GENERALREFERENCES: Albertson,Fluid/Particle Sep. J.,7,IS (1994).
Jewell, Fourie, and Lord, Paste and Thickened Tailings—A Guide, pp.
49–79, Australian Centre for Geomechanics, 2002. Mular, Halbe, and Barratt,Mineral Processing Plant Design, Practice, and Control, vol.
2, pp. 1295–1312 and 2164–2173, SME, 2002. Sankey and Payne, Chemical Reagents in the Mineral Processing Industry,p. 245, SME, 1985. Schweitzer, Handbook of Separation Techniques for Chemical Engineers,2d ed., pp. 4-121 to 4-147, McGraw-Hill, 1988. Wilhelm and Naide, Min. Eng.(Littleton, Colo.), 1710 (1981).
Sedimentation is the partial separation or concentration of suspended solid particles from a liquid by gravity settling. This field may be divided into the functional operations of thickening and clarification.
The primary purpose of thickening is to increase the concentration of suspended solids in a feed stream, while that of clarification is to remove a relatively small quantity of suspended particles and produce a clear effluent. These two functions are similar and occur simultane- ously, and the terminology merely makes a distinction between the primary process results desired. Generally, thickener mechanisms are designed for the heavier-duty requirements imposed by a large quan- tity of relatively concentrated pulp, while clarifiers usually will include features that ensure essentially complete suspended-solids removal, such as greater depth, special provision for coagulation or flocculation of the feed suspension, and greater overflow-weir length.
CLASSIFICATION OF SETTLEABLE SOLIDS AND THE NATURE OF SEDIMENTATION
The types of sedimentation encountered in process technology will be greatly affected not only by the obvious factors—particle size, liquid viscosity, solid and solution densities—but also by the characteristics of the particles within the slurry. These properties, as well as the process requirements, will help determine both the type of equip- ment which will achieve the desired ends most effectively and the testing methods to be used to select the equipment.
Figure 18-93 illustrates the relationship between solids concentra- tion, interparticle cohesiveness, and the type of sedimentation that may exist. “Totally discrete” particles include many mineral particles (usually greater in diameter than 20 µm), salt crystals, and similar substances that have little tendency to cohere. “Flocculent” particles generally will include those smaller than 20 µm (unless present in a dispersed state owing to surface charges), metal hydroxides, many chemical precipi- tates, and most organic substances other than true colloids.
At low concentrations, the type of sedimentation encountered is called
FIG. 18-93 Combined effect of particle coherence and solids concentration on the settling characteristics of a suspension.
GRAVITY SEDIMENTATION OPERATIONS 18-67 particulate settling. Regardless of their nature, particles are sufficiently
far apart to settle freely. Faster-settling particles may collide with slower- settling ones and, if they do not cohere, continue downward at their own specific rate. Those that do cohere will form floccules of a larger diameter that will settle at a rate greater than that of the individual particles.
There is a gradual transition from particulate settling into the zone- settling regime, where the particles are constrained to settle as a mass.
The principal characteristic of this zone is that the settling rate of the mass, as observed in batch tests, will be a function of its solids concen- tration (for any particular condition of flocculation, particle density, etc.).
The solids concentration ultimately will reach a level at which par- ticle descent is restrained not only by hydrodynamic forces but also partially by mechanical support from the particles below; therefore, the weight of particles in mutual contact can influence the rate of sed- imentation of those at lower levels. This compression, as it is termed, will result in further solids concentration because of compaction of the individual floccules and partial filling of the interfloc voids by the deformed floccules. Accordingly, the rate of sedimentation in the compression regime is a function of both the solids concentration and the depth of pulp in this particular zone. As indicated in Fig. 18-93, granular, nonflocculent particles may reach their ultimate solids con- centration without passing through this regime.
As an illustration, coarse-size (45 µm) the aluminum oxide trihy- drate particles produced in the Bayer process would be located near the extreme left of Fig. 18-93. These solids settle in a particulate man- ner, passing through a zone-settling regime only briefly, and reach a terminal density or ultimate solids concentration without any signifi- cant compressive effects. At this point, the solids concentration may be as much as 80 percent by weight. The same compound, but of the gelatinous nature it has when precipitated in water treatment as alu- minum hydroxide, would be on the extreme right-hand side of the fig- ure. This flocculent material enters into a zone-settling regime at a low concentration (relative to the ultimate concentration it can reach) and gradually thickens. With sufficient pulp depth present, preferably aided by gentle stirring or vibration, the compression-zone effect will occur; this is essential for the sludge to attain its maximum solids con- centration, around 10 percent. Certain fine-size (1- to 2-µm) precipi- tates of this compound will possess characteristics intermediate between the two extremes.
A feed stream to be clarified or thickened can exist at any state rep- resented within this diagram. As it becomes concentrated owing to sedimentation, it may pass through all the regimes, and the settling rate in any one may be the size-determining factor for the required equipment.
Sedimentation Testing To design and size sedimentation equipment, reference information from similar applications is pre- ferred. Data from full-scale sedimentation equipment, operating in the application under consideration, are always a first choice for sizing new equipment. However, quite often the application under question deviates sufficiently from reference installations. The characteristics of the feed stream for the new application (i.e., solids characteristics, particle size, viscosities, pH, use of flocculants, etc.) must be identical to the existing application. It is also necessary to know how close to
“capacity” the existing equipment is operating. If the feed characteris- tics and operating conditions are different for the application under question, bench- or pilot-scale testing is recommended to size and design a new sedimentation unit.
To properly design and size sedimentation equipment, several pieces of information are required. Some information is unique to the job site (application, feed rate, etc.), while other data are supplied from similar references or from test work. Site-specific information from the plant site includes
• Application – objectives (underflow, TSS, hardness, etc.)
• Feed rate—design and maximum
• Feed characteristics—solids concentration, chemistry
• Site-specific requirements: seismic zone, weather-related specifica- tions, local mechanical design codes, and the user’s preferred design specifications
• Local operating practices
In the event testing is required to design either a thickener or a clari- fier, the testing must be structured to produce all or some of the following information:
• Feed stream characteristics
• Chemical treatment (type, solution concentration, dose, etc.): coag- ulants and flocculants (organic or inorganic); acid/base for treat- ment and pH correction
• Coagulation and flocculation (mixing time, energy requirements, solids concentration)
• Expected sedimentation objectives: underflow slurry density or concentration; overflow solids concentration (suspended solids and/or turbidity); chemical treatment for soluble components (i.e., hardness, metals, anions, pH, etc.)
• Vessel area and depth
• Settled solids rheology (for raking mechanism design and drive torque specification)
There are three basic approaches to testing for sedimentation equipment:
• Batch bench-scale settling tests The most common procedure requires a relatively small amount of sample tested in a controlled environment using laboratory equipment under static conditions.
• Semicontinuous bench-scale tests Laboratory pumps are used which pump feed slurry and chemicals into settling cylinders from which overflow liquor and underflow slurry are continuously col- lected.
• Continuous piloting A small-diameter thickener or clarifier of the same design as the full-scale equipment being considered is used.
TESTING COMMON TO CLARIFIERS AND THICKENERS Feed Characterization Sample characterization is necessary for both thickening and clarification testing. Without these data included in the basis of design, the sizing and predicted performance cannot be validated for the specified feed stream. Characterization requires the following measurements as a minimum:
• General chemical makeup of the solids and liquor phases
• Feed solids concentration
• Particle size distribution—include coarse (+100µm) and fine (−20µm) particle diameters
• Particle specific gravity
• Liquid specific gravity
• Liquid-phase dissolved materials concentration
• Temperature
• pH
Coagulant and/or Flocculant Selection Coagulants and floc- culants are widely used to enhance the settling rate which reduces thickener and clarifier size and improves overflow clarity and/or underflow slurry density. The terms coagulationandflocculationare sometimes used interchangeably; however, each term describes sepa- rate functions in the particle agglomeration process.
Coagulation is a preconditioning step that may be required to destabilize the solids suspension to allow complete flocculation to occur in clarification applications. Flocculation is the bridging and binding of destabilized solids into larger particles. As particle size increases, settling rate generally increases. The science of flocculation is not discussed here but can be found in numerous texts and litera- ture which are readily available from flocculant vendors.
Both coagulation and flocculation are typically considered in designing clarifiers, whereas flocculation is normally the only step in designing thickeners.
Coagulants may be either organic such as polyelectrolytes or inor- ganic such as alum. Coagulants can be used alone or in conjunction with flocculants to improve the performance of the flocculant or reduce the quantity of the flocculant required. In some systems, where a flocculant has been used in an upstream process, a coagulant may be needed to allow additional flocculant to be effective.
There are two primary types of flocculants:
• Natural flocculants Starch, guar, and other natural materials have historically been used for sedimentation flocculation, but have been replaced by more effective synthetic polymers.
• Synthetic polymeric flocculants There are hundreds of synthetic polymers available developed for specific applications.
Because of the many available flocculants, a screening program is necessary to choose an effective flocculant. The choice of flocculant can be narrowed by considering the following:
• Prior experience with flocculants on the feed stream under evalua- tion is always a good source of data.
• Test one each of the major types of flocculant charge: anionic, non- ionic, and cationic.
• Test one each of the synthetic polymer length: long chain, short chain.
The purpose of the screening test is to select a coagulant or floccu- lant whose generic type will most likely be effective in plant operation, and therefore, suitable for clarifier or thickener testing. Although a thickener or clarifier may be started up on the flocculant selected in the testing, it is very common to conduct further tests on the full-scale machine to further optimize dosage or flocculant type. The flocculant manufacturer can be a source of great assistance in both the testing and the full-scale optimization of flocculant use.
Coagulant or flocculant solutions should be made up according to the manufacturer’s instructions and used within the shelf life recom- mended. The solution concentration recommended for testing is typ- ically more dilute than the “neat” concentration so that the viscosity is lower to make dispersion more rapid during testing.
In the screen tests, each coagulant or flocculant is added to the beaker samples of representative slurry or liquor in a dropwise fash- ion, while the sample is mixed with a spatula, stirrer, or 3-6 jar stirrer mechanism. The amount of coagulant or flocculant required to initiate floc particle formation is noted along with relevant notes as to the size of the floc, capture of fines, resultant liquor clarity, and stability of the floc structure. The dosage is typically noted in g/t solids if the sample is primarily solids (thickener design), or in mg/L liquor if the sample is primarily for clarification and the solids concentration is low.
TESTING SPECIFIC TO CLARIFICATION
Detention Test This test utilizes a 1- to 4-L beaker or similar ves- sel. The sample is placed in the container, flocculated by suitable means if required, and allowed to settle. Small samples for suspended- solids analysis are withdrawn from a point approximately midway between liquid surface and settled solids interface, taken with suffi- cient care that settled solids are not resuspended. Sampling times may be at consecutively longer intervals, such as 5, 10, 20, 40, and 80 min.
The suspended-solids concentration can be plotted on log-log paper as a function of the sampling (detention) time. A straight line usually will result, and the required static detention time tto achieve a certain suspended-solids concentration Cin the overflow of an ideal basin can be taken directly from the graph. If the plot is a straight line, the data are described by the equation
C=Ktm (18-46)
where the coefficient Kand exponent mare characteristic of the par- ticular suspension.
Should the suspension contain a fraction of solids which can be con- sidered “unsettleable,” the data are more easily represented by using the so-called second-order procedure. This depends on the data being reasonably represented by the equation
Kt= − (18-47)
whereC∞is the unsettleable-solids concentration and C0 is the con- centration of suspended solids in the unsettled (feed) sample. The residual-solids concentration remaining in suspension after a suffi- ciently long detention time (C∞) must be determined first, and the data then plotted on linear paper as the reciprocal concentration func- tion 1(C−C∞) versus time.
Bulk Settling Test After the detention test is completed, a bulk settling test is done to determine the maximum overflow rate. This is done by carrying out a settling test in which the solids are first con- centrated to a level at which zone settling just begins. This is usually marked by a very diffuse interface during initial settling. Its rate of descent is measured with a graduated cylinder of suitable size, prefer- ably at least 1 L, and the initial straight-line portion of the settling curve is used for specifying a bulk-settling rate. The design overflow rate generally should not exceed half of the bulk settling rate. From the two clarifier tests, detention time and bulk settling rate, the more
1 C0−C∞
1 C−C∞
conservative results will govern the size of the clarifier.
Clarification with Solids Recycle In many instances, the rate of clarification is enhanced by increasing the solids concentration in the flocculation zone of the clarifier. This is done in a full-scale opera- tion by internally or externally recycling previously settled solids into the flocculation zone where they are mixed with fresh, coagulated feed. The higher population of solids improves the flocculation effi- ciency and clarification rate.
To conduct these tests, a sample of feed is first treated at the chem- ical dosages and mixing intensity determined in the screening tests and flocculated according to the screening test. The solids are allowed to settle, and the supernatant is carefully decanted. The settled solids are then transferred to a new fresh sample, and tests are conducted again, using the same chemical dosages and mixing intensity. Recycle can continue with subsequent tests until the suspended solids in the sample can have concentrations of 1, 2, 3, and 5 g/L. Bulk settling rate, suspended solids, and other effluent parameters are measured with each test until an optimal treatment scenario is found.
In some suspensions, very fine colloidal solids are present and are very difficult to coagulate. In these cases, it is typically necessary to adjust for coagulation mixing intensity and time to obtain coagulated solids that are more amenable to flocculation.
Detention Efficiency Conversion from the ideal basin sized by detention-time procedures to an actual clarifier requires the inclusion of an efficiency factor to account for the effects of turbulence and nonuni- form flow. Efficiencies vary greatly, being dependent not only on the rel- ative dimensions of the clarifier and the means of feeding but also on the characteristics of the particles. The curve shown in Fig. 18-94 can be used to scale up laboratory data in sizing circular clarifiers. The static detention time determined from a test to produce a specific effluent solids concen- tration is divided by the efficiency (expressed as a fraction) to determine the nominal detention time, which represents the volume of the clarifier above the settled pulp interface divided by the overflow rate. Different diameter-depth combinations are considered by using the corresponding efficiency factor. In most cases, area may be determined by factors other than the bulk-settling rate, such as practical tank-depth limitations.
TESTING SPECIFIC TO THICKENING
Optimization of Flocculation Conditions After a flocculant type is selected, the next step is to conduct a range of tests using the selected flocculant, to gather data on the effects of feed slurry solids concentrations on flocculant dosage and settling rate. There are a range of solids concentrations for which flocculation effectiveness is maximized, resulting in improved settling characteristics. Operating within this feed solids range results in smaller equipment sizes, higher underflow slurry densities, better overflow liquor clarity, and lower flocculant dosages.
The tests are conducted using a series of samples prepared at solids concentrations decreasing incrementally in concentration from the expected thickener feed concentration. Typically, the samples are pre- pared in 250- to 500-mL graduated cylinders which give some distance
FIG. 18-94 Efficiency curve for scale-up of batch clarification data to deter- mine nominal detention time in a continuous clarifier.
GRAVITY SEDIMENTATION OPERATIONS 18-69
to measure the settling rate more accurately. For some very fine solids samples (e.g., alumina red mud, clays, leached nickel laterites, etc), it is recommended to also check a sample diluted to 2 to 3 wt % solids.
Begin adding the flocculant solution dropwise; make notes on the dosage at which flocculation begins and the settling velocity. Continue adding flocculant incrementally and noting the floc structure, fines capture, liquor clarity, and settling velocity. Once the settling velocity remains constant for a few tests, sample testing can be stopped. From the tests, the plot shown in Fig. 18-95 can be drawn and the results used to set conditions for the larger and final tests for sizing the thick- ening equipment. The test procedure for the design tests should be structured to span the optimum solids concentration and two points slightly higher and lower. The flocculant dosage should be checked at the optimum and at dosages slightly higher and slightly lower than that determined in the above tests.
Determination of Thickener Basin Area The area require- ments for thickeners frequently are based on the solids flux rates mea- sured in the zone-settling regime. Theory holds that, for any specific sedimentation condition, a critical concentration will exist in the thick- ener which will limit the solids throughput rate. As the concentration in this critical zone represents a steady-state condition, its depth in the settling bed of solids may vary, responding to changes in the feed rate, underflow withdrawal rate, or flocculant dosage. In thickeners operat- ing at relatively high solids retention times and/or low throughput rates, this zone generally does not exist.
Many batch-test methods which are based on determining the solids flux rate at this critical concentration have been developed.
Most methods recognize that as the solids enter compression, thick- ening behavior is no longer a function only of solids concentration.
Hence, these methods attempt to utilize the “critical” point dividing these two zones and size the area on the basis of the settling rate of a layer of pulp at this concentration. The difficulty lies in discerning where this point is located on the settling curve.
Many procedures have been developed, but two have been more widely used: the Coe and Clevenger approach and the Kynch method as defined by Talmage and Fitch (op. cit.).
The former requires measurement of the initial settling rate of a pulp at different solids concentrations varying from feed to final underflow value. The area requirement for each solids concentration tested is calculated by equating the net overflow rate to the corre- sponding interfacial settling rate, as represented by the following equation for the unit area:
Unit area= (18-48)
whereCiis the solids concentration at the interfacial settling velocity viandCuis the underflow concentration, both concentrations being expressed in terms of mass of solids per unit volume of slurry. Using kg/L for the concentrations and m/day for the settling velocity yields a unit area value in m2/(ton/day).
These unit area values, plotted as a function of the feed concentra- tion, will describe a maximum value that can be used to specify the thickener design unit area for the particular underflow concentration Cuemployed in Eq. (18-48).
1Ci−1Cu
vi
The method is applicable for unflocculatedpulps or those in which the ionic characteristics of the solution produce a flocculent structure.
If polymeric flocculants are used, an approach based on the Kynch theory is preferred. In this method, the test is carried out at the opti- mum feed solids and flocculant dose (as determined in tests described earlier) and continued until underflow concentration is achieved in the cylinder. The flocculant solution should be added to the slurry under conditions which promote rapid dispersion and uniform, com- plete mixing with a minimum of shear. In cylinder tests, this is accom- plished by simultaneously injecting and mixing flocculant with the slurry, using an apparatus consisting of a syringe, a tube, and an inverted rubber stopper. The rubber stopper, having a diameter approximately 75 percent that of the cylinder diameter, provides suf- ficient turbulence as it is moved gently up and down through the slurry to cause good blending of the flocculant and pulp. To determine the unit area, Talmage and Fitch (op. cit.) proposed an equation derived from a relationship equivalent to that shown in Eq. (18-49):
Unit area= (18-49)
wheretuis the time, days; C0is the initial solids concentration in the feed, t/m3; and H0is the initial height of the slurry in the test cylinder, m. The term tuis taken from the intersection of a tangent to the curve at the critical point and a horizontal line representing the depth of pulp at underflow concentration. There are various means for select- ing this critical point, all of them empirical, and the unit area value determined cannot be considered precise. The review by Pearse (op.
cit.) presents many of the different procedures used in applying this approach to laboratory settling test data.
Two other approaches avoid using the critical point by computing the area requirements from the settling conditions existing at the underflow concentration. The Wilhelm and Naide procedure (op. cit.) applies zone-settling theory (Kynch) to the entire thickening regime.
Tangents drawn to the settling curve are used to calculate the settling velocity at all concentrations obtained in the test. This permits con- struction of a plot (Fig. 18-96) showing unit area as a function of underflow concentration.
A second, “direct” approach which yields a similar result, since it also takes compression into account, utilizes the value of settling time tx
taken from the settling curve at a particular underflow concentration.
This value is used to solve the Talmage and Fitch equation (18-49) for unit area.
Compression bed depth will have a significant effect on the overall settling rate (increasing compression zone depth reduces unit area).
Therefore, in applying either of these two procedures it is necessary to run the test in a vessel having an average bed depth close to that expected in a full-scale thickener. This requires a very large sample, and it is more convenient to carry out the test in a cylinder having a volume of 1 to 4 liters. The calculated unit area value from this test can be extrapolated to full-scale depth by carrying out similar tests at
tu
C0H0 FIG. 18-95 Data showing that slurry solids concentration affects flocculation
efficiency, thus improving solids settling flux.
FIG. 18-96 Characteristic relationship between thickener unit area and underflow solids concentration (fixed flocculant dosage and pulp depth).