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Colour and turbidity removal with reusable magnetite particles—V
Water Res. Vol. 17, No. 10, pp. 1227-1233, 1983 Printed in Great Britain. All rights reserved
0043-1354/8353.00 +0.00 Copyright © 1983 Pergamon Press Ltd
COLOUR A N D TURBIDITY REMOVAL WITH REUSABLE MAGNETITE PARTICLES--V PROCESS DEVELOPMENT N. J. ANDERSON and A. J. PRIESTLEY CSIRO Division of Chemical Technology, P.O. Box 310, South Melbourne, Victoria, Australia 3205 (Received May 1981)
process design has been developed for a continuous process to clarify turbid and coloured waters with alkali treated magnetite. Critical parts of the process have been investigated in small bench scale equipment, and criteria determined for the proper selection of equipment. A continuous pilot plant which appears to be both technically and economically feasible has been specified from the process design, and will be the subject of further investigations. Abstract--A
INTRODUCTION
In an earlier article in this series (Kolarik, 1983) a description was given of the discovery of the coagulating powers of alkali treated magnetite. The magnetite, of particle size 1-5/zm, when pretreated with 200ml of 0.1 M NaOH per 10ml of wet magnetite slurry (S.G of settled slurry = 2.0), and washed with tap water, was capable of removing turbidity and colour from river water to meet normal potable water standards. Regeneration of the magnetite with lime or acid was shown to be relatively ineffective, while the addition of an oxidant such as H202 with the caustic soda did improve regeneration but was not considered cost-effective. A simple picture of the basic mechanism of the process is illustrated by equations (1)-(3). O
/ \ --M \ /
OH
OH
I
f
M-- + H--OH ~.~--M--O--M--
(1)
0 \M--OH
/
~M--OH
/
+ H ÷ ~-~ \ M - - O H ~
/
+ OH- ~ ~M--O-
/
(2)
+ H20. (3)
Hydrolysis of an arbitrary metal oxide can be described by equation (1). This reaction occurs favourably under alkaline conditions to ~oroduce a highly hydroxylated surface. A charge can then be developed by the addition of acid or alkali as shown in equations (2) and (3). Under acidic conditions the surface is positively charged, and any negatively charged colloids should be attracted and attached to the magnetite surface. When the pH level is raised above 7, the surface becomes more negatively charged, and any previously attached colloids should be repelled. Consequently, the magnetite should act
as a coagulant/adsorbent at low pH, and be capable of regeneration at high pH. This paper describes the development of a fully continuous process that appears both technically and economically viable on a commercial scale. PROCESS DESCRIPTION
A brief description of the basic process steps and associated jar tests was given previously (Kolarik, 1983), but a more complete picture can be obtained from our process block diagram shown in Fig. 1. Initially, the feedwater pH is adjusted to an optimum value for its treatment. This can vary from 5.0 to as high as 8.5 depending on the particular water. Subsequently, the water is dosed with freshly regenerated magnetite slurry at a rate which can vary from 0.5 to 2.0% w/w of the raw water flow. The magnetite must be demagnetized before addition as otherwise it clumps together due to interparticle attraction and does not present its full surface area to the water. The first stage contact time between water and magnetite can vary from 5 to 10 min during which time colour bodies in the water (mainly humic and fulvic acids) are destabilized and adsorbed on to the magnetite surface. There is also some reduction of turbidity. Prior to the second stage, alum or polyelectrolyte can be dosed at a rate which is mainly dependent on the level of turbidity in the raw water. During the second contact period, which can vary from 2 to 4 min, final traces of colour are removed and the remaining turbidity is firmly bound on to the magnetite surface. The total contact time in the process can therefore vary from 7 to 14 rain. The contact stages are followed by separation of the loaded magnetite from the clarified water which now has a turbidity generally less than 1 N T U and colour less than 5 Pt-Co units. The magnetite is then regenerated prior to reuse, and it is this step which makes the process unique. Magnetite has been used
1227
1228
N.J. ANDERSONand A. J. PRIESTLEY
~-
water and Magnetite Magnetite
coagulant
I
i
Solid/LiquidH
Wash Magnetite
separation
Solid/LiquidI Clarified separation water
slurry and coagulant
I
Landed
Magnetite
Contact J I H Solid/Liquid~.~ Magneti te separation with.INNaOH/
l
Liquideffluent
Liquideffluent
Fig. 1. Process block diagram.
4, 5 5
egy adopted will be presented. Table 1 shows the main stages in the process development and the important aspects considered at each stage. In essence, the major aim of the development work was to achieve a fully continuous process, which was not only technically feasible on a large scale, but could also produce a good quality product water at an economic price.
laboratory; continuous
Applicability of the process
Table 1. The stages in process development Aspects considered
Stages
l. Establish main process steps (block diagram) 1 2. Preliminary economic and commercial evaluation 2, 3. Develop process flow diagram 3, 4. Design and select equipment 4, 5. Build and operate pilot plant 2, 6. Detailed economic evaluation 5 7. Optimize process design 3, 8. Design and construct demonstration plant 4, 9. Commercial sales 5 Aspects considered: 1- .~temonstration of process in 2-- applicability of process; 3~conversion from a batch to a process; ~-technical feasibility; 5~=conomic feasibility.
4, 5 4, 5 5 5
before in water treatment but only as a floc weighting agent and, then, only on a once-through basis (De Latour and Kolm, 1976). The magnetite used in our process, after regeneration with caustic soda, acts as a coagulant in its own right and can eliminate the requirement for conventional coagulants such as alum or ferric chloride. In regeneration there are two different steps. Firstly, the turbidity and colour bodies are stripped from the magnetite, and secondly the regenerated magnetite is washed clean. Regeneration requires a pH level in the range 11-12, while the washing step requires a pH level of between 10 and 11. PROCESS DEVELOPMENT
The early laboratory demonstration of the process by Kolarik (1983) was based on the treatment of Yarra River water. It is well known that waters from different geographical locations can vary widely in their character and ease of coagulation with alum. The applicability of this new process in treating a variety of waters was investigated in laboratory jar tests. Results of tests on surface and ground waters of varying turbidity, colour, hardness and pH are given in Table 2. While the ease of treatment varied, Myponga and Mirrabooka being difficult cases, no water was found which was not treatable with the process, the required colour level being achieved simply by increasing the dose of magnetite. It was concluded that the process was at least as broadly applicable as coagulation with alum.
Preliminary economic evaluation
Before proceeding with a detailed description of the process development, a brief outline of the strat-
The main advantages of the new process, as envisaged during early jar test work, were its relative speed
Table 2. The characteristics of various waters before and after treatment
Raw water source Yan Yean, Victoria
T (NTU) 2
Raw water C (Pt~Co) 70
pH 7.0
Alum (mg 1-~) 25 15
Mt. Crosby, Brisbane, Qld 4.2 20 7.9 Mirrrabooka, Western 12 62 6.0 Australia Myponga Resv., South 2 52 7.4 Australia Curries River, Bell Bay, 22 132 6.8 80 Tasmania Yarra River, Victoria 190 37 7.0 *Short chain, high cationic charge type containing quaternary ammonium groups. tShort chain, high cationic charge type containing predominantly weakly basic groups.
Treatment magnetite (g 1-I) 10 10 10 10
0.5*
0.8
14
5.5
LOt 3.0*
4.7 0.2 0.7
10 7.5 5
5.6 5.4 6.0
15 10
T (NTU) 1.3 0.5 0.6 0.8
Product water C (Pt-Co) 2.5 <5 5 13
Polymer (rag 1-~) ~ 0.2* 3.0*
pH 5.0 5.0 7.9 5.0
Colour and turbidity removal with reusable magnetite particles--V Table 3. Jar test economics Chemical
Acid
Polyelectrolyte
Caustic soda
Dose (mgl i)
0 50
0,1 2.0
~400
Cost (cent m -3)
04).27
0.02-0.5
~9.0
Wash water ~ 7 0 0 r o l l -I of product water ND
and simplicity when compared with the conventional alum process. Coagulation/flocculation times were around 7-14 min as compared with 20-30 min for the alum process and floc sedimentation speeds were greatly enhanced, 20-40 m h - ~ compared with 3-5 m h -t. Also, the high quality water produced in jar tests suggested that a filtration step may not be necessary. Consequently, it was predicted that if a cheap technique for regenerating magnetite could be devised, the development of a continuous process would lead to a significant reduction in capital costs for new water treatment plants. With respect to operating costs, the picture was far from clear. Table 3 summarizes the cost of chemicals used in the jar test process, and it is evident that the quantities of caustic soda and washwater are far in excess of economic levels. Chemical cost for conventional water treatment is normally 0.5-1.0cent m 3 (EPA, 1979). To achieve economic operating costs, about a 20-fold reduction in the usage of both caustic soda and washwater would be required in a continuous process, while still maintaining a high quality product water. It was felt that this could be achieved with efficient engineering design. In summary, the economic evaluation indicated that for the process to be successful, it should avoid the need for a filtration step, and should utilize a cheap and efficient process for magnetite regeneration which requires only small quantities of washwater and caustic soda. DEVELOPMENT
OF A CONTINUOUS
PROCESS
Flocculation The flocculation and clarification stages, as carried out in the jar test, were extremely simple, and it was desirable to maintain this property in the continuous process. The simplest option was to scale up the jar test apparatus to plant size, but as the process would operate continuously this would have led to a wide distribution of residence times and, consequently, a significant short-circuiting of the feed water. However, this problem could be largely overcome by having multiple tanks placed in series, although this would then require a corresponding number of stirrers.
Chemical dose Magnetite Alum (~) (mg 1-i) I 30 1 1
WR. 17<10 I~
27 27
1229
Two alternative contactor designs were considered, a solids recirculation clarifier and a simple pipeline or channel through which the magnetite/water slurry flowed in turbulent flow. To test these alternative designs small bench scale models were built and operated; Yarra River water was used as the raw water supply. A representative sample of the results is given in Table 4. The solids recirculation clarifier gave quite a poor performance, especially when compared to the pipeline which had a performance similar to that of the jar test. This was disappointing because it was hoped that, the higher concentration of solids would have resulted in a significant reduction in contact time using this approach. The reason for this failure is not clear at present and a further investigation may be warranted. However, the success of the pipeline meant that an alternative type of contactor to the stirred tank existed. To reduce potential problems with magnetite settling such contactors should have a vertical configuration, and large scale contactors can be built from concrete basins containing vertical partitions. The main advantage of the pipeline contactor is the plug flow condition associated with turbulent flow, which assures that no significant short-circuiting of feed water can occur. Its main disadvantage is that the only control over the agitation level is by variation of the throughput rate, which can be undesirable or even impossible in some situations. The stirred tank on the other hand has independent and direct control over the agitation conditions through the stirrer speed.
Clarification In the clarification step following the solid liquid contact stages, a small quantity of very fine magnetite (1% v/v) is being removed from a large quantity of water. In theory there are many ways in which this separation could be achieved, but if a large quantity of water has to be treated at low cost (e.g. municipal water supplies) then the range of options is reduced to a choice between simple gravity settling and magnetic separation using high gradient separators. High gradient magnetic separators (De Latour and Kolm, 1976) have been used to separate small quantities of very fine, weakly magnetic material from water, but on a unit capacity basis they are relatively expensive, having a capital cost of the order of A$40,000 per M1 day -t. This leaves gravity settling as the simpler and cheaper means of performing the separation. However, magnetic means can be employed to greatly enhance gravity settling. By passing the flow to the
Table 4. Contactor study Feed Contactor Turbidity Colgur type (NTU) (Pt-C'o) Recirculation 31 95 Clarifier Pipeline 69 80 Jar test 69 80
Product Turbidity Colour (NTU) (Pt-Co) 19 25 1. I 1.0
<5 <5
1230
N.J. ANDERSONand A. J. PRIESTLEY
settling tank between the poles of a pair of permanent magnets the magnetite is magnetized and selfflocculates by magnetic attraction to form large flocs which settle rapidly at rates around 40 m h -1. In practice, the clarifier is designed for upflow rates of around 10m h -~ to avoid carryover of partially flocculated magnetite.
Regeneration The development of a successful continuous magnetite regeneration process was difficult because of the need to greatly reduce both caustic soda and washwater usage over that used in the jar test. Firstly, in order to reduce the washwater requirement the principle of countercurrent flow was introduced into the washing process. Simple theory (McCabe and Smith, 1956) demonstrates that the reduction in washwater requirement is in direct relation to the number of theoretical washing stages. However, such theories relate to the washing out of purely soluble species which do not interact with the solid phase. Bench scale experiments were required to determine how countercurrent washing could best be achieved, approximately how many stages would be required, and whether any difficulties arose with interactions between magnetite and colloid particles. The reduction in caustic soda use required a differrent approach as the true caustic soda demand of the process was unknown. Kolarik (1983) had demonstrated that 0.1 M NaOH was superior to 0.05 M NaOH for regeneration, but that the more dilute solution was just as effective if heated to 60°C. The regeneration efficiency could depend on a stoichiometric demand for caustic soda, a requirement for a high pH, or an extended contact time or a combination of these variables. It was decided to stress the regeneration process by looking for a stoichiometric limitation while maintaining the same pH and contact time. This was achieved by reducing the liquid to solids ratio during regeneration to a minimum. This meant that only the void volume in a fully settled magnetite slurry would be raised to the required pH level of around 11.5. If successful this would result in at least a 20-fold reduction in caustic soda use. The two approaches described above were investigated in specially designed bench scale equipment. (i) The regeneration step was investigated by raising the pH level of a fully settled magnetite slurry to 11.5 and then pumping the slurry through a pipeline at a rate sufficient to give a contact time of fifteen minutes. At the end of the tube the magnetite was mixed via a tee junction with washwater at the rate of 4 ml of washwater per ml of slurry, and then passed through another pipeline operating under turbulent flow conditions. The magnetite was then separated from the washwater by an improvised magnetic separator and further washed batchwise before reuse. Three complete clarification/re-
generation cycles were carried out, and the activity of the magnetite after the third cycle was compared to the activity of unused material regenerated in standard jar tests. The results, given in Table 5 show no loss of activity over the three cycles and indicate that the reaction scheme was successful. No stoichiometric limit was detected but, as Kolarik (1983) had demonstrated the need for a high pH, further reduction in the caustic soda dose was delayed until operation of the pilot plant. (ii) Such immediate success, however, was not repeated when attempts were made to separate the magnetite from the non-magnetic colloids stripped from the magnetite surface. Early attempts at washing the magnetite with a reduced washwater volume were carried out in a vertical column with countercurrent flow of water and magnetite. These attempts were unsuccessful as indicated by a rapid deterioration in the product water quality obtained from a bench scale clarification rig using magnetite from the column. The reason for the failure of this approach appeared to be that, although the pH was raised to a level where the colloids should have been repelled from the magnetite surface, they were still physically enmeshed in a relatively thick magnetite slurry. The flow of washwater through the magnetite was laminar with no strong sheafing forces present. Consequently, the magnetite tended to clump together and trap colloidal material within the flocs. This postulate was supported by beaker experiments with reduced washwater volumes where the settling magnetite tended to drag turbidity out of the supernatent. It was concluded that to achieve a good separation some form of turbulence or shearing was required in the separating zone to break open the magnetite flocs and free the colloidal material trapped therein. With the failure of the countercurrent column approach an alternative way of achieving countercurrent flow was required. Another method often used in mineral processing operations is a staged countercurrent decantation system with solid/liquid separation occurring at each stage. A flow diagram for the complete process which uses this approach to washing is shown in Fig. 2. The washing scheme consists essentially of a three stage countercurrent decantation system with a thick slurry phase regeneration step placed in between wash steps 1 and 2. The regeneration step was placed in this intermediate position rather than at the start so that the regeneration effluent could be used to raise the pH of the loaded magnetite thus leading to a significant reduction in the caustic soda requirement. Once this flow diagram was established attention turned to Table 5. Comparison of the activity of new and recycled magnetite Feed Product Turbidity Colour Turbidity Colour (NTU) (Pt Co) (NTU) (Pt- Co) New 130 220 2.9 5 Used 130 220 3.0 5 (3 cycles)
Colour and turbidity removal with reusable magnetite particles--V
1231
Q - t U n i t of flow 5-10min.contact
Feed water
I
Rapid
(IOO o)
Dose Magnetite (IQ)
I
2 - 4 rain,
EM.
mm-
contact with
with vigorous agitation IOIO
Clarified wqfer
I
gentle agitation
(tO0 Q)
Loaded Mognetite (IQ) Alkaline wash (4Q) D,-
Dose
coagulant
1 @
mix at oH 10.5 I
g
(4Q)
~E
(5Q) 15 min. contact of pH 11.5
(50) Wash
(IQ) X
Reoenerotion effluent (4Q)
(IO) 12N NaOH
water ( 4Q )
(SQ)
To recover 0.C.-= Dernognehzing coil F.M. =--Flocculating magnet
'~
--=Solid/Liquid separation
Fig. 2. Process flow diagram.
choosing the right equipment to maximize the process efficiency.
Equipment selection Because of previous experience with the countercurrent column approach it was expected that the crucial step in the proposed flow diagram would be solid/liquid separation. This step had to provide a good separation of the magnetite from the nonmagnetic colloids which would otherwise be returned to the raw water. Possible operating characteristics of the proposed process were examined by a mathematical modeling study which attempted to simulate the washing process for both soluble and nonmagnetic solid materials. The model was based on a series of component mass balances around each washing stage, and took into account the efficiency of separation between the magnetite and the nonmagnetic colloids. This study indicated that the most important factor, apart from a good separation of magnetic and non-magnetic components at each stage, was the concentration of the magnetite slurry from each separation stage, as this determines the quantity of impurities carried over into the next stage. The overall washing efficiency for both soluble and solid phase components was quite sensitive to this variable with improved results being obtained as slurry concentration was increased. The overall washing efficiency was relatively insensitive to the degree of magnetic solids recovery at each stage, the only point in the process where a high degree of magnetics recovery is required being the top separator where the liquid effluent leaves the process. With a 4:1 by weight washwater to magnetite ratio, at least three separate stages were required to achieve
a reasonable washing efficiency, while a fourth stage could be added if a reduced washwater volume was considered important. The addition of a fifth stage gave only a marginal improvement in process performance, and was not considered viable from the viewpoint of incremental return on investment. A variety of solid/liquid separation techniques were considered for application to the proposed process. These were evaluated by a list of criteria developed from the work described above, and a summary of the results is given in Table 6. From Table 6 it can be seen that the only separation technique which satisfies all four selection criteria is the magnetic drum separator. Such separators have been used for many years in the coal washing industry to recover and clean the magnetite used in heavy media separation circuits. They are well developed pieces of equipment which have proved reliable in long term operation, and have a capital cost about an order of magnitude lower than the high gradient separators referred to earlier. Figure 3 illustrates the design of a concurrent style magnetic drum separator in which the drum shell rotates in the same direction as the flow of slurry. Five permanent magnets are positioned as shown with opposite poles
Table 6. Criteria for selection of a solid/liquid separator Criteria*
1
2
3
4
Sedimentation Cyclone Centrifuge Filter Magnetic drum separator
Yes No Yes No Yes
No Yes Yes No Yes
Yes No Yes No Yes
Yes Yes No No Yes
*Criteria: I must be able to separate submicron solids from magnetite particles of size I 10 pm. 2 must have turbulence in the separating zone; 3 must produce a thick solids underflow; , ~ must have a relatively low capital and operating cost.
1232
N.J. ANDERSONand A. J. PRIESTLEY
F d
P
minute was designed into the first washing stage. In early pilot-plant work this mixing time was reduced to a few seconds; the poor quality product water obtained from the adsorption/clarification cycle indicated inefficient regeneration of the magnetite. The required mixing time appears to depend also on the type of polyelectrolyte being used in the process, with longer mixing times and a higher pH level being required for strong base polyelectrolytes having a long chain length. Conventional mixing equipment can be used for the mixing process.
Drum shell
-
~'~"~
Magnetite 0,sohorge
qlv PILOT PLANTDESIGN
Tailings Overflow Fig. 3. Magnetic drum separator.
adjacent and, in some designs, separated by small spacer magnets. This arrangement of magnetic poles causes magnetic substances on the moving drum shell to be continually agitated as the poles are traversed. Non-magnetics are liberated by the agitation caused by the repeated reversals and consequently, the final magnetic discharge product is of high quality. The mixing steps in the proposed washing process are also important. Apart from repulping the magnetite slurry with the alkaline wash water prior to the next separation step, some contact time is required to achieve a good cleaning of the magnetite surface. Beaker experiments indicated that a contact time of at least 30 seconds was required to achieve a satisfactory removal of the colloids from the magnetite surface, and, consequently, a mixing time of one
The process development work described in the previous section has resulted in the production of a process flow diagram, and a rationale for the selection of equipment. The process, as designed, is impossible to simulate in the laboratory, and its suitability or otherwise can only be proved by the construction and operation of a fully continuous pilot plant. The results of the investigations described in this paper can be summarized by the pilot plant design presented in Fig. 4. This illustrates in detail the sequencing of equipment and the flow of water and magnetite through the process in a continuously operating plant. This diagram formed the basis for the construction of two pilot plants, the first of capacity 221min-~, built inside a caravan, and the second, a larger plant of capacity 901min-l, which was skid-mounted. The smaller pilot plant proved unreliable in long term operation, because of blocking and flow control problems associated with the very small size of equipment required to fit inside a
SYMBOLS.
Acid[ ~
Causticsoda
Polymer Sl
$2
Lime
Mixing
$4
S3
$9 .....
II
- ADSORPTION " ~ ~/ FLOCCULATION I I J I I I I I '
R o w wafer
}
'
I
tank with
inlet baffle
'
I
I
I
style
ognetic drum porotor Product water
~
Control valve
Magnetite
r P;,
r ~Ts Wash water
Regenerated '= D3 Magnetite~pi~ x~
Rotometer
i
Paddlestirrer
i
Propellorstirrer
Effluent Peristaltic
F.M.m=Flocculating magnet D C . m Demagnetizing coil
P2
P3 Fig. 4. Pilot plant design.
pump
Centrifugal pump Dosing pump
Colour and turbidity removal with reusable magnetite particles--V caravan. The larger pilot plant, built by the Australian Mineral Development Laboratories under contract to CSIRO, proved reliable in long term operation, and so it was decided to concentrate the longer term development work on this plant. The construction and operation of this plant will be the subject of the next paper in the series.
1233
relatively low level, and the wash water requirement reduced to about 4% of the plant throughput. While this gives the process a good chance of being economically viable, a more accurate answer to the economic question will be given by the long term operation of the larger pilot plant. REFERENCES
CONCLUSION A process design has been devised for a continuous pilot plant to clarify turbid waters with alkali treated magnetite. The design avoids the use of a filtration step and, because the equipment required for the plant is both conventional and commercially available, the process appears to be technically feasible. The caustic soda requirement has been reduced to a
Kolarik L. O. (1983) Colour and turbidity removal with reusable magnetite particles--IV. Alkali activated magnetitc---a new solid, reusable coagulent-adsorbent. Water Res. 17, 141-147. De Latour C. and Kolm H. H. (1976) High gradient magnetic separation--a water treatment alternative. J. Am. Wat. Wks Ass. 68, 325-327. McCabe W. C. and Smith J. S. (1956) Unit Operations of Chemical Engineering. McGraw-Hill, New York. U.S. Environment Protection Agency (1979) Estimating water treatment costs. EPA-600/2-79-162b, Cincinnati, OH.
0043-1354/8353.00 +0.00 Copyright © 1983 Pergamon Press Ltd
COLOUR A N D TURBIDITY REMOVAL WITH REUSABLE MAGNETITE PARTICLES--V PROCESS DEVELOPMENT N. J. ANDERSON and A. J. PRIESTLEY CSIRO Division of Chemical Technology, P.O. Box 310, South Melbourne, Victoria, Australia 3205 (Received May 1981)
process design has been developed for a continuous process to clarify turbid and coloured waters with alkali treated magnetite. Critical parts of the process have been investigated in small bench scale equipment, and criteria determined for the proper selection of equipment. A continuous pilot plant which appears to be both technically and economically feasible has been specified from the process design, and will be the subject of further investigations. Abstract--A
INTRODUCTION
In an earlier article in this series (Kolarik, 1983) a description was given of the discovery of the coagulating powers of alkali treated magnetite. The magnetite, of particle size 1-5/zm, when pretreated with 200ml of 0.1 M NaOH per 10ml of wet magnetite slurry (S.G of settled slurry = 2.0), and washed with tap water, was capable of removing turbidity and colour from river water to meet normal potable water standards. Regeneration of the magnetite with lime or acid was shown to be relatively ineffective, while the addition of an oxidant such as H202 with the caustic soda did improve regeneration but was not considered cost-effective. A simple picture of the basic mechanism of the process is illustrated by equations (1)-(3). O
/ \ --M \ /
OH
OH
I
f
M-- + H--OH ~.~--M--O--M--
(1)
0 \M--OH
/
~M--OH
/
+ H ÷ ~-~ \ M - - O H ~
/
+ OH- ~ ~M--O-
/
(2)
+ H20. (3)
Hydrolysis of an arbitrary metal oxide can be described by equation (1). This reaction occurs favourably under alkaline conditions to ~oroduce a highly hydroxylated surface. A charge can then be developed by the addition of acid or alkali as shown in equations (2) and (3). Under acidic conditions the surface is positively charged, and any negatively charged colloids should be attracted and attached to the magnetite surface. When the pH level is raised above 7, the surface becomes more negatively charged, and any previously attached colloids should be repelled. Consequently, the magnetite should act
as a coagulant/adsorbent at low pH, and be capable of regeneration at high pH. This paper describes the development of a fully continuous process that appears both technically and economically viable on a commercial scale. PROCESS DESCRIPTION
A brief description of the basic process steps and associated jar tests was given previously (Kolarik, 1983), but a more complete picture can be obtained from our process block diagram shown in Fig. 1. Initially, the feedwater pH is adjusted to an optimum value for its treatment. This can vary from 5.0 to as high as 8.5 depending on the particular water. Subsequently, the water is dosed with freshly regenerated magnetite slurry at a rate which can vary from 0.5 to 2.0% w/w of the raw water flow. The magnetite must be demagnetized before addition as otherwise it clumps together due to interparticle attraction and does not present its full surface area to the water. The first stage contact time between water and magnetite can vary from 5 to 10 min during which time colour bodies in the water (mainly humic and fulvic acids) are destabilized and adsorbed on to the magnetite surface. There is also some reduction of turbidity. Prior to the second stage, alum or polyelectrolyte can be dosed at a rate which is mainly dependent on the level of turbidity in the raw water. During the second contact period, which can vary from 2 to 4 min, final traces of colour are removed and the remaining turbidity is firmly bound on to the magnetite surface. The total contact time in the process can therefore vary from 7 to 14 rain. The contact stages are followed by separation of the loaded magnetite from the clarified water which now has a turbidity generally less than 1 N T U and colour less than 5 Pt-Co units. The magnetite is then regenerated prior to reuse, and it is this step which makes the process unique. Magnetite has been used
1227
1228
N.J. ANDERSONand A. J. PRIESTLEY
~-
water and Magnetite Magnetite
coagulant
I
i
Solid/LiquidH
Wash Magnetite
separation
Solid/LiquidI Clarified separation water
slurry and coagulant
I
Landed
Magnetite
Contact J I H Solid/Liquid~.~ Magneti te separation with.INNaOH/
l
Liquideffluent
Liquideffluent
Fig. 1. Process block diagram.
4, 5 5
egy adopted will be presented. Table 1 shows the main stages in the process development and the important aspects considered at each stage. In essence, the major aim of the development work was to achieve a fully continuous process, which was not only technically feasible on a large scale, but could also produce a good quality product water at an economic price.
laboratory; continuous
Applicability of the process
Table 1. The stages in process development Aspects considered
Stages
l. Establish main process steps (block diagram) 1 2. Preliminary economic and commercial evaluation 2, 3. Develop process flow diagram 3, 4. Design and select equipment 4, 5. Build and operate pilot plant 2, 6. Detailed economic evaluation 5 7. Optimize process design 3, 8. Design and construct demonstration plant 4, 9. Commercial sales 5 Aspects considered: 1- .~temonstration of process in 2-- applicability of process; 3~conversion from a batch to a process; ~-technical feasibility; 5~=conomic feasibility.
4, 5 4, 5 5 5
before in water treatment but only as a floc weighting agent and, then, only on a once-through basis (De Latour and Kolm, 1976). The magnetite used in our process, after regeneration with caustic soda, acts as a coagulant in its own right and can eliminate the requirement for conventional coagulants such as alum or ferric chloride. In regeneration there are two different steps. Firstly, the turbidity and colour bodies are stripped from the magnetite, and secondly the regenerated magnetite is washed clean. Regeneration requires a pH level in the range 11-12, while the washing step requires a pH level of between 10 and 11. PROCESS DEVELOPMENT
The early laboratory demonstration of the process by Kolarik (1983) was based on the treatment of Yarra River water. It is well known that waters from different geographical locations can vary widely in their character and ease of coagulation with alum. The applicability of this new process in treating a variety of waters was investigated in laboratory jar tests. Results of tests on surface and ground waters of varying turbidity, colour, hardness and pH are given in Table 2. While the ease of treatment varied, Myponga and Mirrabooka being difficult cases, no water was found which was not treatable with the process, the required colour level being achieved simply by increasing the dose of magnetite. It was concluded that the process was at least as broadly applicable as coagulation with alum.
Preliminary economic evaluation
Before proceeding with a detailed description of the process development, a brief outline of the strat-
The main advantages of the new process, as envisaged during early jar test work, were its relative speed
Table 2. The characteristics of various waters before and after treatment
Raw water source Yan Yean, Victoria
T (NTU) 2
Raw water C (Pt~Co) 70
pH 7.0
Alum (mg 1-~) 25 15
Mt. Crosby, Brisbane, Qld 4.2 20 7.9 Mirrrabooka, Western 12 62 6.0 Australia Myponga Resv., South 2 52 7.4 Australia Curries River, Bell Bay, 22 132 6.8 80 Tasmania Yarra River, Victoria 190 37 7.0 *Short chain, high cationic charge type containing quaternary ammonium groups. tShort chain, high cationic charge type containing predominantly weakly basic groups.
Treatment magnetite (g 1-I) 10 10 10 10
0.5*
0.8
14
5.5
LOt 3.0*
4.7 0.2 0.7
10 7.5 5
5.6 5.4 6.0
15 10
T (NTU) 1.3 0.5 0.6 0.8
Product water C (Pt-Co) 2.5 <5 5 13
Polymer (rag 1-~) ~ 0.2* 3.0*
pH 5.0 5.0 7.9 5.0
Colour and turbidity removal with reusable magnetite particles--V Table 3. Jar test economics Chemical
Acid
Polyelectrolyte
Caustic soda
Dose (mgl i)
0 50
0,1 2.0
~400
Cost (cent m -3)
04).27
0.02-0.5
~9.0
Wash water ~ 7 0 0 r o l l -I of product water ND
and simplicity when compared with the conventional alum process. Coagulation/flocculation times were around 7-14 min as compared with 20-30 min for the alum process and floc sedimentation speeds were greatly enhanced, 20-40 m h - ~ compared with 3-5 m h -t. Also, the high quality water produced in jar tests suggested that a filtration step may not be necessary. Consequently, it was predicted that if a cheap technique for regenerating magnetite could be devised, the development of a continuous process would lead to a significant reduction in capital costs for new water treatment plants. With respect to operating costs, the picture was far from clear. Table 3 summarizes the cost of chemicals used in the jar test process, and it is evident that the quantities of caustic soda and washwater are far in excess of economic levels. Chemical cost for conventional water treatment is normally 0.5-1.0cent m 3 (EPA, 1979). To achieve economic operating costs, about a 20-fold reduction in the usage of both caustic soda and washwater would be required in a continuous process, while still maintaining a high quality product water. It was felt that this could be achieved with efficient engineering design. In summary, the economic evaluation indicated that for the process to be successful, it should avoid the need for a filtration step, and should utilize a cheap and efficient process for magnetite regeneration which requires only small quantities of washwater and caustic soda. DEVELOPMENT
OF A CONTINUOUS
PROCESS
Flocculation The flocculation and clarification stages, as carried out in the jar test, were extremely simple, and it was desirable to maintain this property in the continuous process. The simplest option was to scale up the jar test apparatus to plant size, but as the process would operate continuously this would have led to a wide distribution of residence times and, consequently, a significant short-circuiting of the feed water. However, this problem could be largely overcome by having multiple tanks placed in series, although this would then require a corresponding number of stirrers.
Chemical dose Magnetite Alum (~) (mg 1-i) I 30 1 1
WR. 17<10 I~
27 27
1229
Two alternative contactor designs were considered, a solids recirculation clarifier and a simple pipeline or channel through which the magnetite/water slurry flowed in turbulent flow. To test these alternative designs small bench scale models were built and operated; Yarra River water was used as the raw water supply. A representative sample of the results is given in Table 4. The solids recirculation clarifier gave quite a poor performance, especially when compared to the pipeline which had a performance similar to that of the jar test. This was disappointing because it was hoped that, the higher concentration of solids would have resulted in a significant reduction in contact time using this approach. The reason for this failure is not clear at present and a further investigation may be warranted. However, the success of the pipeline meant that an alternative type of contactor to the stirred tank existed. To reduce potential problems with magnetite settling such contactors should have a vertical configuration, and large scale contactors can be built from concrete basins containing vertical partitions. The main advantage of the pipeline contactor is the plug flow condition associated with turbulent flow, which assures that no significant short-circuiting of feed water can occur. Its main disadvantage is that the only control over the agitation level is by variation of the throughput rate, which can be undesirable or even impossible in some situations. The stirred tank on the other hand has independent and direct control over the agitation conditions through the stirrer speed.
Clarification In the clarification step following the solid liquid contact stages, a small quantity of very fine magnetite (1% v/v) is being removed from a large quantity of water. In theory there are many ways in which this separation could be achieved, but if a large quantity of water has to be treated at low cost (e.g. municipal water supplies) then the range of options is reduced to a choice between simple gravity settling and magnetic separation using high gradient separators. High gradient magnetic separators (De Latour and Kolm, 1976) have been used to separate small quantities of very fine, weakly magnetic material from water, but on a unit capacity basis they are relatively expensive, having a capital cost of the order of A$40,000 per M1 day -t. This leaves gravity settling as the simpler and cheaper means of performing the separation. However, magnetic means can be employed to greatly enhance gravity settling. By passing the flow to the
Table 4. Contactor study Feed Contactor Turbidity Colgur type (NTU) (Pt-C'o) Recirculation 31 95 Clarifier Pipeline 69 80 Jar test 69 80
Product Turbidity Colour (NTU) (Pt-Co) 19 25 1. I 1.0
<5 <5
1230
N.J. ANDERSONand A. J. PRIESTLEY
settling tank between the poles of a pair of permanent magnets the magnetite is magnetized and selfflocculates by magnetic attraction to form large flocs which settle rapidly at rates around 40 m h -1. In practice, the clarifier is designed for upflow rates of around 10m h -~ to avoid carryover of partially flocculated magnetite.
Regeneration The development of a successful continuous magnetite regeneration process was difficult because of the need to greatly reduce both caustic soda and washwater usage over that used in the jar test. Firstly, in order to reduce the washwater requirement the principle of countercurrent flow was introduced into the washing process. Simple theory (McCabe and Smith, 1956) demonstrates that the reduction in washwater requirement is in direct relation to the number of theoretical washing stages. However, such theories relate to the washing out of purely soluble species which do not interact with the solid phase. Bench scale experiments were required to determine how countercurrent washing could best be achieved, approximately how many stages would be required, and whether any difficulties arose with interactions between magnetite and colloid particles. The reduction in caustic soda use required a differrent approach as the true caustic soda demand of the process was unknown. Kolarik (1983) had demonstrated that 0.1 M NaOH was superior to 0.05 M NaOH for regeneration, but that the more dilute solution was just as effective if heated to 60°C. The regeneration efficiency could depend on a stoichiometric demand for caustic soda, a requirement for a high pH, or an extended contact time or a combination of these variables. It was decided to stress the regeneration process by looking for a stoichiometric limitation while maintaining the same pH and contact time. This was achieved by reducing the liquid to solids ratio during regeneration to a minimum. This meant that only the void volume in a fully settled magnetite slurry would be raised to the required pH level of around 11.5. If successful this would result in at least a 20-fold reduction in caustic soda use. The two approaches described above were investigated in specially designed bench scale equipment. (i) The regeneration step was investigated by raising the pH level of a fully settled magnetite slurry to 11.5 and then pumping the slurry through a pipeline at a rate sufficient to give a contact time of fifteen minutes. At the end of the tube the magnetite was mixed via a tee junction with washwater at the rate of 4 ml of washwater per ml of slurry, and then passed through another pipeline operating under turbulent flow conditions. The magnetite was then separated from the washwater by an improvised magnetic separator and further washed batchwise before reuse. Three complete clarification/re-
generation cycles were carried out, and the activity of the magnetite after the third cycle was compared to the activity of unused material regenerated in standard jar tests. The results, given in Table 5 show no loss of activity over the three cycles and indicate that the reaction scheme was successful. No stoichiometric limit was detected but, as Kolarik (1983) had demonstrated the need for a high pH, further reduction in the caustic soda dose was delayed until operation of the pilot plant. (ii) Such immediate success, however, was not repeated when attempts were made to separate the magnetite from the non-magnetic colloids stripped from the magnetite surface. Early attempts at washing the magnetite with a reduced washwater volume were carried out in a vertical column with countercurrent flow of water and magnetite. These attempts were unsuccessful as indicated by a rapid deterioration in the product water quality obtained from a bench scale clarification rig using magnetite from the column. The reason for the failure of this approach appeared to be that, although the pH was raised to a level where the colloids should have been repelled from the magnetite surface, they were still physically enmeshed in a relatively thick magnetite slurry. The flow of washwater through the magnetite was laminar with no strong sheafing forces present. Consequently, the magnetite tended to clump together and trap colloidal material within the flocs. This postulate was supported by beaker experiments with reduced washwater volumes where the settling magnetite tended to drag turbidity out of the supernatent. It was concluded that to achieve a good separation some form of turbulence or shearing was required in the separating zone to break open the magnetite flocs and free the colloidal material trapped therein. With the failure of the countercurrent column approach an alternative way of achieving countercurrent flow was required. Another method often used in mineral processing operations is a staged countercurrent decantation system with solid/liquid separation occurring at each stage. A flow diagram for the complete process which uses this approach to washing is shown in Fig. 2. The washing scheme consists essentially of a three stage countercurrent decantation system with a thick slurry phase regeneration step placed in between wash steps 1 and 2. The regeneration step was placed in this intermediate position rather than at the start so that the regeneration effluent could be used to raise the pH of the loaded magnetite thus leading to a significant reduction in the caustic soda requirement. Once this flow diagram was established attention turned to Table 5. Comparison of the activity of new and recycled magnetite Feed Product Turbidity Colour Turbidity Colour (NTU) (Pt Co) (NTU) (Pt- Co) New 130 220 2.9 5 Used 130 220 3.0 5 (3 cycles)
Colour and turbidity removal with reusable magnetite particles--V
1231
Q - t U n i t of flow 5-10min.contact
Feed water
I
Rapid
(IOO o)
Dose Magnetite (IQ)
I
2 - 4 rain,
EM.
mm-
contact with
with vigorous agitation IOIO
Clarified wqfer
I
gentle agitation
(tO0 Q)
Loaded Mognetite (IQ) Alkaline wash (4Q) D,-
Dose
coagulant
1 @
mix at oH 10.5 I
g
(4Q)
~E
(5Q) 15 min. contact of pH 11.5
(50) Wash
(IQ) X
Reoenerotion effluent (4Q)
(IO) 12N NaOH
water ( 4Q )
(SQ)
To recover 0.C.-= Dernognehzing coil F.M. =--Flocculating magnet
'~
--=Solid/Liquid separation
Fig. 2. Process flow diagram.
choosing the right equipment to maximize the process efficiency.
Equipment selection Because of previous experience with the countercurrent column approach it was expected that the crucial step in the proposed flow diagram would be solid/liquid separation. This step had to provide a good separation of the magnetite from the nonmagnetic colloids which would otherwise be returned to the raw water. Possible operating characteristics of the proposed process were examined by a mathematical modeling study which attempted to simulate the washing process for both soluble and nonmagnetic solid materials. The model was based on a series of component mass balances around each washing stage, and took into account the efficiency of separation between the magnetite and the nonmagnetic colloids. This study indicated that the most important factor, apart from a good separation of magnetic and non-magnetic components at each stage, was the concentration of the magnetite slurry from each separation stage, as this determines the quantity of impurities carried over into the next stage. The overall washing efficiency for both soluble and solid phase components was quite sensitive to this variable with improved results being obtained as slurry concentration was increased. The overall washing efficiency was relatively insensitive to the degree of magnetic solids recovery at each stage, the only point in the process where a high degree of magnetics recovery is required being the top separator where the liquid effluent leaves the process. With a 4:1 by weight washwater to magnetite ratio, at least three separate stages were required to achieve
a reasonable washing efficiency, while a fourth stage could be added if a reduced washwater volume was considered important. The addition of a fifth stage gave only a marginal improvement in process performance, and was not considered viable from the viewpoint of incremental return on investment. A variety of solid/liquid separation techniques were considered for application to the proposed process. These were evaluated by a list of criteria developed from the work described above, and a summary of the results is given in Table 6. From Table 6 it can be seen that the only separation technique which satisfies all four selection criteria is the magnetic drum separator. Such separators have been used for many years in the coal washing industry to recover and clean the magnetite used in heavy media separation circuits. They are well developed pieces of equipment which have proved reliable in long term operation, and have a capital cost about an order of magnitude lower than the high gradient separators referred to earlier. Figure 3 illustrates the design of a concurrent style magnetic drum separator in which the drum shell rotates in the same direction as the flow of slurry. Five permanent magnets are positioned as shown with opposite poles
Table 6. Criteria for selection of a solid/liquid separator Criteria*
1
2
3
4
Sedimentation Cyclone Centrifuge Filter Magnetic drum separator
Yes No Yes No Yes
No Yes Yes No Yes
Yes No Yes No Yes
Yes Yes No No Yes
*Criteria: I must be able to separate submicron solids from magnetite particles of size I 10 pm. 2 must have turbulence in the separating zone; 3 must produce a thick solids underflow; , ~ must have a relatively low capital and operating cost.
1232
N.J. ANDERSONand A. J. PRIESTLEY
F d
P
minute was designed into the first washing stage. In early pilot-plant work this mixing time was reduced to a few seconds; the poor quality product water obtained from the adsorption/clarification cycle indicated inefficient regeneration of the magnetite. The required mixing time appears to depend also on the type of polyelectrolyte being used in the process, with longer mixing times and a higher pH level being required for strong base polyelectrolytes having a long chain length. Conventional mixing equipment can be used for the mixing process.
Drum shell
-
~'~"~
Magnetite 0,sohorge
qlv PILOT PLANTDESIGN
Tailings Overflow Fig. 3. Magnetic drum separator.
adjacent and, in some designs, separated by small spacer magnets. This arrangement of magnetic poles causes magnetic substances on the moving drum shell to be continually agitated as the poles are traversed. Non-magnetics are liberated by the agitation caused by the repeated reversals and consequently, the final magnetic discharge product is of high quality. The mixing steps in the proposed washing process are also important. Apart from repulping the magnetite slurry with the alkaline wash water prior to the next separation step, some contact time is required to achieve a good cleaning of the magnetite surface. Beaker experiments indicated that a contact time of at least 30 seconds was required to achieve a satisfactory removal of the colloids from the magnetite surface, and, consequently, a mixing time of one
The process development work described in the previous section has resulted in the production of a process flow diagram, and a rationale for the selection of equipment. The process, as designed, is impossible to simulate in the laboratory, and its suitability or otherwise can only be proved by the construction and operation of a fully continuous pilot plant. The results of the investigations described in this paper can be summarized by the pilot plant design presented in Fig. 4. This illustrates in detail the sequencing of equipment and the flow of water and magnetite through the process in a continuously operating plant. This diagram formed the basis for the construction of two pilot plants, the first of capacity 221min-~, built inside a caravan, and the second, a larger plant of capacity 901min-l, which was skid-mounted. The smaller pilot plant proved unreliable in long term operation, because of blocking and flow control problems associated with the very small size of equipment required to fit inside a
SYMBOLS.
Acid[ ~
Causticsoda
Polymer Sl
$2
Lime
Mixing
$4
S3
$9 .....
II
- ADSORPTION " ~ ~/ FLOCCULATION I I J I I I I I '
R o w wafer
}
'
I
tank with
inlet baffle
'
I
I
I
style
ognetic drum porotor Product water
~
Control valve
Magnetite
r P;,
r ~Ts Wash water
Regenerated '= D3 Magnetite~pi~ x~
Rotometer
i
Paddlestirrer
i
Propellorstirrer
Effluent Peristaltic
F.M.m=Flocculating magnet D C . m Demagnetizing coil
P2
P3 Fig. 4. Pilot plant design.
pump
Centrifugal pump Dosing pump
Colour and turbidity removal with reusable magnetite particles--V caravan. The larger pilot plant, built by the Australian Mineral Development Laboratories under contract to CSIRO, proved reliable in long term operation, and so it was decided to concentrate the longer term development work on this plant. The construction and operation of this plant will be the subject of the next paper in the series.
1233
relatively low level, and the wash water requirement reduced to about 4% of the plant throughput. While this gives the process a good chance of being economically viable, a more accurate answer to the economic question will be given by the long term operation of the larger pilot plant. REFERENCES
CONCLUSION A process design has been devised for a continuous pilot plant to clarify turbid waters with alkali treated magnetite. The design avoids the use of a filtration step and, because the equipment required for the plant is both conventional and commercially available, the process appears to be technically feasible. The caustic soda requirement has been reduced to a
Kolarik L. O. (1983) Colour and turbidity removal with reusable magnetite particles--IV. Alkali activated magnetitc---a new solid, reusable coagulent-adsorbent. Water Res. 17, 141-147. De Latour C. and Kolm H. H. (1976) High gradient magnetic separation--a water treatment alternative. J. Am. Wat. Wks Ass. 68, 325-327. McCabe W. C. and Smith J. S. (1956) Unit Operations of Chemical Engineering. McGraw-Hill, New York. U.S. Environment Protection Agency (1979) Estimating water treatment costs. EPA-600/2-79-162b, Cincinnati, OH.