Treatment of water

A water quality improvement process is provided for acid water containing sulphate and/or sulphite ions. The water is continuously fed into a fluidized bed containing calcium carbonate particles. The water consumes calcium carbonate and has its pH increased while calcium sulphate CaSO.sub.4 and/or CaSO.sub.3 are produced.

THIS INVENTION relates, broadly, to the treatment of acid water to improve 
the quality thereof. More particularly, the invention relates to a process 
for the treatment of water, which may be environmentally undesirable by 
virtue of its having a low pH, together with a high sulphate ion content 
and possibly a high heavy metal cation content of metals such as Fe and 
Mn, so as to improve its environmental acceptability. 
According to the invention, there is provided a process for the treatment 
of acid water containing anions selected from the group consisting of 
sulphate ions and sulphate ions, to improve the quality thereof, the 
process being continuous and including the step of feeding said acid water 
into at least one fluidized bed of particles comprising calcium carbonate, 
the water fed to each bed passing through the bed where it consumes the 
calcium carbonate of the particles, which consumption is associated with 
an increase in the pH of the water, said anions in the water reacting with 
calcium ions from the calcium carbonate according to a reaction selected 
from: 
EQU H.sub.2 SO.sub.4 +CaCO.sub.3 .fwdarw.H.sub.2 O+CaSO.sub.4 +CO.sub.2 ; and 
EQU H.sub.2 SO.sub.3 +CaCO.sub.3 .fwdarw.H.sub.2 O+CaSO.sub.3 +CO.sub.2 
and a product water being produced with a pH which is higher than that of 
the acid feed water. 
When acid water has a sulphate content above 2000 mg/l, particularly above 
2500 mg/l, calcium sulphate may precipitate as gypsum (CaSO.sub.4 
.multidot.2H.sub.2 O) crystals Furthermore, if the acid water has a 
sulphite ion content which is sufficiently high, calcium sulphite can also 
precipitate as crystals. 
Each bed is thus a fluidized bed, the particles of the bed being fluidized 
by the water passing therethrough. 
If desired, e.g. when a higher rate of water flow up through each fluidized 
bed is required for fluidizing purposes, than is available in the feed of 
water to be treated, a proportion of the water which has passed through 
each bed may be recirculated to the water feed to said bed, being passed 
therethrough, together with the acid feed water, to assist in fluidizing 
the bed. 
The particles in each bed may be selected from limestone particles, 
dolomite particles (comprising a proportion of magnesium carbonate) and 
mixtures thereof. When dolomite is used, the MgCO.sub.3 therein will react 
with any sulphate or sulphite ions to provide some MgSO.sub.4 or 
MgSO.sub.3 in solution, i.e. CaMg(CO.sub.3).sub.2 +2H.sub.2 SO.sub.4 
.fwdarw.CaSO.sub.4 +MgSO.sub.4 +2CO.sub.2 +2H.sub.2 O with a similar 
reaction for any sulphite ions of CaMg(CO.sub.3).sub.2 +2H.sub.2 SO.sub.3 
.fwdarw.CaSO.sub.3 +MgSO.sub.3 +2CO.sub.2 +2H.sub.2 O. The MgSO.sub.3 is 
substantially more suluble than the CaSO.sub.3 and will remain in 
solution, providing the useful advantage of separating CaSO.sub.3 and 
MgSO.sub.3 for use as potentially valuable by-products. Magnesium is 
present in the dolomite as part of a minor proportion of 
CaMg(CO.sub.3).sub.2 mixed with a major proportion of CaCO.sub.3. Any 
MgSO.sub.4 which precipitates as crystals will typically form part of any 
gypsum or calcium sulphite precipitated, but in practice essentially all 
the MgSO.sub.4 remains in solution in the treated water, together with 
such MgSO.sub.3 as is present. 
The calcium carbonate inventory in each bed, and the rate at which the 
water is passed therethrough, will generally be selected so that the water 
undergoes the desired degree of quality improvement as it passes through 
the bed. 
As the process is continuous, fresh calcium carbonate inventory, may be 
continuously or at least intermittently fed into the bed, spent inventory, 
when the calcium carbonate feed is not pure calcium carbonate, being 
purged from the bed continuously or at least intermittently. In other 
words, calcium carbonate-containing particles being fed to the bed, at 
least intermittently, spent particles being purged from the bed, at least 
intermittently, when calcium carbonate therein has been consumed. The 
inventory feed may be at or adjacent the top of each fluidized bed, 
purging of inventory being at or adjacent the bottom of the bed. 
Furthermore, if desired, the water being treated may be passed through 
several said fluidized beds employed in series with regard to water flow, 
each being operated in similar fashion and being charged with fresh 
calcium carbonate inventory. Instead, or in addition, spent material 
purged from the fluidized bed in the series may be charged, intermittently 
or continuously, into an upstream fluidized bed, e.g. the immediately 
upstream bed, to consume residual calcium carbonate therein, so that the 
material finally purged from the series is fully spent and contains 
substantially no available calcium carbonate. Accordingly, in a particular 
embodiment, the water being treated may pass through a plurality of 
fluidized beds arranged in series, fresh particles being fed into the last 
bed of the series and the spent particles purged from each bed other than 
the first bed in the series being fed to the preceding bed in the series, 
the spent particles purged from the first bed in the series containing 
substantially no calcium carbonate being discarded. 
The process of the present invention will, as the water being treated is 
acid, act at least partially to neutralize and raise the pH thereof. 
Usually the rate of water feed, bed height and/or hydraulic retention 
time, may be selected so that water leaving each bed, has, within limits, 
a desired pH. When each bed is a fluidized bed, the overall rate of 
inventory feed may be balanced with the rate of water feed, so that a 
stoichiometric amount of calcium carbonate is fed to the bed or series of 
beds, sufficient to neutralize the acid in the water being fed 
therethrough. The water feed rate and/or bed sizes are conveniently such 
that the water has a total residence time in contact with the calcium 
carbonate inventory of about 1-3 minutes or more. Preferably such rate is 
no more than is required reliably to fluidize each bed, the average upflow 
velocity of the water, based on the empty volume of each fluidized bed 
(i.e. flow rate in m.sup.3 /hr divided by the cross-sectional area of the 
bed in m.sup.2) being 30-40 m/hr, e.g. 36 m/hr. 
The neutralization reaction can be expressed by: 
EQU CaCO.sub.3 +2H.sup.+ .fwdarw.Ca.sup.2+ +CO.sub.2 +H.sub.2 O. 
As the CO.sub.2 produced can dissolve in the water to form carbonic acid, 
the process is capable of raising the pH of the water to no more than 
about 5.5-7.6, any further pH increase requiring the addition of CaO, 
Ca(OH).sub.2, NaOH or the like alkaline material. If desired, CO.sub.2 
produced in this reaction can be recovered as a by-product. 
For each fluidized bed, a particle size for the calcium carbonate inventory 
may be employed in the broad range of 150-10,000 .mu.m, e.g. 250-600 
.mu.m, more specifically 300-420 .mu.m. As the water being treated reacts 
with the calcium carbonate in each fluidized bed, a size reduction of the 
calcium carbonate-containing particles takes place, any non-calcium 
carbonate insoluble residue, depending on its particle size, either being 
eluted from the bed or purged therefrom. Any gypsum or calcium sulphite 
will be formed in the form of fine crystals, and the Applicant has found 
in practice that gypsum crystals can easily be caused, depending on the 
water flow rate, to separate substantially from the calcium carbonate 
inventory of each bed (due to the differing settling velocities thereof 
and slow rate of crystallization of the gypsum), to form an upper 
fluidized gypsum layer, above a layer of fluidized calcium carbonate 
inventory in the bed. The same is in principle possible with calcium 
sulphite crystals. It will be appreciated in this regard that, in each 
fluidized bed of calcium carbonate particles, there is substantial 
attrition of the calcium carbonate particles. Any tendency for a coating 
of calcium sulphite or a layer of gypsum to form thereon is thus 
counteracted, and the attrition tends to prevent the build-up of any such 
coating. 
Optionally, the water feed rate may be selected such that any gypsum or 
calcium sulphite crystals are not excessively eluted from each fluidized 
bed together with the water leaving the bed. In this embodiment, such 
crystals may be withdrawn from said layer of crystals in each bed and 
dewatered, the water separated therefrom optionally being returned to the 
process. Any eluted crystals may be separated, e.g. by settling, which can 
employ an organic flocculant, followed by filtration, such as 
ultra-filtration, to form a potential by-product which, however, can, if 
necessary, by dumped as an environmentally acceptable solid waste product. 
In another embodiment, the crystals may be deliberately eluted and then 
separated from the water, e.g. as described above, between the fluidized 
beds in the series and after the last fluidized bed in the series. The 
Applicant has found that, surprisingly, while a portion of any dissolved 
calcium sulphate formed by reaction of sulphate ions with calcium ions in 
the inventory precipitates relatively quickly as gypsum crystals which can 
be withdrawn or eluted from each fluidized bed, a portion thereof is 
resistant to precipitation as crystals, and remains dissolved. Thus, when 
gypsum crystals which have been withdrawn or eluted are separated from 
water by settling, a proportion of the settled crystals may advantageously 
be recirculated to the settling step to act as seed crystals to promote 
further precipitation of dissolved calcium sulphate as gypsum. This can be 
effected in two tanks, namely a first tank in which crystallization of 
gypsum takes place, and a second tank which acts as a settling tank, into 
which the first tank discharges, in which gypsum crystals are settled, 
crystals been recirculated as seed crystals from the second tank to the 
first tank. Instead, or in addition, the water containing dissolved 
calcium sulphate can be passed through a fixed or fluidized bed of gypsum 
crystals whose crystal growth takes place to reduce the concentration of 
dissolved calcium sulphate in the water. 
Instead, or in addition, naturally, the separation of gypsum and other 
solids from the water may, if desired, be effected by flotation and/or 
centrifuging, and the flotation can be used to separate at least some of 
such solids from one another. 
When the acid water to be treated contains heavy metal cations, e.g. 
transition metal cations such as Fe.sub.2.sup.+, Fe.sub.3.sup.+ or the 
like, the process may include the step, prior to passing the water through 
any said bed of calcium carbonate particles, and e.g. in a suitable 
reactor such as a mixed bed reactor upstream, relative to water flow, of 
the particles, of treating the water with an alkali, to cause 
precipitation of heavy metals therefrom in hydroxide form. This alkali may 
be CaO, Ca(OH).sub.2 or indeed CaCO.sub.3 in fine powder form. Thus 
precipitation resists coating of the calcium carbonate particles in the 
fluidized bed with Fe(OH).sub.3 or particularly Fe(OH).sub.2, which 
coating can render the calcium carbonate in the fluidized bed inaccessible 
to hydrogen ions. 
When the acid feed water contains Fe.sup.2+ ions, the process may include 
the step, prior to passing the water being treated through any bed 
containing calcium carbonate, of oxidizing the Fe.sup.2+ ions to 
Fe.sup.3+ ions. Indeed, other heavy metal cations such as manganese 
cations, can similarly be oxidized, this oxidation in general reducing 
solubility of the hydroxides thereof. This is because the Applicant has 
found that, surprisingly and unexpectedly, when the acid feed water 
contains Fe.sup.2+ ions, these ions tend to precipitate as Fe(OH).sub.2, 
at a pH of about 4-7, in a fashion such that a coating of Fe(OH).sub.2 is 
formed on the inventory particles, masking the calcium carbonate therein, 
and inhibiting the reaction of hydrogen ions with the calcium carbonate. 
In contrast, when the acid feed water contains Fe.sup.3+ ions, these ions 
tend to precipitate as Fe(OH).sub.3, at a pH of about 3-4, in a fashion 
such that separate flocs of Fe(OH).sub.3 are formed, which do not coat the 
inventory particles, and have little, if any, inhibiting effect on the 
rate of reaction of sulphate ions with the calcium carbonate. 
It is therefore desirable, as indicated above, to convert any Fe.sup.2+ to 
Fe.sup.3+ cations, e.g. before the pre-treatment with alkali to 
precipitate heavy metals, so that they precipitate at a low pH of 3 or 
somewhat higher during this pre-treatment. The Fe(OH).sub.3 flocs formed 
can instead pass through the bed, and although they will not form an 
inhibiting coating on the inventory, their formation will contribute to 
the acidity of the water. Preferably, however, this precipitate of 
Fe(OH).sub.3 is removed from the water, e.g. by settling or the like, 
water separated therefrom being at a pH of about 3-4 and being fed to the 
particles for further neutralization. This precipitation takes place in 
accordance with the reaction: 
EQU 2Fe.sup.3+ +6H.sub.2 O.fwdarw.2Fe(OH).sub.3 +6H.sup.+ 
This reaction, as indicated above, tends to re-acidify the water somewhat, 
prior to the neutralization by the inventory. This pre-treatment promotes 
the removal of all the Fe.sup.2+ and Fe.sup.3+ in the water upstream of 
the inventory, so that little, if any, Fe(OH).sub.3 precipitation takes 
place in the inventory. The method accordingly contemplates dosing the 
water with oxidizing agents, e.g. air, oxygen, KMnO.sub.4, Cl.sub.2, 
O.sub.3, H.sub.2 O.sub.2, MnO.sub.2 or the like, in said mixed bed 
reactor, together with addition of the alkali, or upstream of the 
pre-treatment with alkali. Instead, biological oxidation may be employed 
for the purpose of oxidizing Fe.sup.2+ ions to Fe.sup.3+ ions, upstream 
of the pre-treatment with alkali, according to the reaction: 
EQU 2Fe.sup.2+ +2H.sup.+ +1/2O.sub.2 .fwdarw.2Fe.sup.3+ +H.sub.2 O. 
For this biological oxidation, one or more species of microorganisms 
selected from, for example, Thiobacillus thiooxidans T. neapolitanis and 
T. ferroxidans can be employed at a pH of 0.8-3, e.g. 1.5-2.5, in a 
suitable reactor such as a mixed tank or mixed pond, or a packed tower, 
the water in the reactor optionally being oxygenated and having a 
metabolizable carbon source added thereto. Furthermore, biological 
oxidation can be effected by so-called wetland oxidation whereby the acid 
feed water is passed through a bed of growing plants in a wetland zone. 
These plants can also remove heavy metals from the water. 
In this regard it should be noted that, if the water contains excessively 
high proportions of Fe.sup.2+ and/or Fe.sup.3+ ions associated with 
sulphate ions, as can be the case of certain biologically acidified mine 
waste water obtained e.g. from gold mines, so that up to 75% or more of 
the SO.sub.4.sup.2- ions are associated with Fe.sup.2+ or Fe.sup.3+ 
ions, and the balance thereof are associated with acid H.sup.+ ions, the 
process of the present invention is suitable for the treatment thereof, 
provided Fe.sup.2+ ions are converted to Fe.sup.3+ ions and preferably 
precipitated as Fe(OH).sub.3 which is removed prior to feeding the water 
through the inventory. It is further to be noted that, in principle, other 
heavy metal cations can be removed from the water in analogous fashion to 
that described above for removing Fe.sup.2+ or Fe.sup.3+ ions, i.e. by 
oxidation, optionally biologically, and precipitation as the hydroxide 
prior to the treatment of the water in the fluidized bed. 
Naturally, as the precipitation of Fe(OH).sub.3 in the inventory does not 
affect the rate of reaction of hydrogen ions with the calcium carbonate 
unaccceptably, the pretreatment with alkali to precipitate Fe(OH).sub.3 
before the water passes through the inventory can be omitted, provided 
Fe.sup.2+ ions are oxidized to Fe.sup.3+ ions. In this case Fe(OH).sub.3 
merely forms as flocs at a pH of 3-4 in the inventory and can join the 
gypsum in the upper fluidized layer of crystals, after which it can be 
separated from the treated water after it is eluted with the crystals from 
the inventory. This option can be employed when the crystals produced are 
to be discarded and not recovered as a by-product, the Fe(OH).sub.3 
conveniently being discarded with the crystals after optionally being 
separated therewith from the product water. Instead, the Fe(OH.sub.3) can 
be separated from the water by settling or filtration, prior to crystals 
precipitation. 
It is expected that a major application of the process of the present 
invention will be in the treatment of certain industrially produced 
effluent waste waters, such as those produced by explosives manufacturers 
or uranium refiners. Waste waters from uranium refiners can have pH's of 
less than 2.5. In such cases, i.e. when the acid feed water has a pH of 
less than 2.5, the method may include the step, prior to the precipitation 
of the heavy metals therefrom, of increasing the pH of the water to at 
least 2.5. Such waters can be treated in a fluidized bed containing 
calcium carbonate to increase the pH to 2.5; they can then be dosed, as 
described above, with alkali to increase pH sufficiently (e.g. to a value 
of 3-4) to precipitate Fe(OH).sub.3, preferably with prior oxidation of 
Fe.sup.2+ to Fe.sup.3+ ions as described above, and followed by further 
pH increase in one or more further, similar, fluidized beds containing the 
calcium carbonate inventory. 
In the case where the water to be treated contains manganese ions in the 
form of Mn.sup.2+ ions, and when dolomite is used to provide the calcium 
carbonate inventory in the fluidized bed, the process of the present 
invention can, in principle at least, reduce the concentration of these 
Mn.sup.2+ ions by the reaction thereof with the magnesium carbonate in 
the fluidized bed inventory according to the reaction: 
EQU CaMg(CO.sub.3).sub.2 +Mn.sup.2+ .fwdarw.MnCO.sub.3 +CaCO.sub.3 +Mg.sup.2+ 
The MnCO.sub.3 produced is substantially less soluble in water than the 
MgCO.sub.3 consumed, resulting in the precipitation of MnCO.sub.3 and the 
release of Mg.sup.2+ ions into the water. This is beneficial, as 
environmentally acceptable water is usually permitted to contain 
substantially higher concentrations of Mg.sup.2+ ions (e.g. about 100 
mg/l) than Mn.sup.2+ ions (e.g. about 0.5 mg/l). Any residual Mn.sup.2+ 
ions in the water can, if desired, be removed by oxidising the Mn.sup.2+ 
ions to Mn.sup.4+ to obtain a precipitate in the form of MnO.sub.2. This 
oxidation can be effected by Cl.sub.2 at a pH of about 7, according to the 
reaction: 
EQU Mn.sup.2+ +Cl.sub.2 +2H.sub.2 O.fwdarw.MnO.sub.2 +2HCl+2H.sup.+ 
Instead, at a pH of greater than 9.5, the oxidation can be effected by 
using oxygen (treatment with air, O.sub.2, O.sub.3, H.sub.2 O.sub.2 and/or 
KMnO.sub.4), according to the reaction: 
EQU 2Mn.sup.2+ +O.sub.2 +2H.sub.2 O.fwdarw.2MnO.sub.2 +4H.sup.+ 
These oxidizing reactions require relatively high pH's as indicated above, 
which can be obtained, as described above, by treating the water issuing 
from the fluidized bed with e.g. CaO, Ca(OH).sub.2 or NaOH. The oxidizing 
agent employed therefor can be introduced into the, or the last, fluidized 
bed, or downstream thereof. 
Any MnCO.sub.3, MnO.sub.2 or indeed any Fe(OH).sub.3 produced in the 
inventory can be separated from the water being treated together with the 
gypsum, when the gypsum is separated from the water as described above, 
and, if desired, they can be at least partially separated from one another 
by flotation, the MnO.sub.2 and MnCO.sub.3, in addition to the CaSO.sub.4 
.multidot.2H.sub.2 O and CaSO.sub.3 being potentially valuable 
by-products, or, at worst, environmentally acceptable waste products if 
they are dumped to waste in solid form. 
With regard to the utility of the present invention, it should be noted 
that calcium carbonate is, in fully mechanically mixed bed reactors, 
difficult to use to treat acid waste waters. The neutralization reaction 
is slow, and much of the calcium carbonate can be lost with the treated 
stream. Using the fluidized bed process of the present invention, however, 
promotes full and effective use of all the calcium carbonate fed to the 
process, with reduced danger of such loss. Furthermore, the large surface 
area provided by the particles of calcium carbonate inventory in the 
fluidized bed, which are constantly rubbed free of any gypsum produced, 
leads to an acceptably rapid rate of reaction Full advantage can 
accordingly be taken of the relatively low cost of limestone or dolomite 
as reagents, compared e.g. with CaO, Ca(OH).sub.2, NaOH or the like, which 
are otherwise often employed for pH reduction of acid waters to improve 
the quality thereof.

In FIG. 1, a schematic flow diagram of the process of the present invention 
is generally designated by reference numeral 10. The flow diagram 10 is 
selected to show a representative embodiment of the process, and to 
illustrate a number of optional features of the process. It should be 
appreciated that, in practice, a number of features illustrated in FIG. 1 
may frequently be omitted, if they are not required. 
In the drawing, a raw water feed line 12 and an alkali feed line 14 are 
shown feeding into a fully mixed tank 16 constituting an alkali dosing 
stage. An oxidizing agent feed line 18 is also shown feeding into the tank 
16. A flow line 20 leads from the tank 16 to a settling stage in the form 
of a settling tank 22 having a solids discharge line 24 and a liquid 
discharge line 26. The liquid discharge line 26 is shown feeding into a 
feed line 28 for a fluidizing stage constituted by a fluidized bed reactor 
30 provided with the usual fluid distributor 32. The reactor 30 has a 
spent inventory discharge line 34 and a solids inventory feed line 36, 
together with a liquid discharge line 38. The liquid discharge line 38 
leads to a settling stage constituted by a settling tank 40. 
The settling tank 40 has a solids discharge line 42 and a liquid discharge 
line 44. A liquid recirculation line 46 leads from the settling tank 40 
and joins the flow line 26 feeding into the flow line 28. 
The flow line 44 leads to a further fluidized bed reactor 48, and feeds 
into the reactor 48 via flow line 50. The arrangement of the reactor 48 is 
substantially similar to that of the reactor 30, in that it has a solids 
inventory feed line 52 and a spent inventory solids discharge line 54 
which leads into the flow line 36. The reactor 48 has a liquid discharge 
line 56 which is shown leading into a settling stage in the form of a 
settling tank 58 having a solids discharge line 60 and a liquid discharge 
line 62. A recirculation flow line 64 leads from the settling tank 58 into 
the flow line 50 feeding into the reactor 48. 
A further, substantially similar, fluidized bed reactor 68 is shown 
arranged in series, with regard to the flow of water being treated, 
downstream of the reactor 48. This again has a solids inventory feed line 
70 and a spent solids inventory discharge line 72 which feeds into flow 
line 36. It has a liquid feed line 74 fed by the flow line 62 from the 
settling tank 58. The reactor 68 has a liquid discharge line 78. 
Liquid discharge line 78 leads to a settling tank 80 which has a solids 
discharge line 82, a liquid discharge line 84 and a recirculation 
discharge line 86 leading into the flow line 74. 
Each of the reactors 30, 48 and 68 is shown with a gas outlet flow line, 
designated 88. 
A representative process in accordance with the present invention will now 
be described with reference to the flow diagram 10. 
The process is for treating an acid waste water containing Fe.sup.2+ and 
Fe.sup.3+ cations, and SO.sub.4.sup.2- anions, having a pH of somewhat 
less than 2.5 and a SO.sub.4.sup.2- ion content of somewhat higher than 
2500 mg/l. 
In accordance with the process, a raw water feed is fed along flow line 12 
into the mixing tank 16. At the same time, a suitable alkaline material, 
in the case of this example Ca(OH).sub.2, is fed into the tank 16 via flow 
line 14, while a suitable oxidizing agent, in this example H.sub.2 
O.sub.2, is fed into the tank 16 via flow line 18. Sufficient Ca(OH).sub.2 
is fed to raise the pH at least to about 3, i.e. above 2.5. 
In the tank 16 the H.sub.2 O.sub.2 reacts with ferrous ions to produce 
ferric ions in accordance with the following reaction: 
EQU 4Fe.sup.2+ +2H.sub.2 O.sub.2 +4H.sup.+ .fwdarw.4Fe.sup.3+ +4H.sub.2 O 
Sufficient H.sub.2 O.sub.2 is fed along flow line 18 to ensure that all the 
ferrous ions are converted to ferric ions in the tank 16. At the same 
time, calcium hydroxide from the feed line 12 reacts with the ferric ions 
produced in the acid environment in the tank 16, when the pH reaches about 
3, in accordance with the reaction: 
EQU 3Ca(OH).sub.2 +2Fe.sup.3+ .fwdarw.2Fe(OH).sub.3 +3Ca.sup.2+ 
The ferric hydroxide forms a precipitate, and the treated water from the 
tank 16, together with this ferric hydroxide, flows from the tank 16 along 
flow line 20 to the settling tank 22, where it settles and is discharged 
as a solid to waste along flow line 24. Water from the tank 22 which has 
been separated from ferric hydroxide leaves tank 22 along flow line 26. 
If desired, the ferric hydroxide solid discharge from tank 22 along flow 
line 24 may be dewatered, the solid ferric hydroxide being recovered as a 
by-product or dumped to waste and the water separated therefrom being 
returned to the flow line 26. 
Flow line 26 feeds via flow line 28, as a fluidizing fluid, into the 
fluidized bed reactor 30 below the distributor 32. Solids inventory is 
simultaneously fed, in this example continuously, into the reactor 30 via 
flow line 36. Spent solids inventory is discharged to waste and dumped 
from the reactor 30 via flow line 34, after optional dewatering, the water 
separated therefrom being returned, e.g. to flow line 26. 
From the description which follows hereunder, it will be understood that 
the solids inventory fed to reactor 30 via flow line 36 contains a 
relatively low proportion of calcium carbonate, and the solids inventory 
leaving the reactor 30 via flow line 34 has substantially no calcium 
carbonate in it whatsoever. 
In the reactor 30, the calcium carbonate in the solids inventory in the 
reactor, which solids inventory is designated by reference numeral 89, 
acts to neutralize water entering the reactor via flow line 28 in 
accordance with the reaction: 
EQU CaCO.sub.3 +2H.sup.+ .fwdarw.Ca.sup.2+ +CO.sub.2 +H.sub.2 O 
The carbon dioxide produced issues from the reactor 30 via flow line 88, 
and can be recovered as a by-product of the process. 
As the raw water contains sulphate ions, these sulphate ions react with the 
calcium ions liberated by the neutralization, to form a precipitate of 
gypsum or CaSO.sub.4 .multidot.2H.sub.2 O crystals. These crystals form a 
fluidized layer, designated by reference numeral 90, in the reactor 30, 
above the fluidized solids inventory 89. 
Partially neutralized water issues from the reactor 30, together with 
eluted gypsum crystals, via flow line 38 and the flow along flow line 38 
enters the settling tank 40 where the gypsum is settled as shown at 92, 
together with such other solid materials, e.g. residual ferric hydroxide, 
as are in the water. 
The solids settled in the tank 40 issue along flow line 42 to a dewatering 
stage (not shown), in this example a filtration stage, where the gypsum is 
separated as a by-product, separated water being returned to flow line 44. 
Water issuing from the tank 40 passes along flow line 44, except for a 
proportion thereof which is recirculated along flow line 46 which feeds 
into flow line 28, to provide a sufficient upward water flow rate in the 
reactor 30 for the fluidization. This flow rate is no more than is 
sufficient reliably to fluidize the solids inventory 89 in the reactor 30. 
It will be appreciated that, as the use of calcium hydroxide fed along flow 
line 14 to tank 16 is expensive, no more calcium hydroxide will be fed 
along flow line 14, than is necessary to precipitate all the ferric ions 
in the raw water as ferric hydroxide in the tank 16. This occurs at a pH 
at or slightly above 3. Furthermore, as indicated above, the inventory 
feed along flow line 36 to the reactor 30 will contain relatively little 
calcium carbonate, as the purpose of the reactor 30 is to ensure that all 
the calcium carbonate fed to the process is consumed. There will 
accordingly not be a substantial pH increase in the water being treated as 
it passes through the reactor 30, so that this water is still undesirably 
acid for release to the environment. 
The water from flow line 44 accordingly passes into the fluidized bed 
reactor 48 for further pH increase. The water enters the reactor 48 via 
flow line 50 below separator 32, and fresh calcium carbonate-containing 
inventory is fed to the reactor 48 along flow line 52, in this example in 
the form of dolomitic limestone, comprising about 80-90% by mass carbonate 
as calcium carbonate and magnesium carbonate, the magnesium carbonate 
comprising the minor proportion thereof, so that the carbonate is present 
as a major proportion of CaCO.sub.3 and a minor proportion of 
CaMg(CO.sub.3).sub.2. 
In the reactor 48 the calcium carbonate in the inventory, again designated 
89, reacts in similar fashion to that described above for the reactor 30, 
to produce carbon dioxide which issues along flow line 88 and gypsum which 
forms as the fluidized layer 90. 
The rate of inventory feed along flow line 52 is selected so that calcium 
carbonate is provided at a rate, stoichiometrically based, sufficient to 
neutralize to a pH of 7 the water entering the reactor 48 along flow line 
44 without a stoichiometric excess of calcium carbonate. Owing to the 
production of carbon dioxide and the presence of carbonic acid in the 
reactor 48, however, this neutralization cannot achieve a pH much higher 
than about 5.5-6. Accordingly, not all the calcium carbonate fed to the 
reactor 48 can be consumed 
As there is a continuous feed of fresh inventory along flow line 52, there 
is a corresponding continuous purging of spent inventory along flow line 
54 which, as described above, contains a residual proportion of calcium 
carbonate. Flow line 54 feeds into flow line 36 and then into reactor 30 
where said residual calcium carbonate is reacted as described above. As 
indicated above, the purpose of reactor 30 is to ensure that all the 
calcium carbonate in the spent inventory from the reactor 48 is consumed 
In a fashion similar to the water from reactor 30, the neutralized water 
from reactor 48 issues along flow line 56 to settling tank 58 where 
settled solids 92 are discharged along flow line 60 for dewatering in a 
fashion similar to that described above to settling tank 40. Once again 
there is recirculation of clarified water from settling tank 58 along flow 
line 64 via flow line 50 to the reactor 48, to ensure sufficient upflow 
for reliable fluidization in the reactor 48. 
Product water issues from settling tank 58 along flow line 62 to reactor 
68. 
The reactor 68 is provided to increase the pH of the process water as high 
as possible, so that it issues finally at a pH in the region of 5.6-7.6, 
controlled only by the presence of carbon dioxide in the reactor 68 and as 
close as possible to 7.6. 
Accordingly, flow from flow line 62 enters the reactor 68 below its 
separator 32, along flow line 74, while dolomitic lime inventory feed is 
fed to the reactor 68 along flow line 70, spent inventory issuing from the 
reactor 68 along flow line 72 which feeds into flow line 36 for use in the 
reactor 30. Carbon dioxide issues along flow line 88 and treated water 
along flow line 78. 
Once again, the water from flow line 78 is clarified, by passing into 
settling tank 80, settled solids 92 issuing along flow line 82 for 
dewatering as described above with reference to settling tank 40. Product 
water issues along flow line 84, and a proportion of the clarified water 
from the tank 80 is recirculated along flow line 86 to flow line 74, to 
provide sufficient upflow in the reactor 68 for reliable fluidizing. 
It is possible that the raw water fed along flow line 12 in this example 
can contain Mn.sup.2+ cations. In this case it is believed that, in each 
of the reactors 30, 48 and 68, these Mn.sup.2+ cations can in principle 
possibly react with magnesium carbonate in the dolomitic limestone 
according to the reaction: 
EQU CaMg(CO.sub.3).sub.2 +Mn.sup.2+ .fwdarw.MnCO.sub.3 +CaCO.sub.3 +Mg.sup.2+ 
The MnCO.sub.3 is relatively insoluble, and the major proportion thereof 
can in principle precipitate in the reactors, the precipitate, being in 
finely divided form, being eluted from the reactors together with the 
gypsum, and being settled together with the gypsum in the settling tanks 
40, 58 and 80, so that this MnCO.sub.3 issues from the process, in each 
case, together with the gypsum and together with such residual ferric 
hydroxide as is not settled in the tank 22. 
If desired, MnCO.sub.3 and/or Fe(OH).sub.3 can be separated from the gypsum 
and/or from each other, e.g. by flotation in flotation stages (not shown) 
associated respectively with the dewatering stages which are in turn 
associated with said settling tanks 40, 58 and 80. 
It should be noted with regard to the reactors 30, 48 and 68, that the 
respective inventory feed lines 36, 52 and 70 are arranged to feed into 
said reactors 68 at substantially the same elevation as the interfaces 94 
between the fluidized inventory 88 and the fluidized layer of gypsum 90. 
As any manganese carbonate produced is not entirely insoluble, product 
water issuing along flow line 84 can contain Mn.sup.2+ ions, while being 
at a pH of 5.5-7.6. In this example these Mn.sup.2+ ions can be removed 
from the water by an alkali dosing/oxidation stage, shown in broken lines 
at 96 in the form of a stirred tank. In this example calcium hydroxide is 
fed into the tank 96 along flow line 98, while an oxidizing agent in the 
form of air is fed into the tank 96 via flow line 100. The calcium 
hydroxide added is sufficient to raise the pH of the water from flow line 
84 which enters the tank 96, to a pH of above 9.5. At this pH the 
Mn.sup.2+ ions can be oxidized by the oxygen in the air accordance with 
the reaction: 
EQU Mn.sup.2+ +2H.sub.2 O.fwdarw.MnO.sub.2 +2H.sup.+ 
The MnO.sub.2 would be insoluble and form a fine precipitate, while the 
calcium hydroxide, in further neutralizing and raising the pH of the 
process water, can react according to the following reaction: 
EQU Ca(OH).sub.2 +H.sub.2 SO.sub.4 .fwdarw.CaSO.sub.4 +H.sub.2 O. 
Further gypsum can thus be produced and this gypsum, in fine crystal form 
together with the MnO.sub.2, can be finally separated from the process 
water in a settling tank 102 fed by flow line 104 from tank 96. Water, 
from which substantially all the iron (Fe.sup.2+ and Fe.sup.3+) cations 
and possibly some of the manganese (Mn.sup.2+) cations have been removed, 
and which has been neutralized to a pH of about 9-10 finally issues from 
the tank 102 via flow line 106, for release to the environment. 
Solids settled in the tank 102 pass along flow line 108 to a dewatering 
stage where any MnO.sub.2 and the gypsum are dewatered as described 
hereinabove with reference to solids issuing from tank 40 along flow line 
42. Water separated therefrom can be returned to flow line 108. If it is 
desired to obtain the MnO.sub.2 as a by-product, this can be separated 
from the gypsum by flotation in a flotation stage (not shown) 
FIG. 2 shows a modification of the flow diagram 10 of FIG. 1, and this 
modification is generally designated 110. Unless otherwise specified, the 
same reference numerals refer to the same parts as in FIG. 1. 
There are two principal differences between the flow diagram of FIG. 1 and 
that of FIG. 2. The first is that the fully mixed tank 16 of FIG. 1 and 
feed lines 14 and 18 leading thereto are omitted, together with the flow 
line 20. Instead, a biological oxidation stage such as a reactor 112 is 
fed by the feed line 12 and feeds into the line 26. 
The second is that, between each fluidized bed reactor (respectively 30, 48 
and 68) and its associated respective settling tank (respectively 40, 58 
and 80), there is provided a crystallization stage in the form of a 
fluidized bed (respectively 114, 116 and 118), containing a gypsum crystal 
inventory 120. 
The discharge line 38 of the reactor 30 feeds into the crystallization 
stage 114, which in turn feeds via flow line 122 to settling tank 40. 
Similarly, flow line 56 from reactor 48 feeds to crystallization stage 116 
while flow line 78 from reactor 68 feeds to crystallization stage 118; and 
stage 116 feeds via flow line 124 to tank 58 while stage 118 feeds via 
flow line 126 to tank 80. 
A seed crystal feed line 128 feeds from solids discharge line 42 from tank 
40 into flow line 38. Similarly seed crystal flow lines 130 and 132 feed 
respectively from solids discharge lines 60 and 82 into flow lines 56 and 
78 respectively. Organic flocculants dosing lines 134-138 are shown 
respectively feeding into tanks 40, 58 and 80. 
Furthermore, the alkali dosing stage 96, settling tank 102 and associated 
flow lines 98, 100 and 104-108 of FIG. 1 are omitted, the flow diagram 110 
of FIG. 2 being specifically intended for water having no Mn.sup.2+ 
cations, and essentially only Fe.sup.2+ and Fe.sup.2+ cations as heavy 
metal cations therein. Furthermore, it is not intended to recover gypsum 
as a by-product. 
The process of the present invention, as carried out in the flow diagram of 
FIG. 2, is broadly similar to that of FIG. 1, except, once again, for two 
major differences, one being that the chemical oxidation and ferric 
hydroxide precipitation in tank 16 and ferric hydroxide settling in tank 
22 are replaced by a biological oxidation using T. ferrooxidans at a pH of 
1.5-2.5 in the reactor 112. The other is that crystallization of gypsum is 
promoted in each of the fluidized beds 114-118. 
Accordingly, Fe.sup.3+ ions precipitate as Fe(OH).sub.3 flocs in the 
reactor 30, and to a lesser extent in the reactors 48 and 68, the flocs 
joining the gypsum crystals in the fluidized layers 90. Partially 
neutralized water from the reactor 30 with eluted gypsum crystals and said 
Fe(OH).sub.3 flocs, passes along flow line 38 to fluidized bed 114. In 
fluidized bed 114, whose inventory 120 is principally gypsum crystals, 
further crystallization of gypsum from the water is promoted, the 
inventory crystals acting as seed crystals. In the tank 40, a suitable 
organic flocculant is used to flocculate Fe(OH).sub.3 flocs and gypsum 
crystals fed to the tank 40 along flow line 122 from fluidized bed 114. A 
proportion of the settled solids 92 passing along flow line 42 is 
recirculated via flow line 128 to fluidized bed 114 to provide seed gypsum 
crystals Water flowing from reactor 48 along flow line 56 and from reactor 
68 along flow line 78, with Fe(OH).sub.3 flocs and gypsum crystals, is 
dealt with in similar fashion respectively by fluidized beds 116 and 118 
and their respective tanks 58 and 80, product water issuing directly from 
tank 80 via line 84, without any Mn.sup.2+ ion removal. 
It is an advantage of the invention that, particularly as described with 
reference to the drawings, it provides a process for the improvement of 
the quality of acid waste waters containing sulphate ions and also the 
metal cations Fe.sup.2+, Fe.sup.3+ and possibly Mn.sup.2+ ions. Carbon 
dioxide, gypsum, manganese carbonate, manganese dioxide and ferric 
hydroxide are produced as at least potentially usable by-products, which, 
on the other hand, if desired, are environmentally acceptable for dumping 
to waste. In particular, it is an advantage of the invention that it 
provides a process whereby relatively inexpensive limestone, dolomite or 
dolomitic limestone can be employed to increase the pH of such waters, at 
reduced cost compared with the use of certain other alkaline materials 
such as CaO, Ca(OH).sub.2 and NaOH which can be used for the same purpose. 
Finally, it should be noted, with reference to FIG. 1, that, instead of 
separating eluted gypsum crystals in the settling tanks 40, 58 and 80, 
gypsum crystals may be withdrawn directly from the layers 90 via separate 
flow lines to similar settling tanks, from which separated water can be 
returned to the process Instead of feeding fresh limestone into both 
reactors 48, 68, fresh limestone can be fed only to reactor 68, spent 
limestone from reactor 68 being fed to reactor 48. Finally, if the raw 
water is sufficiently acid, the oxidizing and settling stages respectively 
in the tanks 16 and 22 of FIG. 1 can be moved downstream, to a suitable 
position between two of the reactors 30, 48, 68 where the water has an 
appropriate pH therefor. 
Although the invention with reference to the drawings has been described 
with reference to an acid feed water containing sulphate ions, it will be 
appreciated that the process of the invention can be carried out in 
substantially analogous fashion when the acid water contains sulphite ions 
instead of or in addition to sulphate ions.