or 
copolymerized with other unsaturated monomer such as acrylic acid, 
methacrylic acid, styrenesulfonic acid, their salts, dimethylacrylamide, 
styrene, methyl methacrylate, methyl acrylate, acrylamide, or 
methacrylamide. Preferably, the copolymerization is carried out such that 
the content of the allylamine monomer having a dithioic acid group(s) as a 
substituent(s) is 5 mol % or more, more preferably 15% or more. 
The above polymerization reaction can be carried out in a solvent such as 
water, dimethylformamide, and toluene in the presence of a water-soluble 
polymerization initiator such as 2,2'-azobis(2-amidinopropane) 
hydrochloride, potassium persulfate, and ammonium persulfate or a radical 
polymerization initiator such as acryloisobutyronitrile and benzoyl 
peroxide at 40.degree. to 100.degree. C. 
When waste water is treated with the present metal scavenger,the present 
metal scavenger can be used alone, but when a sodium sulfide such as 
sodium hydrogensulfide, sodium monosulfide, a sodium polysulfide, for 
example, sodium disulfide, sodium trisulfide, sodium tetrasulfide, and 
sodium pentasulfide is used additionally, an excellent metal collecting 
effect can be obtained. 
The present metal scavenger is suitable for treating metals in waste water 
discharged, for example, from plating plants, incineration plants, 
laboratories, hospitals, industrial waste disposal processing plants, and 
electronic part plants as well as suitable for treating metals contained 
in incineration and flying ash discharged from incineration plants, metals 
contained in slag discharged from mines and metal refineries, metals 
contained in sludge discharged from waste water treatment plants, and 
metals contained in solid waste such as contaminated soil. The present 
metal scavenger can also be used in scavenging metals contained in exhaust 
gases discharged from incineration plants. 
To treat waste water, for example, after the pH of the waste water is 
suitably adjusted (generally to 3 to 12), the present metal scavenger is 
added to the waste water, the formed flock is settled to separate from the 
waste water and is removed, and the waste water is discharged. To treat 
solid waste, for example, the present metal scavenger is added into the 
solid waste to immobilize and insolubilize metals contained in the solid 
by the metal scavenger, and then the solid waste is solidified with 
concrete and is discharged into the sea or is used for reclamation.

EXAMPLES 
Now, the present invention will be described in detail with reference to 
the following Examples. 
Example 1 
200 g of polyacrylonitrile was dissolved in 800 g of dimethylformamide, 
Raney nickel as a hydrogenation catalyst was added, and the hydrogenation 
was carried out under conditions having a hydrogen pressure of 3.5 
kg/cm.sup.2, thereby obtaining a polyamine having aminomethyl groups 
(yield: 90%; average molecular weight: 20,000). 55 g of this polyamine, 
400 g of pure water, and 40 g of sodium hydroxide were charged into a 
four-necked flask with a reflux condenser, and after 76 g of carbon 
disulfide was added dropwise thereto over 4 hours with the temperature 
kept at 40.degree. C. with stirring, the reaction was continued by heating 
for 10 hours at 80.degree. C. with stirring to obtain an aqueous solution 
containing a metal scavenger. 
Waste water containing 20 ppm of copper(II), 15 ppm of zinc, 10 ppm of 
nickel, and 2 ppm of mercury was treated with the thus obtained aqueous 
metal scavenger solution. The aqueous metal scavenger solution was added 
in such an amount that 50 mg of the metal scavenger was for 1 liter of the 
waste water, and after the mixture was stirred for 10 min, the mixture was 
allowed to stand, and the time required for the flock to settle was 
measured, which is shown in Table 1. The settled flock was separated and 
removed and the concentrations of metals remaining in the waste water were 
measured. The results are shown in Table 1. 
TABLE 1 
__________________________________________________________________________ 
Time required for the 
Concentration of remaining metal ions (ppm) 
flock to settle (min) 
Copper (II) 
Zinc 
Nickel 
Mercury 
__________________________________________________________________________ 
Example 
1 4 &lt;0.5 2.0 1.5 0.003 
2 3 &lt;0.5 0.5 1.0 0.002 
3 4.2 &lt;0.5 2.5 1.6 0.003 
4 2.9 &lt;0.5 0.5 0.9 0.001 
Comparative 
1 5 1.2 3.0 2.9 0.010 
Example 
2 8 1.8 5.0 3.5 0.015 
__________________________________________________________________________ 
Example 2 
100 g of polyacrylamide was dispersed into 900 g of tetrahydrofuran and was 
reduced in the presence of 80 g of sodium methylate to obtain a polyamine 
having aminomethyl groups (yield: 90%; average molecular weight: 10,000). 
55 g of this polyamine, 40 g of pure water, and 60 g of sodium hydroxide 
were placed in a four-necked flask with a reflux condenser, and after 114 
g of carbon disulfide was added dropwise over 4 hours with the temperature 
kept at 40.degree. C. with stirring, the reaction was continued for 10 
hours by heating at 80.degree. C. with stirring to obtain an aqueous 
solution containing a metal scavenger. 
The same waste water as that used in Example 1 was treated with the thus 
obtained aqueous metal scavenger solution. The results are also shown in 
Table 1. 
Example 3 
28.5 g of a polyamine (having an average molecular weight of 30,000) 
obtained by polymerizing allylamine, 775 g of pure water, and 14 g of 
sodium hydroxide were placed in a four-necked flask with a reflux 
condenser, then 26.6 g of carbon disulfide was added dropwise over 6 hours 
with the temperature kept at 30.degree. C. with stirring, and the reaction 
was continued for 5 hours by heating at 60.degree. C. with stirring to 
produce an aqueous solution containing a metal scavenger. 
The same waste water as that used in Example 1 was treated with the thus 
obtained aqueous metal scavenger solution. The results are also shown in 
Table 1. 
Example 4 
57 g of allylamine, 80 g of sodium hydroxide, and 400 g of pure water were 
placed in a four-necked flask with a reflux condenser, then 152 g of 
carbon disulfide was added dropwise over 4 hours with the temperature kept 
at 20.degree. C. with stirring, and the reaction was continued for 10 
hours by heating at 80.degree. C. with stirring to produce mono (dithioic) 
allylamine sodium salt. Then 60 g of this monomer was dissolved in 40 g of 
pure water, 1 g of 1-[(1-cyano-1-methylethyl)azo] formamide was added 
thereto, and the polymerization was carried out at 100.degree. C. for 20 
hours under a flow of nitrogen to produce an aqueous metal scavenger 
solution. 
The same waste water as that used in Example 1 was treated with the thus 
obtained aqueous metal scavenger solution. The results are also shown in 
Table 1. 
Comparative Example 1 
430 g of an aqueous solution containing 10% of a polyethyleneimine (having 
an average molecular weight of 100,000) and 240 g of 5% sodium hydroxide 
were placed in a four-necked flask with a reflux condenser, then 22.8 g of 
carbon disulfide was added dropwise over 2 hours at 32.degree. C. with 
stirring, and the reaction was continued for 6 hours at 50.degree. C. to 
produce an aqueous solution containing a metal scavenger formed by 
introducing sodium dithioate groups as substituents to the nitrogen atoms 
of the polyethyleneimine. 
The same waste water as that used in Example 1 was treated with the thus 
obtained aqueous metal scavenger solution. The results are also shown in 
Table 1. 
Comparative Example 2 
An aqueous solution obtained by dissolving 11 g of sodium hydroxide into 
110 g of a 10% aqueous sodium hypochlorite solution was added dropwise to 
350 g of an aqueous solution containing 10% of a polyacrylamide (having an 
average molecular weight of 4,500) with the temperature kept at 50.degree. 
C. and then the reaction was continued for 30 min at that temperature to 
produce an aqueous solution containing a polyamine having amino groups. 50 
g of 10% sodium hydroxide was added to the thus obtained aqueous solution, 
then 11.2 g of carbon disulfide was added dropwise thereto over 3 hours at 
30.degree. C. with stirring, and the reaction was continued for 5 hours at 
70.degree. C. to obtain an aqueous solution of a metal scavenger having 
sodium dithioate groups as substituents. 
The same waste water as that used in Example 1 was treated with the thus 
obtained aqueous metal scavenger solution. The results are also shown in 
Table 1. 
Effects of the Invention 
As described above, when the present metal scavenger is used for treatment 
of waste water or the like, without using a polymer coagulant 
additionally, the settling speed of the formed flock is high and the 
treatment of waste water can be performed efficiently. Further, since the 
number of functional groups in the metal scavenger can be increased 
easily, even if the amount of the metal scavenger used is small, metals 
can be collected and removed efficiently. 
Further, according to the present process, a metal scavenger excellent in 
settling property of flock and collecting property of metals can be 
obtained even if the molecular weight of that metal scavenger is 
approximately the same as that of conventional metal scavengers and a 
metal scavenger having excellent properties can be produced efficiently 
since, for example, it is not required to increase the molecular weight by 
carrying out troublesome steps such as crosslinking with epichlorohydrin.