Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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REMOVAL OF IRON FROM CHELANT SOLUTIONS
This invention pertains to a procPss for
removing dissolved iron from an a~ueous li~uid con-
taining i.ron chelated by alkylenepolyamine polyacetic
acid. More particularly, this invention pertains to
a treatment process for removing dissolved iron from
certain liquid waste.
The alkylenepolyamine polyacetic acid chelan-ts
form a known class of compounds having many members.
The most common of these are ethylenediaminetetraacetic
acid (EDTA) and diethylenetriaminepentaacetic acid
(DEPA); and of these, EDTA is by far the most widely
used compound on a commercial scale. Because -the
materials are relatively insoluble in the acid form,
such compounds are normally used as their soluble salts.
The alkali metal (e.g. sodium) salts and -the ammoniated
salts are the best known.
There are many chelant uses for the alkylene-
polyamine polyacetic acids, but one such u~ility is in
cl~aning iron oxide containing scale from steam gener-
ating equipment. In almost any type of metal equipment
29,653-F -l-
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in which water is evaporated or heat transfer occurs,
insoluble salts deposit upon the surface to form a
scale. The composition of the scale will vary
depending upon the water which has been used, the
5 type of equipment, the operating temperature, etc.
The scale may be very dense or coa.rse, tightly bound
ko the base metal or not. In most steam generating
units, the scale usually consists of o~ides of iron,
which include magnetite together with red iron oxide.
And when the steam generating unit is fabricated
using parts containing copper or copper alloys, as
for example, in the condenser units, the scale will
normally contain copper and/or contains copper
oxide(s~.
Several investigators have addressed the
problem of scale removal. Lesinksi described in
U.S. Patent No. 3,308,065 a unique way of removing
iron oxide containing scale from ferrous metal sur~
faces and for passivating the clean surface. Lesinski
discovered that ammoniated salts of alkylenepolyamine
polycarbo~ylic acids were particularly efficien-t in
scale removal when used at an alkaline pH (generally
above about a pH of 8). Ammoniated EDTA used at a
pH of from 8 to ll, preferably about 9, was said to
be a preferred solvent.
Harriman et al. in U.S. Patent No. 3,438,811
discovered that an aqueous solution of a ferric chelate
of an alkylenepolyamine polycarboxylic acid at an
alkaline pH was unusually effective in removing copper-
-containing encrus-tations of elemental ccpper from
ferrous metal surfaces.
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Teumac ln U.S. Patent No. 3,413,160 then
descxlbed a method of passivatlng ferrous metal sur-
faces which had been cleaned using an alkylenepoly
amine polycarboxyl1c acid at an alkaline pH to remove
5 iron-oxide containing scale and/or copper encrusta~
tions from steam generating equipment. In this pro
cess, an o~idlzing agent was added at the end of the
cleanlng process and the oxidation potential of the
aqueous solution was carefully monitored to achieve
the desired deyree of passlvation.
The technology described by Lesinski,
Harrlman, and Teumac represent the state of the art.
All use alkylenepolyamine polycarboxylic acids or
sal~s ~hereof at an alkaline p~ and all generate
waste solutions which contain dissolved iron as an
iron chelate with the alkylenepolyamin~ polycar-
boxylic acids. ~nd, in most instances, the a~ue~us
solu~ions also contain dissolved copper as a copper
chelate.
Waste-disposal is a significant problem in
today's industrial e~vironment. Thus, the problem
of how to safely deal with a liquid waste containlng
dissolved complexes of iron and/or copper and Qther
heav~ metals with alkylenepolyamlrle polyacetlc acid
chelates is a ~uestion that must be faced after each
cleaning job. It has been demonstrated that the
liquid was-te can be safely and efficiently incinerated,
but with increasingly stringen~ contxols on incin-
eration, this is also becoming less acceptable.
Thus, there exists a strong and incre~sing need for
a safe and efficient method for removing heavy met~ls
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_4.
from such a~ueous liquids prior to other trea-tment
methods.
The invention resides in a process for
removing iron from an aqueous liquid containing
dissolved iron in the form o~ a soluble chelate
with an alkylenepolyamine polyacetic acid and/or
soluble chelating salts of said acid, said process
comprising the steps of: ~a) adjusting the pH of
said aqueous liq~lid to at least about 12.5, (b)
aclding sufficient quantities of a soluble calcium
salt to facili-~ate the growth of insoluble colloidal
iron hydroxide par~icles, and (c) separating the
resulting insoluble mass containing iron hydroxide
from the liquid effluent.
The novel process is highly efective in
reducing the amount of dissolved iron in solu~ion
and, in addition, the insoluble ma~s resul-ting from
the process is easily separated from -the liquid
effluent.
.
Iron hydroxide is normally formed as a
gelatinous mass which is difficult to deal with.
In this process, however, the iron hydroxide is
formed as a floc and is easily separable from the
liquid.
~5 The invention also resides in a waste treat~
ment p.rocess for removin~ iron and copper ions from
an aqueous liquid waste in whi.ch said ions exist as
~oluble chelates with ethylen2diaminetetraace-tlc acid
~EDTA) and/or a soluble chelating sal-t of EDTA, said
process comprising the steps of: (a) adjusting the
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pH of said aqueous liquid waste to at leas-t abou-t
12.5 with sodium hydroxide, (b) adding from 0.25
to 1 weight percent calclum hydroxide as an aqueous
slurry, (c) adding a substantially stoichiometric
amount of sodium N,N-diethyldithiocarbamate related
ko the amount of copper ions present in said aqueous
liquid waste, (d) adding from 2 to 10 ppm of an anionic
water soluble polyacrylamide, and (e~ separating the
solids in said liquid waste from the liquid effluent.
The general class of alkylenepolyamine
polyacetic acid chelants is, of course, a well known
class of compounds. The types of metals that are
chelated by these compounds are likewise well known
and include, iron, chromium, copper, nickel, etc.
The instant process is highly effec-tive in removing
dissolved iron from solutions of such chelant com-
plexes.
Step (a) is a pH adjustment st~p in which
the pH of the solution is adjusted to a-t least about
12.S. This is conveniently done by adding an alkali
metal oxide or hydroxide to the aqueous liquid.
Sodium hydroxide or potassium hydroxide would typically
be used; ancl, sodium hydroxide would be the alkali
me-tal hydro~ide of choice based upon commercial avail-
ability and cost.
In step ~b) a soluble calcium salt is addedin amounts sufficient to facilitate the particle
growth of the iron hydroxide floc. Usually amounts
of from 0.25 to 5 weight percent are satisfactory.
Substantially any inorganic calcium salt can be used,
but calcium chloride, calcium bromide, and calcium
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hydroxide are common calcium salts and -they would
typically be used. Calcium hydroxide is the calcium
salt of choice because it is alkaline and the present
process is conducted at a basic pH. An anionic water-
5 -soluble, high molecular weight polymer such as poly~
acrylamide and the like, can also be added to facilitate
rapid flocculation; such addition is preferred. Usually
the anionic polymer is added in the amounts of from 2 to
10 parts per million.
In step ~c) the solid mass is separated
from the liquid ef1uent. This can be an active
process or a passive process. For example, one can
use high pressure filtration techniques, centri-
fugation techniques, settling ponds, etc. The par-
ticular means used in step ~c) will largely depend
upon the eguipment available to the ar-tisan.
If one desires to also remove copper ions
from the aqueous liquid, dimethyl and/or diethyl-
dithiocarbamate is added in essentially stoichiometric
29 amounts, relative to the amount of dissolved copper.
The reaction with the dialkyldithiocarbamate is almost
instantaneous and the copper is removed in almost
quantitative yield~ The solid precipitate containing
-the copper is then removed from the liquid effluen-t
by techniques as described or step (c).
If one desires to purify the effluent even
further, the liquid effluent can be passed through a
bed of activated charcoal to remove organics and to
decolorize the liquid. The activated charcoal is
also effective in removing trace amounts of heavy
me-tals. After this "polishing" step, the liquid
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effluent contains the alkylenepolyamine polyacetic
acid, usually as the primary ingredient dissolved
in water. The alkylenepolyamine polyacetic acid
can be removed from solution by lowering the pH. For
example, ethylenediamin~-tetraacetic acid will pre~
cipitate at a pH of approximat~ly 1.8 or below.
Experimental
The following e~periments will further
illustrate the invention.
Example 1
A representative "waste" solution was prepared
using ammoniated EDTA buffered at a p~ of about 9.2
with ammonia to dissolve iron sulfate (5100 ppm iron)
and copper sulfate (1000 ppm copper). The pH of the
solution was raised to about 12.5 using 50 percent
aqueous sodium hydroxide, followed by the addition
of two weight percent calcium hydroxide, followed
by the addition of one weight percent sodium dlethyl-
dithiocarbamate. The precipitated solids were removed
by filtration and the liquid effluent was tested for
dissolved metals using conven~ional analytical tec~miques
(eOg. atomic absorption). It was found that the clear
liquid effluent contained less than one part per
million dissolved iron and less than one part per
million dissolved copper.
Example 2
A representative "waste" was prepared by
dissolving iron sulfate (1500 ppm iron) wlth an
ammoniated EDTA buffered at a pH of from 4 to 6 with
formic acid. This solution was then treated with
50 percen-t sodium hydroxide to a pH of about 12.5,
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followed by the addition of one weight percent calcium
hydroxide. The solid precipitate was removed by fil-
tration and the clear liquid effluent analyzed for
dissolved metal content. It was found that the effluent
contained less than one part per million dissolved
iron.
Exam ~e 3
The waste from cleaning an industrial boiler
at a utility company in California had been discharged
to a holding pond. The commercial solvent used to
clean the steam generating equipment was an inhibi~ted
ammoniated EDTA solution buffered with ammonia at a
pH of approximately 9.2. The liquid waste conkained
chelated iron, chromium, copper, and nickel. Treat-
ment of this waste to remove metals was a continuoustreatment in which the liquid waste was pumped from
the holding pond into a mixing tank at an average
rate o about 35 yallons per minute. The treatment
chemicals (50 percent aqueous sodium hydroxide, 35
weight percent calcium chloride, and 25 weight per~
cent sodium diethy.ldithiocarbamate~ were likewise
continuously added to the mixing tank. The waste
and the trea-tment chemicals were mixed in this 700
gallon mixing tank and allowed to reac-t and overflow
into the bottom of a 30,000 gallon clarifier. In
the clarifier, the treated mixture was con-tacted wi-th
a polyacrylamide (AP 273 from The Dow Chemical Company)
to enhance flow formation and ~ettling rates. As
the solids coagulated in the clarifier, the solids
separated from the treated solution by gravi-ty. The
clarified solution rose to the top of the clarifier
and overflowed to a polishing filter. The solids
- were withdrawn from the bottom of the clarifier slowly
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i~ order to maintain a st~ble sludge "blanket". The
~ sludge blanket was quite dense (4-5 weight percent
solids) and acted as a "sieve" to provide efficient
solid/liquid separation.
The solids were transferred into a thickener uni-t
- (basically a holdlng tank) where the solids continu2d
to condense and precipitate. After the solids were
accumulated, they were processed in-to filter cakes using
a commercial filter press and disposed of safely.
The clear liquld effluent flowing from the
clarifier was decolori~ed in the polishing filter.
Me-tal analysls (by atomic absorptlon spectroscopy)
on the polished ef~luent provided the information in
Table I.
TABLE I
Before ~fter %
MetalsTreatment Trea~men-t Removal
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Iron1040 ppm 0.09 ppm 99.99
Copper 114 0.04 99.96
Chromium0.73 0.13 82.2
Nic~el 80 14 82.5
The total treatment chemicals used to treat
150,000 gallons of waste were: 1350 gallons of 50
weight percent sodium hydroxide, 4500 pounds of calcium
hydroxide (dry weight basis), and 370 gallons of 25
weight percent sodium diethyldithlocarbama-te.
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