Regeneration of adsorbents

A process for the regeneration and recovery of a loaded adsorbent which comprises the steps of: PA0 (a) contacting the loaded adsorbent with an alkaline solution for a period sufficient to effect regeneration of the adsorbent; PA0 diluting the resulting slurry of the adsorbent in the alkaline solution with recycled overflow liquid from one or both of the subsequent separation steps, to thereby reduce the concentration of adsorbent in the slurry to at least the level where unhindered settling can occur; PA0 (c) separating the slurry to give a first liquid overflow and a first solids underflow containing the adsorbent and discharging at least a portion of the first liquid overflow to waste, the remainder (if any) being returned to step (b); PA0 (d) slurrying the first solids underflow from step (c) with washwater and recycled second liquid overflow from the subsequent separation step; PA0 (e) separating the thus formed slurry to give a second liquid overflow and a second solids underflow containing the adsorbent and returning a portion of the second liquid overflow to step (d) and the balance to step (b); PA0 (f) recovering the second solids underflow containing the regenerated and washed adsorbent.

This invention is concerned with a method for regenerating adsorbents, in 
particular the so-called "coagulant/adsorbents" used in the water 
clarification process described in our Australian Patent No. 512,553 and 
patent application No. 40032/78. 
The patent describes how suspended impurities and coloured substances can 
be removed from water by contact with a "coagulant/adsorbent," that is a 
finely divided particulate mineral material, the individual particles of 
which have a thin hydroxylated surface layer which has a positive zeta 
potential at the adsorption pH, i.e., the pH of the water under treatment. 
The patent application shows that the operation of this process is 
improved by the addition of a polyelectrolyte during the treatment. 
Coagulant/adsorbent which has become loaded with impurities and coloured 
substances during water treatment is regenerated by a simple treatment 
with aqueous alkali. This releases from the surface of the 
coagulant/adsorbent the impurities and coloured substances which pass into 
the alkali solution and can thus be separated from the 
coagulant/adsorbent. After washing the coagulant/adsorbent can be recycled 
for use in the water treatment process. 
Because the coagulant/adsorbent must be in the form of very fine particles 
(less than 10 microns) the most preferred mineral materials for the 
coagulant/adsorbent are magnetic or magnetisable materials, in particular 
magnetite because of the relative ease with which such materials can be 
handled, i.e., by magnetic means, compared to non-magnetic particles of 
similar size. 
The commercial viability of the water clarification process described in 
our patent depends largely on the successful regeneration and reuse of the 
magnetite used as the preferred coagulant/adsorbent. The patent describes 
a process involving a three-stage, countercurrent-flow, alkali 
regeneration which makes use of magnetic separators and has been 
successfully applied in process trials. Further development of the process 
to the commercial plant scale, however, has delineated several 
disadvantages of the existing recovery process. Firstly, it is relatively 
expensive for small scale applications; the regeneration train can 
represent about half of the total capital cost of the plant. Because of 
this fact the existing clarification process is uneconomic for plants with 
a capacity less than 10 Ml/day. Also, the process produces a liquid 
effluent with a volume of not less than 5% of the total plant throughput, 
compared with 1/2% to 1% of plant throughput for the effluent volume from 
a conventional alum flocculation process. This difference can result in 
additional costs for effluent disposal in the process of the patent. 
The present invention has as its main objective, the provision of an 
improved recovery and regeneration procedure for the aforesaid 
coagulant/adsorbents, especially magnetite, which avoids the problems just 
referred to. 
During our investigations of alternative magnetite regeneration processes, 
we attempted to replace the magnetic separators used in our patented 
process with simple settling tanks but this gave unsuccessful results. At 
the water to magnetite ratios which we had used with magnetic separators 
in the regeneration stages of our patented process (18% to 25% w/w 
magnetite) we found although settling occurred, much of the turbidity and 
colour released from the surface of the magnetite following alkali 
treatment remained trapped in the settled solids bed rather than appearing 
in the supernatant liquid. This reduced the overall efficiency of the 
regeneration and washing stages and consequently the capacity of the 
regenerated magnetite to effect clarification of water when recycled in 
accordance with the patented process. 
We attribute this behaviour to the phenomenon we have termed "hindered 
settling," wherein the boundary layers surrounding adjacent particles 
interact and modify the velocity gradients in the vicinity of the particle 
surface. Thus, adjacent particles indirectly modify the forces acting on 
each other, and the settling velocity is greatly reduced from that of a 
single particle. The settling velocity of a particle no longer depends on 
its particle size, but only on the concentration of other particles in its 
immediate vicinity. 
Because of the relatively high concentration of solids in the slurry phase 
and the steep velocity gradient in the boundary layer surrounding each 
particle, colloidal particles in the interstitial liquid are much more 
likely to impinge on a particle surface. Consequently, the smaller 
particles of colloidal material become trapped in the interstices between 
the larger magnetite particles and are carried down with them, with the 
result that all the solids settle in the form of a blanket with a sharp 
interface between the slurry and the clear liquid phase. This is the prime 
reason for the apparent unsuitability of settling tanks in the present 
regeneration process. 
We have now found that the above problems can be overcome by reducing the 
slurry concentration to a level where individual particles do not interact 
and discrete unhindered settling can take place. For the magnetite-water 
system we have found this occurs below a slurry concentration of about 10% 
w/w magnetite and that by operating at slurry concentrations of this level 
it is possible to employ settling tanks in place of the magnetic 
separators of our patented process. 
However, using settling tanks and such a dilute slurry but retaining the 
flow scheme described in our patent, would require a very significant 
increase in the washwater consumption and consequently in the volume of 
effluent water for disposal. 
We have found that by suitable redesign of the regeneration and washing 
stages of the flowscheme we can not only avoid increasing washwater 
consumption, but we can actually reduce it, and the effluent for disposal, 
to levels comparable with those obtainable in a conventional 
alum-flocculation process. This new flowscheme forms the basis for the 
present invention. 
According to one aspect of the present invention, there is provided a 
process for the regeneration and recovery of a loaded adsorbent (as 
hereinafter described) which comprises the steps of: 
(a) contacting the loaded adsorbent with an alkaline solution for a period 
sufficient to effect regeneration of the adsorbent; 
(b) diluting the resulting slurry of the adsorbent in the alkaline solution 
with recycled overflow liquid from one or both of the subsequent 
separation steps, to thereby reduce the concentration of adsorbent in the 
slurry to at least the level where unhindered settling can occur; 
(c) separating the slurry to give a first liquid overflow and a first 
solids underflow containing the adsorbent and returning a portion of the 
first liquid overflow to step (b), the remainder being discharged to 
waste; 
(d) slurrying the first solids underflow from step (c) with washwater and 
recycled second liquid overflow from the subsequent separation step; 
(e) separating the thus formed slurry to give a second liquid overflow and 
a second solids underflow containing the adsorbent and returning a portion 
of the second liquid overflow to step (d) and the balance to step (b); 
(f) recovering the second solids underflow containing the regenerated and 
washed adsorbent. 
It is preferred that the separations in steps (c) and (e) are carried out 
in settling tanks, but the advantages of the process can be realized with 
other separating apparatus, such as magnetic separators. 
The term "adsorbent" is used herein for convenience to refer to 
finely-divided particulate magnetite or any other suitable adsorbent used 
as a coagulant/adsorbent in the process of our patent or patent 
application. "Loaded" implies the coagulant/adsorbent has been used in the 
water-clarification process. 
It will be evident to those skilled in the art that the regeneration and 
recovery process of the invention may also have other applications, i.e., 
those associated with water-clarification by methods other than those 
described in our patent and patent application. 
The preferred time for regeneration step (a) is about 10 minutes. 
As already indicated the preferred adsorbent is magnetite for which, in 
step (b) of the process the concentration after dilution should be less 
than about 10% w/w. 
The invention also includes a process for water clarification which 
includes the regeneration and recovery process defined above but is 
otherwise in accordance with the teachings of our patent or patent 
application.

As shown in the flowscheme, raw water (A) and regenerated 
coagulant/adsorbent (magnetite) are admixed at 1 and fed to a contactor 2 
which may be a pipe as described in our aforementioned Patent No. 512,553, 
or any other suitable apparatus. If desired the polyelectrolyte (B) may be 
added to the mixture (in accordance with our patent application No. 
40032/78) as it leaves the contactor 2. The mixture then passes to a 
solids clarifier 4 which may be of any suitable type, for example, those 
described in our patent. Clarified water (C) is taken off as the overflow 
from the clarifier 4. 
The solids underflow (D) from the clarifier 4 consists of a loaded 
coagulant/adsorbent, i.e., associated with the colloidal and other 
impurities which have been removed from the water. This underflow passes 
to the regeneration mixing stage 5 where it is mixed with dilute caustic 
soda to raise the pH to a level sufficient to free the coagulant/adsorbent 
from the impurities (e.g., about pH 10 to 11) and thence to another mixer 
6 where it is mixed with overflow liquid from one or both of the 
subsequent separator stages (lines E2 and G2) to reduce the solids content 
to a suitable level, less than 10% w/w in the case of magnetite. The 
mixture is then passed to a first settling tank 7. The overflow stream (E) 
from the tank 7 is split into two streams (E1 and E2), the first of which 
passes to waste and the second to the mixer 6. The underflow stream (F) 
containing the coagulant/adsorbent passes to a second mixer 8 where it is 
mixed with washwater (I) and overflow liquid from the subsequent separator 
stage (line G1). The mixture passes from mixer 8 to the second settling 
tank 9. 
The liquid overflow (G) from tank 9 is split into two streams (G1) and 
(G2). Stream (G1) is returned to the second mixer 8 and stream (G2) to the 
first mixer 6. 
The underflow stream (H) containing the regenerated washed magnetite is 
recycled for admixture at 1 with raw water. 
It will be obvious that process conditions can be adjusted by varying the 
ratio between streams (G1) and (G2) and (E1) and (E2). Such adjustments 
may include the complete elimination of some of the streams (G1) (G2) or 
(E2). 
It will be noted that in the present flowscheme the loaded magnetite from 
the clarifier (4) is regenerated in a single stage treatment with alkali 
(caustic soda). This differs from the flowscheme of our patented process 
which employs a three-stage alkali regeneration. 
As in our previous process, there are two washing stages after 
regeneration, but these have been altered as regards the provision of 
washwater recycle loops. By this revision of the washwater recycle 
arrangements, the solids concentration in the solid/liquid separation 
steps can be reduced to the point where discrete unhindered settling of 
the solid phase can occur without resulting in an increase in the total 
washwater requirement. In fact, it has been found possible to reduce the 
washwater consumption to surprisingly low levels (about 1% of plant 
throughput) with this new flowscheme. 
The process of the invention provides significant advantages over our 
earlier process, namely: 
1. Magnetic separators can be replaced with settling tanks. For small scale 
plants (&lt;10 Ml/day) this allows the use of simple cheap hopper bottom type 
clarifiers. 
2. The washwater makeup can be reduced to around 1% or less of plant 
throughput. This not only reduces operating costs but also greatly reduces 
the effluent disposal problems. 
3. The total number of steps for regeneration and washing has been reduced 
from 5 to 3. 
Those skilled in the art will appreciate that a further washing step or 
steps may be added to the above-described process, if this should be 
considered necessary. 
In Jar Test studies of the kind described in our patent and patent 
application, we investigated the performance of a magnetite 
coagulant/adsorbent over many cycles of water treatment, using Yarra River 
(Vic.) water, and regeneration, using mixing and settling procedures which 
simulated the process of the present invention. After 37 cycles the 
magnetite was still performing adequately as a coagulant/adsorbent and its 
performance was approaching a steady state, i.e., its performance before 
and after a regeneration stage was substantially unchanged. This indicates 
that the treatment/regeneration cycle could be repeated almost 
indefinitely. 
The invention is further illustrated by the following Examples. 
Examples 1 to 3 relate to a series of pilot plant experiments performed to 
study the regeneration and recovery of a magnetite coagulant/adsorbent 
which had been used at the rate of 12.5 g/l to clarify Yarra River (Vic) 
water having a turbidity of 60 NTU and a colour of 43 Pt-Co units. When 
fresh magnetite was used to clarify this water in accordance with the 
teachings of our patent and patent application, the clarified water had a 
turbidity of 0.5 NTU and a colour of 4 Pt-Co units. 
EXAMPLE 1 
The loaded magnetite was regenerated as described in our patent except that 
settling tanks were used in place of magnetic separators. The 
concentration of magnetite in the washing stages was 30% w/w and the wash 
water bleed rate was 3% i.e., the volume of water used to wash the 
regenerated magnetite was 3% of the total product water volume from the 
clarification stage. 
The regenerated and washed magnetite was then used to treat a further 
quantity of the river water, after which the magnetite was again 
regenerated. After two such cycles it was found that the turbidity of the 
clarified water had risen to 14 NTU and its colour to 9 Pt-Co units, i.e., 
the regenerated magnetite was substantially inferior to fresh magnetite in 
effecting clarification. 
EXAMPLE 2 
The procedure of Example 1 was repeated except that regeneration and 
recovery of the magnetite was carried out in accordance with the method of 
the present invention, the concentration of the magnetite in the washing 
stages being 8% w/w. After 35 cycles the clarified water had a turbidity 
of 0.5 NTU and a colour of 4 Pt-Co units, showing that the performance of 
the regenerated magnetite was equal to that of fresh magnetite. 
EXAMPLE 3 
The procedure of Example 2 was repeated except that the wash water bleed 
rate was reduced to 1%. After 35 cycles the clarified water had a 
turbidity of 0.8 NTU and a colour of 8 Pt-Co units, showing that a 
substantial reduction in the wash water bleed rate had only a minor effect 
on the quality of the product water. 
EXAMPLE 4 
The effect of varying the wash water feed rate on the quality of the 
clarified water was further examined in a pilot water clarification plant 
in which regeneration of the loaded adsorbent (magnetite) was performed as 
described in our patent and magnetic drum separators were used to carry 
out the separation stages during regeneration and washing of the 
magnetite. The concentration of the magnetite in the washing stages was 
30% w/w. It was found that when the wash water bleed rate was reduced from 
5% to 1% the turbidity of the product water rose from 1.0 NTU to 3.3 NTU. 
EXAMPLE 5 
The procedure of Example 4 was repeated in another water clarification 
plant using magnetic drum separators but in which the regeneration and 
washing stages were constructed and operated in accordance with the 
teachings of the present invention. The concentration of the magnetite in 
the washing stages was 8% w/w. When the wash water bleed rate was reduced 
from 5% to 1% the turbidity of the product water rose from 0.9 NTU to only 
1.3 NTU. 
This example when taken in conjunction with Example 4 demonstrates the 
greater efficiency of the regeneration and washing procedure of the 
present invention compared with the procedure disclosed in our patent even 
when magnetic separators are used. It also shows that the present 
invention allows a reduction in the wash water bleed rate to about 1% 
without significant decrease in the quality of the clarified water.