Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~2~ 76
PROCE~S OF PREPARIMG
A PREFERRED FERRIC SULFATE SOL~TION, AND PRODUCT
~ackground of the Invention
The present inven-tion relates to the production of iron
(III) sulfate and in particular to the produc-tion of
preferred iron (III) sulfate solutions for use in water
treatment and water purification facilities. In this
specification, iron (III) sulfate refers to an iron salt
with the iron in the ferric form, and iron (II) sulfate
refers -to an iron salt with the iron ,in the ferrous form or
ferrous oxidation state.
It is well known that various hyclroxides may be used as
coagulating or floc producing agents in water purification
and treatment facil:ities. The floc formed from such agents
may be used to trap undissolved materials in the water such
as organics, inorganic ~recipitates, and various biological
matters. I'he floc precip.itate generally effectively
attracts and absorbs even very fine contaminat:ing part.icles.
Duriny the t:reatment process, the floc grows in si~e, arld
ultimately is removed from the water by set-tling, skimming
or filtering. Such agents may also be used in -the treatment
of water for the presence of phosphates, and as sludge
conditioners in sewage treatment, whereby the sludge is
rendered more capable of being filtered and treated.
The better known flocculating agents or floc producing
agents are probably ferric hydroxide and alurninum hydroxide.
In the past, aluminum hydroxide has often been preferred.
However, recently the presence of aluminum hydroxides in
~.
trea~ed water has caused some concern as a potential health
and/or environmental risk. Thus, more recently, attention
has been focusing on -the use of ferric hydroxides as
water purifying agents.
A well known process for the production of ferric
hydroxide, in water treatmen-t facilities, has been by the
addition of ferric sulfate to the water to be treated.
Generally, the alkaline content of the water rapidly induces
hydroxide formation and ultimate floc production, after
ferric sulfate is added to the water. If necessary, the pl-l,
or hydrogen ion content of the water, may be adjusted for
preferred hydroxic~e formation.
In the past, numerous problems have resulted from the
utilization of ferric sulfake as the hydroxide producing
agent. Generally, these can be traced to problems arising
during the initial :Eerric sulfate production, and in the
cornpositi.on of -the ferric sulfate agent used, rather than
any inherent prohlern in the use of ferric sulEate itself.
For example, ferric sulfate is general:Ly produced from the
oxidation of ferrous sulfate. If the oxiclation does not go
to completion, then some ferrous sulfate may stil:L be
pre~ent in the water treatment compound. While Eerric
sulfate is highly soluble in water, and rapidly forms
relatively insoluble ferric hydroxide, ferrous sulfate forms
ferrous hydroxide, which is somewhat soluble. Thus, in the
past, the water being treated may have introduced therein,
during the hydroxide forming step, the presence of soluble
ferrous hydroxide that is not separated out by the floc.
The ferrous hydroxide would then contaminate the treated
water.
r,~
PV It 7
Further, ferrous sulfate is less soluble in water than
is ferric sulfate. The presence of ferrous sulfate in the
solutions to be used at water treatment facilities can
cause problems, for example by precipitation from solution
in the equipment of the plant.
Another problem with ferric sulfate compounds made
according to prior processes has been the presence of excess
acidity, generally free sulfuric acicl, in the ferric sulfate
product. Generally, when ferrous sulfate is oxidized, the
resulting products are ferric oxide and ferric sulfate.
However, if the oxidation takes place in the presence of
sulfuric acid, the resulting product is primarily ferric
sulfate, with relatively lit-tle of the oxide present.
Generally, in past productions of ferric sulfate, according
to such a general reaction scheme, excess acid would be
present in the resulting ferric sulfate product. When such
a product has been used in a water treatmen-t facility, it
has often been necessary to add alkaline compounds to the
water, to adjusk the p~l of the solution. This not only can
be costly, but also the temporary excess aciclity, and the
length of time taken for neutralizatlon, may inhibit good
floc formation. That is, generally, good floc formation has
been found to be related to a relati~ely short period of
time for hydroxide formation. If the length of time is
increased, floc formation, in water purification, may not be
as efficient. Further, the temporary excess acidity may be
harmful to the pipes or machinery in the treatment plant.
Also, the addition of alkaline materials to adjust pH may be
undesirable under some circumstances.
Conventional methods of forming ferric sulfate
76
generally result in a product which is a slurry, sludge or
solid. Such products have, in the past, posed several
problems. First, the material may be relatively difficult
to handle or dissolve, especially if in the form of a rather
solid cake. Further, impurities from the initial iron
source may be present in the solid product, resulting in the
addition of impurities to the water. Further, such a
product may be relatively difficult to handle and package at
the site of formation, especially since solid ferric sulfate
may be relatively hygroscopic.
Objects of the Invention
Therefore, the objects of -the present invention are: to
provide an iron (III) sulfate product especially suited as a
water trea-tmen-t product for use in generating floc formation
and coagulation during water treatment; to provide such a
product which is characterized by a selected and minimal
free acid content; to provide such a product which includes
less than 0.5% by weight free ac:id; to provicle such a
product in which the iron content is substantially 99.9~ by
weight Eerric ion, present as ferr1c sulfate; to provide
such a procluct wh:ich has less than 0.1 of 1~ by we:lght
insolubles present therein; to provide such a product which
generally comprises an aqueous solution oE ferric sulEate in
which the tota] iron content, by weight, is preferably
greater than about 10~; to provide a process of producing
such a product; to provide such a process wherein the iron
starting material may be scrap metal iron or iron oxides; to
provide such a process wherein during an initial step the
~2~ 6
iron oxicles or scrap metal iron are dissolved in sulfuric
acid, to form iron sulfates; to provide such a process in
which during dissolving of the scrap metal and/or iron
oxides, the amount of sulfuric aeid used is maintained
within desired limits for yielding a final produet without
excess aciclity; to provide such a process in which oxidation
of ferrous sulfate in the reaction mixture, to ferric
sulfate, is conducted in two oxidation steps, the first step
being oxidation in the presence of molecular oxygen and, the
second step being final oxidation in the presence of a
ehemical oxidizing agent seleetec~ from a group of non-02
oxidizing agents; to provide sueh a process in whieh the
ehemieal oxidizing agent is hydrogen peroxide; -to provide
sueh a proeess in whieh the desired iron compounds in the
reaction mixture are substantially in solution throughout
the proeess, so that by deeantation or Piltration undesired
insolubles, from the starting ma-terials or otherwise, may be
rernoved; to provide sueh a proeess in which a catalyst may
be used duriny oxidation; to provide sueh a proeess in whieh
the catalyst is selee-ted from a group of copper catalysts
ineluding eopper su:Lfate ancl eopper arnmonium sulfate; to
provide sueh a process in whieh the resul-tincJ prc)cluct from
the :final oxidation step ineludes a concentrat.ion of iron,
by weight, of greater than about 10% so that the produet,
without further treatment or coneentration, eontains an
appropriate iron (III) sulfate eoneen-tration for use as a
water -treating agent, to provide such a proeess whieh
produees a produet that is stable with respeet to
preeipitation over a relatively wide temperature range,
ineluding normal shipping or use temperature ranges; to
:12~76
provide such a process which is relatively inexpensive to
effect to provide such a process which is relatively easy
to effect; to provide such a process which may take
advantage of relatively readily available starting
materials; and to provide such a process which is
particularly well adapted for the preparation of iron (III)
sulfate products for water treatment and similar uses.
According to the present invention there is provided a
process for the preparation of a ferric sulfate solution;
said process including the steps of: (a) dissolving iron in
an aqueous sulfuric acid solution containing between about
l.0 and l.l molar equivalents of sulfuric acid per molar
equivalent of iron to be dissolved, to form an initial
reaction solution so as to produce ferrous sulfate; (b)
conducting a first s-tage of oxidation by reacting the
.initial reaction solution with oxygen added to and dissolved
in the reaction solution, while adding sufficient sulfuric
acid to balance the requirement for sulfate in conver-ting
ferrous sulfate in the reaction solu-t:ion to ferric sulfate
and while main-tai.ning the addition o:E sulfuric ac.id at a
sufficiently slow rate relative to t'he :rate of reac-tion
between the sulfuric acicl ancl .iron to prevent the
precipitation of iron sulfate in -the final. product and
continuincJ unti'l. less t.han about 10% Oe t'he :iron original.ly
dissolvecl remai.ns in a ferrous oxida-tion state and at least
90% of the iron is a ferric product, the ferric product
being substantially ferric sulfa-te in solution; (c)
subsequent to the first stage of oxidation, conducting a
second stage of oxidation by addition of hydrogen peroxide
to the product solution of paragraph b, in the presence of
sulfuric acid so as to further convert ferrous sulfate to
ferric sulfate; the total amount of sulfuric acid added in
paragraphs a, b and c being such that the molar equivalents
of sulfate reactions with iron is less than about 1.5 molar
equivalents per molar equivalent of iron, (d) said step of
conducting a second stage of oxidation including ocnducting
said second stage of oxidation until at least 99% of the
iron dissolved to form the initial solution has been
converted to the ferric oxidation state and a final product
having a free sulfuric acid content of less than 0.5% by
weight.
Also according to the present invention there is
provided a process for the preparation of a ferric sulfate
solution, from a mixture containing ferrous oxide and ferric
oxide; said process including the steps of: (a) dissolving
the mixture of ferrous oxide and ferric oxide in an aqueous
sulfuric acid solution to form an initial reaction solution;
sulfuric acid being added to said solution as necessary to
conver-t from ferrous sulfate to ferric sulfate and slowly
enough to allow reaction between the iron and acid and to
prevent substantial prec:ipitation of the iron sulfates in
the :E.inal product; (b) conducting a first stage o:f oxi.da-tion
by reacting the in.iti~l reaction solution w:ith dissolved
oxygen added to and dissolved in the reaction solution in
the presence of sulfuric acid until at least about 90% of
the iron in a resulting intermedia-te product solution is in
the form of dissolved ferric sulfate; (c) subsequently
conducting a second stage of oxidation, following said first
stage of oxidation, by reacting the intermediate product
solution, from said first stage of oxidation, with hydrogen
peroxlde in the presence of sulfuric acid until at least
about 99% of all iron in a resulting final product solution
is in the form of dissolved ferric sulfate; and (d) the
total amount of sulfuric acid added being such that the
final solution contains less than or equal to about 1.5
molar equivalents of sulfate ion per molar equivalent of
iron.
Further according to the present invention there is
provided a process for the preparation of a ferric sulfate
solution by the oxidation of ferrous sulfate in the presence
of sulfuric acid; said process including the steps of: (a)
providing an aqueous solution having ferrous sulfate
dissolved therein as an initial reaction soluti.on; (b)
conducting a first stage of oxidation by reacting the
aqueous solution of ferrous sulfate with molecular oxygen
dissolved in -the solution in the pressence of sufficient
sulfuric acid to generate at least a 90~ conversion of all
iron in the ferrous oxidation state to iron in the ferric
oxidation state; said first stage of oxidation including
providing a suf:ficient amount of sulfuric acid in the
a~ueous solution to obtain said ferric iron in the form of
dissolvecl ferric sulfate, while simultaneously providing a
total amount of molar equivalents of sulfate ion present of
less than or equal to about 1.5 molar equivalents per molar
equivalent of ferrous ion present prior to said first stage
of oxidation; the sulfuric acid being added slowly enough -to
maintain a relative].y low concentration of sulfuric acid in
the final product so as to avoid substantial precipitation
of iron sulfate; (c) conducting a second stage of oxidation,
following said first stage of oxidation, by oxidative
76
treatment of a product solution of paragraph b with a non-
molecular oxygen oxidizing agent selected from -the group
comprising: hydrogen peroxide; chlorine dioxide; chlorine;
and ammonium persulfate; (d) said steps of conducting a
first stage of oxidation and a second stage of oxidation
including providing a final total amount of molar
equivalents of sulfate ion present of equal to about 1.5
molar equivalents per molar equivalent of ferrous ion
originally in the initial reaction solution; (e) said step
of conducting a second stage of oxidation including
conducting said second stage of oxida-tion until at least 99%
of the dissolved ferrous sulfate from the initial reaction
solution has been converted to dissolved ferric solution.
Further according to the present invention there is
provided a process for the preparation of a ferric sulfate
solution by the oxidation of ferrous sulfate in the presence
of sulfuric acid; said proeess including the steps of: (a)
providing an aqueous solution having ferrous sulfate
dissolved therein as an initial reaction solution; (b)
eondueting a first stage of ox.idation by reacting the
aqueous solution of ferrous sulfate with mol.ecular oxygen
dissolved :in -the aqueous solution in the presenee of
sufficient sulfuric aeicl adcled aq needed for converting from
ferrous sul:Eate to :Eerric sulfate to generate at least a 90%
conversion o:E all iron in the Eerrous oxidation state; said
sulfuric acid being added at sueh a rate to allow reaction
of the acid and iron to limit free sulfuric acid and to
avoid substantial precipitation of the iron sulfate in the
final produet; the total amount of molar equivalents of
sulfate ion presen-t at any time during the process being
7~
less than or egual to about 1.5 molar e~uivalents per molar
equivalent of ferrous ion present prior to said first stage
of oxida~ion; and (c) conducting a second stage of
oxidation, following said first stage of oxidation, by
treatment of a product solu-tion from step (b) addition of
ozone to the aqueous solution including ferrous and ferric
sulfate with the addition of sulfuric acid being as needed
to convert from ferrous sulfate to ferric sulfate; and (d)
said step of conducting a second stage of oxidation
including conducting said second stage of oxidation until at
least 99~ of the dissolved Eerrous sulfate from the initial
reaction solution has been converted to dissolved ferric
sulfate.
Further according to -the present invention there is
provided a process for the preparation of a Eerric sulfate
solution by the oxidation of ferrous sulfate in the presence
of sulfuric acid said process inc].uding -the steps of: (a)
providing an a~ueous solution hav.ing ferrous sulfate
dissolved therein, as an initial reaction solution; (b)
conducting a first stage of oxlda-tion by reacting the
aqueous solution of ferrous sulfate with mol.ecular oxygen
dissolved in the so:Lution in the presence of su:Efic.ien-t
sul.Euric acicl to generate at least a 9Q% conversion of all
iron in the ferrous oxidation state to :iron in -the ferric
oxidation ~tate in an intermediate solution; said sulfuric
acid being added as necessary to convert from ferrous
sul:Eate to ferric sulEate and being added sufficien-tly
slowly to allow reaction so as to prevent substantial
precipitation of the iron sulfate in the final product; (c)
conducting a second stage of oxidation, following said first
~lZ~4~;
stage of oxidation, by additlon of a non-molecular oxygen
oxidizing agent selected from the ~roup comprising: hydrogen
peroxide; chlorine dioxide; chlorine; and ammonium
persulfate to said intermediate solution; (d) said step of
conducting a second stage of oxidation including conducting
said second stage of oxidation until at least about 96% of
the dissolved ~errous sulfate from the initial reaction
solution has been conver-ted to dissolved ferric sulfate, and
(e) the total amount of sulfuric acid added being such that
the sulfate ion present in the final product is less than or
equal to about 1.5 molar equivalents per molar equivalent of
ferrous ion present initially.
Other objects and advantages oE this invention will
become apparent from the following descriptions wherein are
set forth by illustration and example certain embodiments of
this invention.
Detailed Description of the Preferred Embodiments
As required, de-tailed embodimentfl of the presen-t
invention are di~cloqed herein; however, it i8 to be
understood that the disclose~ embodiments are merely
exemplary of the :inven-tion which may be embodiecl in various
forms. '~herefore, specific details di~closed herei.n are not
to be interpreted as limitincJ, but rather merely as a basis
for the claims and as a representa-tive basis ~or teaching
one skilled in the art to variously employ the present
invention in virtually any appropriate manner.
As used herein, all temperatures are in Fahrenheit
(F.); all percentages (~) or parts per million (ppm) are by
~L2~ ,J6
weight; and all pressures (PSI) are gauge, unless otherwise
noted. In addition, the term equivalents (or equivalent~ is
understood to mean mole equivalents. Also, unless otherwise
noted, the terms "oxygen" or "dissolved oxygen", when used
in conjunction with solutions herein, are understood to mean
molecular oxygen dissolved in such a solution, which ox~gen
may be derived by adding or bubbling air, purified oxygen
(such as from a liquid oxygen source) or any other suitable
source of oxygen into the solution.
rrhe process of the presen-t invention concerns the
production of preferred iron (III) sulfate solutions; i.e.
aqueous solutions of ferric sulfate. I'he preferred
solutions have little, if any, free acid content, i.e. less
than 0.5~ by weight; have relatively little ferrous ion
present; and include a minimal insolubles con-ten-t. The
reaction product is also characterized by being an aqueous
solution of ferric sulfate, with an iron content of at least
10~ and preferably at least 12~, by weight. The process is,
in part, characterized by being relatively easy to effect
and by permitting the use of readily available starting
materials.
General1.y, a variety of soLlrces of :iron may be used
according to the present :invention. It is foreseen that
preferred sources of iron would be scrap metal iron, which
primarily inclucles iron in an unoxidized state, or various
iron oxide compositions, such as ore, which usually include
mixtures of iron (II) oxides and iron (III) oxides.
Initially, the process will be described using scrap
metal iron as the starting material. For the most part, the
process is the same for the use of iron oxides, however
76
preferred modifications will be discussed below.
Scrap metal iron, which is available from many
commercial sources in a variety of purities and
compositions, generally includes iron in an unoxidized
state, and sometimes includes contaminating silicates and
carbon compounds. According to the process oE the present
invention, the scrap metal iron is first dissolved in
sulfuric acid. Preferably, the total amount of iron present
in the metal used will have been initially assayed, ancl a
sufficient amount of sulfuric acid will be added to
completely dissolve the metal, and yield approximately one
mole equivalent of sulfuric acid, per mole equivalent of
iron. The resulting solution is substantially an aqueous
solution of iron (II) sulfate, i.e. ferrous sulfate. In the
preferred embodiment, zero to a sligh-t excess (that is, more
than is necessary to dissolve the components) of sulfuric
acid, 0~ to about 10% by weight, may be used at this
dissolving stage.
Preferably, the dissolving of the scrap iron, in the
aqueous sulfuric acid solution, is done within a -temperature
range of 160 E'ahrenheit (P.) to about 230~ F., and most
preferably at approximately 200 F. It has been found that
under such conditions, when careEully control:Lecl, and w'hen
only suff:icient acid solution is addecl to make the iron
comp].etely solu'ble, the result:in~ iron solut.ion includes a
total iron concentration, by weight, of at least 10%.
Generally, the acid solution is heated prior to dissolving,
and the mi~ing may be either by addition of the acid to the
metal, the metal to the aeid, or simultaneous addition to
the reaction vessel. While the acid dissolving might be
13
6~
done out~ide of the preferred -temperature range, the
probability of problems from precipitates would be expected
to be increased.
It will be readily understood that the leng-th of time
required to dissolve the iron will vary, in part depending
upon the particle size of the iron scraps used. Generally,
for scrap metal available for most scrap iron sources, a
total dissolving time of from a few hours to about twelve
hours may be necessary.
At the end of the dissolving process, the resulting
solution contains, generally: ferrous sulfate; a relatively
low percentage of excess sulfuric acid (preferably O to 0.5
by weight); and water. Also, insoluble impurities, such as
silicates, from the scrap metal iron may be suspended in the
solution.
'rhe next step in the process is the partial oxidation
of the ferrous sulfate to ferric sulfate. When conducted in
the presence of sulfuric acid, the oxidation of ferrous
sulEate generally yields ferric sulEate. According to the
present process, the reac-tion solution is placed in a
vessel, preferably under pressure, in the presence of a
dissolved oxygen source such as comprefised air or liquid
oxygen. An aqueous sulfuric aciA so:lution is slowly added
to the reaction vessel, during the oxidation. Care is taken
to avoid the addition o excess sulfuric ac:id, which might
lead to precipitates or a final product having excess or
free acidity. Rather, the Einal molar ratio of added
sulfuric acid is pre-calculated so that the final, total,
concentration of added sulfuric acid or sulfate ion in -the
mixture, rela-tive to iron ion, is approximately 1.5 to 1.
14
76
That is, if complete oxidation were achieved, the solution
would contain ferric sulfate wi-th substantially no excess or
free sulfuric acid presen-t.
~ he addition of the sulfuric acid is done relatively
slowly (for example, adclition may be evenly divided over the
entire period of first stage oxidation), in part, in order
to avoid the precipitation of ferrous sulfate from the
solution and -to control the reaction rate. The pressure at
which the reaction vessel is maintained and the temperature
of the reaction are primarily economic considerations; that
is, the operator may choose a combination of the pressure
and temperature which yields a preferred rate of conversion.
The reaction vessel is pre~erably maintained under a
pressure of approximately 100 pounds per square inch and the
reaction is run at a temperature of between about 180 F.
and 220 F. It has been found that temperature ranges
outside of the preferred range may result :in the
precipitation of products such as iron oxide or the
precipitation of some starting materia:l ferrous sulfate,
from the so].ution.
Generally, the precisQ cond.i-tions are determined by the
operator, ad hoc, who controls the reackion to minim.ize
.. . ..
precip.i.tation and maximize rate. I'hroughout the reacti.on,
the disappearance of ferrous 5ul fate may be monitored, with
suff:icient sulfuric acid being addecl to maintain the
reaction at a desired rate. Again, a primary concern is to
avoid the addition of excess acid beyond a 1.5 to 1 ratio of
total sulfuric acicl, or sulfate ion, to iron.
In a typical production run, about 5,000 gallons of
reaction solution, weighing about 1~.5 pounds per gallon as
~36~76
a result of dissolved iron and sulEuric acid, may be used~
The reaction vessel would be pressurized -to about 100 pounds
per square inch, under oxygen introduced from a liquid
oxygen source. The oxidation/acidification step, under such
circumstances, would take abou~ 8-12 hours, when a copper
catalyst, such as copper sulfate or copper ammonium 9ul fate,
is used. It will be understood that w~ile a catalyst may be
used, it. is not necessarily required :Eor oxidation. The
length of time will vary, of course, depending primarily
upon the rate of addition of the aeid and the extent of
agitation. Ayain, the rate is generally limited by adding
the aeid a:t such a rate that the excess acidity does not
lead to precipitation of insolubles. That is, as long as no
precipitate is noted, generally acid is added until the
limit is reached.
Following the above described initial oxidation step,
the reaction solu-tion includes both ferrous sulfate and
ferric sulfate. That is, under general react:ion conditions
it is found that only abou-t 95~ oxidati.on takes place. It
has been found, experimentally, that :it is cluite difficult
to achi.eve a final, complete, oxixlation using air or liquid
oxyyen. Preferably, the Eirst oxidation is run unti:l abou-t
95~ oxidation (i.e., ferrou~ to ferric) is aehievecl, with
any longer reaetion time, in an attenlpt to achieve grea-ter
oxiclation, being generally ineffective and uneconomical.
In order to complete the oxidation, the process of the
present invention includes a second step of oxidation
wherein a non-molecular oxygen oxidizing agent, such as a
peroxide, is added to the reaetion mixture. The agent
hydrogen peroxide has been found to be preEerable.
16
Generally, the oxidation is conductecl wi-thout outside
heating of the reaction mixture, preferably with the
temperature having fallen to about 130 F., and under
atmospheric pressure. However, a wide range of temperatures
would be expected to yield good conversion.
The above described conditions yield substantially
compleke oxidation in about three hours. It will be
understood that the hydrogen peroxide solution used, which
is preferably an aqueous solution including between about
30~ and 40~ hydrogen peroxide, is preferably added
relatively slowly and cautiously to minimize the potential
danger from explosive oxidation. A variety of techniques
for handliny large amounts of hydrogen peroxide are well
known and documented.
Generally, for reasons of safety, efficiency and
convenience, excess hydrogen peroxide is avoided. Also,
excess hydrogen peroxide may lead to problems with
precipitates. Throuyhout the process, the arnount of ferrous
sulfate present may be assayed, with the hyclrogen peroxide
adclition continuing until essentially no ferrous sulfate is
left in the solution, i.e. un-til oxidation is complete.
At -this point, the reaction solution t~vpically consis-ts
essentially of ferric sul:Eate in water. Any solid
precipitates can be filtered out, by Eiltering through an
industrial filter such as a cheese cloth or the like. It
will be understood that filtration may have been done at
almost any convenient point following the initial dissolving
of the metal~
Preferably, dilutions have been selected such that by
the end of the final conversion, the total concentration of
~Z~6~ 6
iron ion present is at least about 10% hy weight, in the
product solution. In this manner, a preferred product for
use in water treatment facilities is taken directly from the
second oxidation step reaction vessel, withou-t further
concentration. That is, it has been found that the process,
when conducted in the concentration ranges preferred to
achieve such a final product concentration, not only
proceeds efficiently but also produces a preferred final
product that does not require final dilution and/or
concentration. Again, it is noted that a ferric sulfate
solution having a concentration in the preferred range is
relatively stable to any substantial precipitation over a
wide temperature range, including the temperature range over
which most products would be shipped, stored or used in a
treatment facility.
If a mixture of iron oxides is used, such as an ore,
basically the same procedure may be followed. ~lowever, at
the initial dissolving stage generally all of the sulfuric
acid needed to reach a 1.5 to 1 ratio with the iron may be
added. That is, generally, acid i8 not added during the
oxidation. It is observed that generally, with oxides,
there is less problem with precipitation from the reaction
mixture.
Except as outlined above, the two stage oxidation
process would follow an analagous course to that described
for scrap iron, with careful control of total added acid to
avoid excess acidity in a final reaction product mixture,
and to avoid prec:ipitation. Again, the principal parameters
to be controlled are temperature and pressure, for control
o~ reaction rate and maintenance of solubility of starting
~ 18
~364~;
materials and products.
As alluded ~o above, it is foreseen that a variety o~
chemlcal oxidizing agents may be used during the second step
of the oxidation reaction. Hydrogen peroxide is foreseen to
be the preferred oxidizing agent, since it is does not
result in any undesired contamination of the product.
However, it is foreseen that such agents as: ozone,
peroxides other than hydrogen peroxide; ammonium persulfate;
chlorine dioxide; and chlorine, may also be usable.
Generally, mining sources can provide adequate
iron oxides, for use in the modification of the process
which begins with iron oxides rather than scrap metal iron.
In some instances, ores of mixed iron oxides as mined may be
of such high quality that substantial purification is not
necessary prior to the initial dissolving in the sulfuric
acid solution.
As suggested above, with respect to both the starting
materials and product mixtures, it is often necessary to
assay the amount of iron present as the ferrous iron or the
ferric iron. Qenerally, conventional rnethods Oe assay may
be used, for example wherein the amount Oe ferrous ion
present is determined by -titration with potassium
dichromate. Following SUC}I a potassium dichromate
oxidation, all iron ion present would generally be in the
form of the erric ion. The total ferric ion content would
then be determined with conventional techniques such as by
reduction with stannous chloride or similar compounds.
Thus, the amount of ferric ion present in the initial
ferric/ferrous mixture can be determined by calculation.
The product prepared by the above process preferably
19
7~
includes less than 0.5% by weight free acid. It also
preferably has at least 99.9~ by weight of the iron present
in the ferric form. Further, i-t has been found that
relatively little, less than 0.1 of 1% by weight, insolubles
are present, the amount of insolubles being controlled, in
part, by filtering. rrhe product solution generally has a
reddish-brown color, but is otherwise clear. rrhat is, it
includes relatively little particu]ate ma-tter in
suspension.
As indicated above, duriny the oxidation step a
catalyst may be used. Although the art teaches the
utilization of nitrogen oxides as a catalyst, their use in
preparing compounds for water treatment may be undesirable,
due to their potential toxicity. Preferred catalysts Eor
use with the present process are copper salts, preferably
copper sulfate or copper ammonium sulfatel present in a
concentration of about 200 parts per million by weight
during the oxidation. Generally, such catalysts, being
present in the final produc-t in such a low concentration
ratio, are believed not to pose a s:igniEicant contamination
problem.
Exarnples of the process in accordance with the
invention which Eollow are eor the purpose of demonstratiny
~3pecific procesl3es in accordance with the inven-tion and are
not intended to be limiting in scope on the invention or
claims. rIrhe following examples a.e exemplary syntheses of
numerous actual runs.
Example 1
6~
It is foreseen that a process, according to the present
invention, for conversion of scrap metal iron to a preferred
iron (III) sulfate solution may be conducted as follows:
~ 4,500 gallon solution of ferrous sulfate weighing
about 12 1/2 pounds per gallon is prepared. Generally, the
solution is prepared from dissolved scrap metal iron, having
between about 1 and 1.1 equivalents of sulfuric acid per
equivalent oE iron. Generally, the solution will have been
made by a slow addition of -the hot sulfuric acid solution,
at about 200 F., until the iron is completely dissolved to
form ferrous sulfate in solution~
The reaction solution is then placed in a 5,000 gallon
reaction vessel, that can be pressurized and which is formed
from a substance non~reactive to the acidic reaction
solution. Liquid oxygen is pumped into the reaction vessel,
and the pressure of the vessel is brought up to
approximately 100 pounds per square inch. Copper ammonium
sulfate catalyst, present in solution in a concentration of
about 200 parts per mi]lion, by weight, i~ utilized to
increase -the rate of oxidation.
The first stage oxiclatiorl is preferably conduc-ted in a
temperature ranye o between about 18n F. and 200 F.
Generally, sulfuric acid is slowly aclded to the reaction
vessel, as the oxi~ation proceeds. The reaction solu-tion is
vigorously agitated, so as to increase the rate of
oxidation. Care is taken to avoid the addition of excess
sulfuric acid; that is, the addition of sulfuric acid beyond
105 equivalents of sulfuric acid or sulfate ion per iron ion
present is avoided. The purpose of this is to ensure -that
at complete oxidation, i.e. complete formation of iron tIII)
21
sulfate, none, or very little excess sul~uric acid is
present.
Generally, the first step or stage of oxidation is
conducted until the rate of oxidation has slowed to a point
as to appear to stop, determined by analysis at various time
intervals. Generally, this has been found to be when about
95~ of the iron in solu-tion has been oxidized to the ferric
ion, with about 5~ remaining as ferrous ion. At this point,
oxidation is sufficiently slow, or stopped, so that 100%
oxidation cannot be readily achieved. Generally, the first
stage of oxidation takes about 12-20 hours under these
conditions.
The solution is then prepared for the second
stage of oxidation, by depressurizing to atmospheric
pressure, such that the contents of the vessel are under
air. Without addition of an external heating source, a
hydrogen peroxide solution is carefully added to complete
oxidation of the ferrous iron to ferr:ic ion, i.e. oxidation
of ferrous sulfate to ferric sulfate. Preferably, an
a~ueous solution of hydrogen peroxide including about 30% to
about A0~ hydrogen peroxide, by weight, is used. Also,
preferably, excess hydrogen peroxide :is avoided. Generally,
caution must he used to ensure that the oxidation proceeds
at a generally safe, and slow, rate. Techniques ~or
handling hydrogen peroxide under similar conditions are
known in the art. Generally, the temperature to which the
solution cools during processing will be acceptable, if the
rate of oxidation is acceptable, however a good temperature
for the peroxide oxidation has been found to be about 130
F., which results in substantially complete oxidation in
69L76
about three hours.
Following the second step of oxidation, the reaction
solution is cooled and, if precipitates or materials in
suspension appear, the product solution may be filtered as
by passing through a conventional industrial Eilter press or
the like. Without further treatment and/or concentration,
the product solution is generally in condition to be
contained and shipped for use in a water treatment
facility.
Preferably, solution concentrations are selected so
that the final iron concentration of the product solution is
at least about 10~ (preferably 12~ or greater) by weight.
Such a solution is relatively concentrated for an iron
sulfate solution, and is, in part, believed possible due to
the absence of any suhstantial amount of ferrous ion. That
is, the product solution is, in part, characterized by
having generally at 1east about 99.9~ by weight of the iron
present in the solution in the ferric sta-te.
~ lso, clS indicated above, care i9 taken to maintain
that excess acid is not added to the solution. That is,
reactant concentra-tion~ and amounts are selected such -tha-t a
total of 1.5 equivalents of sulfuric acid, or sulfate ion,
are present per equivalent of iron. In this manner, the
product solution is characterized by having less than about
0~5~ by weight free acid present.
It i9 noted -that during the first stage of oxidation of
ferrous sulfate to ferric sulfate, at least early in the
oxidation, a slight excess of sulfuric acid may be present.
That is, when the iron is present as ferrous sulfate, the
ratio oE equivalents of sulfuric acid to ferrous ion may
23
7~
preferably be in the range of about 1/1 to about 1.1/1. It
is further noted, however, that as the tota] amount of
sulfuric acid added increases, and the oxidation proceeds
near completion, excess acid is avoided so that the final
solution never includes substantially more than about 1.5
equivalents of added sulfuric acid or sulfate ion per
equivalent of iron, as indicated above.
Example 2
It is foreseen that a mixture of iron (II) oxides and
iron (III) oxides may be converted to the preferred iron
(III) sulfate solution, according to the present invention,
as follows:
11,800 pounds of iron ore of approximately 62~ by
weight iron and comprising minor impurities and iron oxides
in a ratio of ferric oxide to ferrous oxide of about 2 to 1
are mixed with about 22,000 pounds of water. 20,400 pounds
of 66 Baume sulfuric acid are slow]y added to dissolve the
oxides and form sulfates. Generally, thi~ solutation takes
about three hours, at about 180 F. to 220 F., with
ayitation. The reaction mixture including the dissolved
sulfates is filtered throug~ an industrial filter press to
remove any impurities such as silicates or other compounds
from the initial ore.
The reaction solution is placed in a pressure vessel,
pressurized with liquid oxygen -to about 100 pounds per
square inch, with a temperature maintained at about 170 F.
to 200 F. With agitation, the oxidation proceeds, however
the rate of oxidation may be increased with the addition of
24
~2~ 76
200 parts per milllon by weight of copper ammonium sulEate
or copper sulfate, as a catalyst. The reaction is monitored
until approximately ~5~ of the ferrous ion has been
converted to ferric ion. A typical first stage oxidation
for the above reported reaction mixture will take about 8 to
12 hours.
E`ollowing the first stage o~ oxidation, the reaction
vessel is preferably isolated from any outside heating
source, with -the reaction mixture slowly cooling. An
aqueous solution of about 35% hydrogen peroxide is slowly
added, to complete oxidation to ferric sulfate. Generally,
the reaction is monitored and only sufEicient hydrogen
peroxide is added to achieve substantially complete
oxidation.
Generally, the oxida-tion is permitted to proceed at the
temperature to which the reaction mixture has cooled prior
to hydrogen peroxide addition. It has been found that at
about 130 F., the oxidation with h.ydro~en peroxide will run
fa.irly smoothly and be complete in about three hours.
However, it wi:L.L be understood tha-t during the peroxide
oxidation, careful control. of tempera-ture i.8 generally not
necessary, as lony as the oxidation can be conclucted
relatively safely. Generally, duri.ny the peroxide
oxidation, the reaction ïn:ixture is not maintained under
pressure above atmospheric, and the mixture is maintained
under air.
It i5 to be understood that while certain forms of the
present invention have been illustrated and described
herein, it is not to be limited to the specific forms or
arrangement of parts described and shown.
