introduction

since williamson first created a system to combine phosphatation and flotation in l9l9^[1], which process was termed “phosflotation” by saranin^[2], many advances have been achieved and new flotation-clarification processes have been developed in the sugar industry. these processes have improved the quality of the sugar produced, and in some cases, increased the capacity of sugar plants. one important advance is the extensive application of highly efficient polyacrylamide flocculants, which greatly increased the velocity and the stability of the flotation-separation process. another advance is the application of special cationic surfactants such as “talofloc”^[3], or dioctadecyl dimethyl ammonium chloride, which combines with the negatively charged colorants (the major portion of color) as well as other cationic impurities, which then precipitate together.

by this means, the efficiency of color removal and clarification has been increased to a high level. however, these chemicals are still too expensive for many sugar plants, especially when the world price of sugar is low.

in china, most cane sugar factories produce plantation white sugar using a double sulphitation or double carbonatation process. after the grinding season, some of them play the roe of a refinery and produce white sugar from imported raw sugar. since the 1970's, the author and his colleagues have carried out intensive research on phosflotation and other methods of flotation-clarification. based on this research, a new, highly efficient and low cost system has been developed and brought into use in several sugar factories in the guangdong province of china. it is a process combining phosflotation with either sulphitation or carbonatation in which all of the precipitate is removed from the liquor by flotation. compared with decolorization of about 30% for the simple phosphatation process, this new system has a color removal of 50 to 70%, depending on the manner of combination and the working conditions of the process, which can be selected flexibly according to the quality of the raw sugar treated and the level of c1arification efficiency desired.

there exists a special problem for this new system, as caso₃, and caco₃, precipitates are heavier and more difficult to float than calcium phosphate and, as the amount of these precipitates is much larger than that in the phosphatation process, it is quite difficult to make such heavy precipitate float steadily and quickly. to solve this problem some new equipment for aerating the liquor and for creating good flocculation and flotation have been designed by the author. they have given satisfactory results and enabled this system to run smoothly. 

1. combination of phosflotation and sulphitation

when the phosflotation process was first applied in our sugar factories to clarify raw sugar liquor, so₂ was not used. it has been proved in practice, however, that the quality of sugar produced can be improved by addition of small amount of so₂, into the clarified liquor after phosflotation, and the effect is even better when a higher level of sulphitation is applied prior to phosflotation. in the latter method, the caso₃ precipitate forming in the liquor can be removed together with calcium phosphate by flotation, and both functions of color removal - by so₂ (mainly based on chemical reaction) and by caso₃(main1y based on adsorption) - can be utilized, giving a higher decolorization of 50-65%, depending on the level of so₂ added. the flow sheet of this combined process is shown below:　

the main operating conditions are:

the efficiency of color removal by the process increases with the amount of so₂ added, and thus can be adjusted to the desired level according to requirement. when the raw sugars treated are of good quality, for instance, with a color value below 4000 mau, the addition of 0.4-0.8 g/l of so₂, is sufficient to produce white sugar of normal quality (90-l20 mau); if it is increased to l.0 or even l.4 g/l, superior white sugar with a low color of 80 or down to 60 mau can be produced. on the other hand, when the raw sugars are poor in quality, such as with a high color of over 8000 mau, considerable problems would occur in the refining process, so a high level of sulphitation is necessary for these raws to keep the process running normally. for instance, in l987, mei-san sugar factory in guangdong province (with a crushing capacity of 6000 tons of cane per day) received imported raws with a dark brown color. during the two months refining period, by controlling the leve1 of so₂ at l.2-l.4 g/l, the quality of white sugar was kept normal. some color figures during this period were as follows:

during the milling season in our sugar factories, phosflotation is also used for syrup clarification, combined with juice clarification in which l.2-l.4 g/l of so₂ is added. nevertheless, it has been proved that, when the clarified syrup absorbs a small amount of so₂ of about 0.2 g/l again, the effect is even better. superior white sugar with the following quality index has been produced in zhong-san sugar factory by this process:

 moreover, it is a significant phenomenon that these sugars are of good keeping quality. some samples made in l983 have been stored in white glass bottles, without sealing to make them airtight. at the present time, although they have absorbed moisture from the air and have turned wet and sticky, they still remain quite white and bright, without apparent yellowing or darkening. in this process, both juice and syrup sulphitation are necessary. according to our experience over a long period of time, both sulphitation and phosphatation each has its own function of removing colorants and other impurities. they can complement each other, but the one cannot completely replace the other. this is also true of the juice and syrup clarification process.

the effectiveness of phosphatation has been studied in detail by many researchers^[4] and has been accepted commonly in the sugar industry; but on the function of sulphitation, different evaluations exist. in l984, based on a series of researches, shore pointed out^[5] " so₂ is effective in inhibiting the color forming reactions which occur during the storage of sugar as well as during the processing stages", and  " the major role of so₂ as used in the factory process is that of inhibiting the non-enzymic browning reactions", and "so₂ also has a role in inhibiting color formation by enzymic reactions". these conclusions are also confirmed by our experience.

it is worthwhile to mention the fact that, in most sugar factories in guangdong province, both juice and syrup sulphitation are performed respectively using tubular reactors, which work under the condition of a slight vacuum created by juice or syrup injection. this equipment is of simple construction, having a high absorption efficiency of over 92% and with only a very short retention time of a few minutes.

2. combination of phosflotation and carbonatation

the carbonatation process is well known to be more effective in removing colorants and other impurities than sulphitation; however, it requires a great deal of capital investment for equipment and produces a large amount of alkaline filter mud, the disposal of which is becoming increasingly difficult because of pollution problems.

to make use of the advantages of carbonatation but avoid its shortcomings, a new system, consisting of low-level carbonatation and phosflotation, has been developed in guangdong and put into use with satisfactory results. this process involves two steps of treatment and flotation as shown in flow sheet 2:

the main operating conditions are as follows:

although the raw sugar is of a very dark color, this process yields a white sugar of low color and sparkling appearance.

in this process, the amount of lime added is reduced considerably and hence, the amount of filter mud is also much less than with traditional carbonatation. since the mud contains phosphate and is lower in alkalinity, it is a suitable fertilizer for acid soil, and the problem of pollution is decreased to a minimum.

the co₂ used in this process may be obtained from either alcohol fermentation or flue gas, but the former is much better in this application. when fermentation gas is used, the equipment of this system can be considerably simplified because this gas is nearly pure co₂. a tubular reactor used for co₂ saturation of sugar liquor designed by the author has resulted in good performance and high efficiency of co₂ absorption. the gas enters the tubular reactor and mixes with the liquor for only a few seconds, during which about 70% of the co₂ is absorbed; the mixture then enters a small tank where co₂ is further absorbed to approximately 90% in about ten minutes. this process is easy to control, the equipment and gas pipe are of small size and quite simple, and the power consumption is small. the greatest benefit is obtained when this process is applied in a plant, which has adjacent alcohol production. investment cost for this simplified carbonatation process is much lower than with traditional carbonatation. on the other hand, if flue gas is used to provide co₂, the saturator, washing equipment and gas piping are larger and more complicated, and the power consumption is much bigger than when using fermentation gas.

the precipitated caco₃ formed in the liquor is around l0 g/l, and such an amount of heavy precipitate particles is very difficult to float. to solve this problem is of vital importance. in this area, some effective measures in aeration and flocculation have been applied which will be described below.

this system involves two stages of flotation, and the primary stage needs more flocculent and air bubbles for floating more precipitate. to serve the two stages of treatment, a newly designed double-layer clarifier of shallow type is used, and the retention time of each layer is l4 to l8 minutes, depending on the amount of liquor treated.

usually, good and complete flocculation and flotation can be achieved in both steps of this process when running under normal conditions, and both clarified liquors are transparent. but if the working conditions of carbonatation are unstable or unsuitable, the first flotation would worsen and the primary clarified liquor would be turbid. however, this residual suspended matter can be removed at the secondary flotation, which works under more favorable conditions (with less precipitate mainly comprising calcium phosphate which is easier to flocculate and float); thus the final liquor is still clean and bright.

it appears that this process can be further simplified into one step of flotation, by prior addition of phosphoric acid, controlling the ph at around 8.0, and providing with automatic devices to maintain the temperature, ph and chemical dosage suitable and stable. then the system will be more beneficial and economical.

3. mechanism of flotation process

the modern flotation process is a highly efficient technology for separating solid particles in liquid to remove or recover them through addition of suitable air bubbles and flocculent. by comparison with the traditional method of sedimentation that is still in common use in the sugar industry and many other areas, the flotation process has a much higher separation velocity so that the flotation equipment can be of smaller volume. most solid particles in sugar juices settle under gravity at such a low velocity that many factories have to install sedimentation tanks (clarifiers) with a very large volume. an additional advantage for the sugar industry of increasing the separation velocity is shortcoming of the retention time and, hence, reduced sugar loss.

the flotation process works on the princip1e of forming low-density aggregates of particles and bubbles, and the lower the density, the more quickly they float. it is obvious therefore that the most important factor for this process is to have all solid particles attached firmly to sufficient air bubbles, which in turn is determined by many physical-chemical factors and hydrodynamic parameters.

physical-chemically, the properties of the solid particle surface can be divided into two classes: hydrophobicity and hydrophilicity. particles with hydrophobic surfaces themselves repel the water from their surfaces and tend to adhere to air bubbles by their own nature, so they can become firmly attached to the bubbles and rise together spontaneously. on the other hand, hydrophilic particles have surfaces with affinity for water, so they do not adhere readily to bubbles and are difficult to float.

as pointed out by gochin^[6]: "almost all naturally occurring solid particles and most inorganic chemical precipitates have surfaces with a strong affinity for water (hydrophilicity) and they are invariably unflotable". this is or liquor: most insoluble matters are coagulates of hydrophilic organic colloids and various calcium salts and are typical substances having hydrophilic surfaces. experience shows that it is not easy to make them float.

in order to further examine this problem and the relative mechanism of the flotation process, the author has carried out a series of researches and some important phenomena have been studied. the first problem is to find out the properties of calcium phosphate - a major chemical constituent in the phosflotation process. a test was made as follows: sodium phosphate and calcium chloride solutions were added to distilled water, equivalent to the mixture containing 300 ppm p₂o₅, and 400 ppm cao. calcium phosphate was precipitated in the form of many tiny solid particles. these gradually combined into floccules of a slightly larger size and settled slowly. subsequently this liquid was aerated by the addition of some aerated water, which is made by pressurization of the mixture of water and injecting compressed air under 6 kg/cm2 pressure with a retention time of 3 minutes to make the air dissolve in the water. this aerated water, called "dissolved air" water, liberates a great many minute air bubbles when it comes out from the pressurized vessel, because the solubility of air in water decreases at low pressure. when this water was mixed with the liquid containing the phosphate particles in the above-mentioned test, although lots of minute air bubbles were released, they did not adhere to the solid particles and rose by themselves. none of the solid particles were floated, and all of them continued settling gradually. this demonstrated that the phosphate precipitate has hydrophilic surfaces. further tests showed that if some surface-active agents were added to the liquid before the aeration, the air bubbles would attach to the solid particles and cause them to float together. this is because of the orientation of the adsorbed surfactant molecules on the surface of the particles, whereby the hydrocarbon chains of the surfactant make the surface of particles hydrophobic. however, it is impossible to use this method in the production of sugar.

calcium phosphate has a characteristic in that it can flocculate spontaneously into forms having a loose structure, with plenty of cavities inside. in the course of forming floccules, some other solid particles such as the impurities in sugar liquor can be trapped inside and these settle together. this is why phosphate is very effective in removing suspended matter (including chemically inert particles). similarly, in the course of flocculation, calcium phosphate particles can also trap minute air bubbles forming floccules having a lower density than the liquid. this can be proved by a test, which is like that described above but in a different sequence as follows:

water containing dissolved air is continuously added at the same time as sodium phosphate solution to a liquid that contains calcium chloride. calcium phosphate is precipitated, and gradually forms floccules that have air bubbles inside or on the surface that can be seen clearly. they float upwards at different velocities depending on the size of the floccules and the amount of air occluded. this type of flotation is based on the flocculation of the solid particles, and it has different characters from that of hydrophobic particles.

when the air bubbles float up singly, the larger bubbles rise more quickly and the smaller ones slowly. in the case of flocculation-flotation, only minute bubbles can be trapped and are effective, whereas the bigger bubbles are wasted and may even be harmful. in the system, the major factors that have been found to be most important in determining the success and efficiency of flotation-separation are the parameters of the bubbles and the creation of flocculation. 

4.  air bubble parameters

air bubbles provide the lifting force for flotation of the solid floccules; their size and number have great influence on the stability and velocity of flotation. a basic physical-mathematical analysis has been made, from which some fundamental rules can be shown.

the force f₁, causing a body to float in a liquid is:

f₁ = v (d₂ -- d₁)                                                (l)

where v is the vo1ume of the body, d₂ the density of the liquid, d1 the density of the body.

the resistance f₂ to a body in motion is given by

f₂ = cd₂ av₂ / 2g                                              (2)

where c is the coefficient of motion resistance, a the sectional area of the body, v the moving velocity of the body, and g the acceleration due to gravity.

the resistance coefficient varies with some other factors. under the conditions to be examined, it can be expressed as:

c = 24 / re                                                   (3)

where re is the reynolds number of  the system examined.

the rising velocity of an air bubble in a liquid is determined by its size and the properties of the liquid, which can be calculated according to the above formulae and the relative parameters. some figures of these velocities for air bubbles of various diameters in water and in a 60ºbrix sugar solution at various temperature are shown in figure l. they coincide with the results of practical measure.  

 it can be seen from the figure that the rising velocity of air bubbles increases rapidly with their size, and is approximately proportional to the square of the diameter, under same other conditions. from this relationship, the size of bubbles can be estimated roughly by observing their rising velocity. for the sugar flotation process, minute air bubbles smaller than 50 micrometers are advisable; their floating velocity in 60ºbrix sugar solution at 60º-80ºc is less than 2 cm/min. if aerated sugar liquor is held at rest for about two minutes then plenty of bubbles remain, the aeration effect is satisfactory.

in a high brix sugar solution, the rising velocity of small air bubbles is quite low. but in a good flotation process, the floccules can rise at higher speed.  therefore, after most of the  floccules rise, there are still some minute bubbles left in the clarified liquor that makes the liquor look somewhat turbid. the floccules have a much higher density than that of the air bubbles but, because they are much larger than the bubbles, when their density is lower than the liquid by occlusion of many air bubbles, the floccules will rise faster than individual bubbles.  

calculating the rising velocity of floccules of different size and different density by the above formulae can also show this. the results of such calculations are shown in figure 2, which  gives the rising velocity of spherical bodies having densities of 0.9. 1.0 and l.1, respectively, and having diameters of 0.l to 0.8 mm, floating in a 60ºbrix sugar solution at 60ºc (with density of l.264 and viscosity of 9.69 centipoises). it can be seen that the rising velocity of these bodies increases rapidly with their sizes; this is the same as in the sedimentation process (only replacing rising by settling). particles larger than 0.5 mm and with densities lower than l.l can rise at more than 10 cm/min that is much faster than that of small air bubbles.

the above figures are calculated for spherical bodies, whereas, in practice, floccules have different, complicated shapes. this affects the coefficient of motion resistance and the rising velocity to some extent, but the above correlation between the relative parameters is still applicable on the whole.

the size and density of the floccules are two major factors in determining their rising velocity in a certain liquid. it can be seen from figure 2 that, if the density decreases by 0.1, the rising velocity increases by 40 to 70%. since the densities of the solid and the liquid show little variation, the density of the floccules is mainly determined by the amount of air bubbles occluded. this discussion explains the important role of modern flocculation technique in considerably improving the flotation-separation velocity through forming floccules with large size and low density by occluding bubb1es. through a series of researches, the following factors have been found to be essential in achieving good results.

(1) air bubbles must be of microscopic size. as pointed out by saranin^[2]: "the bubbles need to be of sufficiently small size as to be easily enmeshed into the floccules of the precipitate". generally, sizes smaller than 30 microns are preferable, and smaller than 50 microns are acceptable. larger bubbles are wasted and may even be harmful, because they can bring about detrimental turbulence in the clarifier and interfere with the flotation of floccules.

(2) the quantity of bubbles should be sufficient but not too much; as the floccules can only enmesh a certain quantity of bubbles, excessive bubbles are useless and harmful. in a good system using a high-efficiency flocculent where air bubbles can be utilized effectively, the quantity of bubbles required for lifting these floccules is not large. in the simp1e phosflotation and sulphitation-phosflotation process, a volume ratio of bubbles to liquor between 0.5 and 1% is sufficient. taking into account the amount of insoluble solids in the liquor, including the precipitate formed by chemical treatment is only l-3 g/l, the bubbles of the above-mentioned volume of air amount to 3-5 ml per gram of solid. if the bubbles and solids can mix together by themselves, the mixture will have a density as low as 0.2-0.3. it is obvious that, in this case, the key factor is to utilize the bubbles effectively, but not to supply too many. on the other hand, too many bubbles increase the volume of floating scum and, hence, decrease the effect of a certain amount of flocculent in the floccules and the scum.

(3) good flocculation is of great help in the occlusion of bubbles by the floccules. during the flocculation of calcium phosphate, named "primary flocculation", they can enmesh some bubbles. based on this function, the phosflotation process has been applied in many sugar refineries for some fifty years. however, this effect is limited, the process is therefore not too stable, and the practical results are not very good. the application of polyacrylamide greatly improves flocculation in what is known as "secondary flocculation", when floccules are formed of much larger size, often reaching several millimeters, while they have many more bubbles occluded inside. these make the flotation process more stable and of much higher efficiency. in this aspect aeration at the right time to coordinate both flocculation to achieve the best effect is also important.

5.  method of aeration

many aeration methods have been used in the flotation process in the sugar processing and other industries. in the early years, the aeration of liquor was done by injecting compressed air into it, or pumping either the whole or a part of the liquor through an injector to suck air in, but these methods formed many large bubbles. in the 1950's, the so-called "dissolved air" method was introduced in some sugar refineries. this method can make minute and uniform air bubbles and has been used extensively in other industries. saranin examined its application in the sugar industry in detail[2]. however this aeration system is somewhat complicated. another method is the use of an aerating pump, usually a modification of a centrifugal pump, with some changes in the construction to increase the breaking effect on the bubbles. in general, these machines provide bubbles that are not so good as those produced by the "dissolved air" method. according to the above-mentioned mechanism, the author has designed a new aeration system. the aerator is of multi-knife style, consisted of a rotor having 20 knife blades running at high speed (about 2900 rpm) inside a cylindrical shell. the knives are machined to make their edges very sharp, and both ends are made tortuous in a special shape to increase the cutting effect on the bubbles; the shell is machined to form hundreds of small troughs with sharp edges on the inner surface. the annular clearance between the rotor and the shell is very small. the treated liquor or syrup with air flows through the passage formed by this clearance, and is cut by knives and ground by the shell, producing numerous minute bubbles. all larger bubbles are broken down and eliminated. microscopic observation shows that the bubbles formed are in the size range 10 to 30 microns, as good as those liberated in the "dissolved air” method, and much more suitable than those produced by other methods. the aerator is equipped with a 7 kw electric motor and can provide sufficient aeration for a refinery with a capacity of 1000 tons raw sugar per day.

the aerated liquor or syrup from this aerator is a yellowish emulsion, containing 10-30% by volume of minute bubbles, depending on the composition of the liquor. genera1ly speaking, washed raw sugar liquors contain less surface-active substances and bubble-forming action is not too strong. on the other hand, during the crushing season, cane syrups contain much more surface-active substances, such as nitrogenous compounds; they often form many bubbles which are very stable and can stand for a long time. some syrup samples have been found to be able to form an aerated emulsion containing as much as 40% by volume of stable minute bubbles.

since the aerated liquors contain a great many bubbles, it is not necessary to put all the liquor through the aerator. if a part of the liquor is treated by the aerator and then mixed with the rest, by adjusting the proportion of this first part the amount of bubbles in the whole liquor can easily be controlled at a suitable level. usually, this proportion is 15-25% for cane syrup and  25-40% for washed raw sugar liquor. since some air bubbles will disappear (merge and break) in the course of the process before they pass into the clarifier, the proportion of aerated liquor should be controlled according to the flotation in the clarifier. 

6. flocculation and the use of flocculent

the efficiency of modern flotation process has been raised greatly by using polyacrylamide, of which the chemical constitution and the relative parameters, as well as the preparation and application method, have great influence on the effect of flotation.

most flocculants used in the sugar industry are co-polymers of acrylamide and acrylic acid, the latter component usually comprising about 20-30%. some other flocculants have other components containing other chemically active groups. a chemical plant in guangzhou has made many species of flocculent samples according to our requirements and, through a series of tests and comparisons, some highly effective flocculants have been selected for sugar industry application. in general, polyacrylamides of higher molecular weight have higher efficiency in flocculation. the flocculants we use now have a molecular weight of over ten million and also contain some other special active groups. they should be carefully dissolved in warm soft water using a low-shear stirrer to give a 0.1% solution.

the dosage of flocculent has great influence on the flotation velocity of floccules. some results of our laboratory tests shown in figure 3 present the effect of flocculent dosage on the rising velocity of floccules in cane syrup treated by phosflotation. it can be seen that the rising velocity obviously increases with the dosage of flocculent, since bigger floccules containing more bubbles are formed.

the flocculent dosage required for the process is not only dependent on the variety of flocculent used and its properties, but also on the arrangement of the process. for example, in the cane syrup phosflotation process, aeration should be arranged before phosphate flocculation, so that the primary flocculation of phosphate can play a useful role in enmeshing air bubbles. this action decreases the density of floccules, thus diminishing the load of flocculent and the dosage required. by this means, combined with good treatment, 3 ppm flocculent on solids in the laboratory and 5 ppm in production are sufficient to obtain good and fast flotation. on the other hand, when treating raw sugar liquors, even if aerated earlier, some of the bubbles will disappear in the course of treatment. so, the principal aeration mainly depends on the action of the secondary flocculation through adding the flocculent, and more flocculent is therefore needed. as a rule, 8-10 ppm on solids for simple phosflotation, l0-15 ppm for sulphitation-phosflotation, and 20 ppm for carbonatation-flotation are required.

the thorough mixing of flocculent and 1iquor is another aspect that must be emphasized but may often be neglected in practice. since the flocculent solution is very viscous, it is not easy to disperse it uniformly in the liquor that is also viscous. incomplete mixing often leads to uneven distribution and distinctly decreases the effect of the flocculent. in some systems, flocculent solution is added into the liquor pipe prior to the entry of the clarifier, as the pipe is quite short and without a stirrer, the flocculent hardly ever disperses to the whole liquor. in l some other systems, flocculent is added to the overflow-exit of a high-level container prior to the c1arifier; since the pipe is longer and the greater level difference leads to a stronger turbulence, the mixing effect is better. but this method often brings about another problem: sucking in air from the opening of the overflow, which produces many large bubbles that seriously disturb the flotation       process in the clarifier.

in the new system designed by the author, a special mixer is equipped beside the clarifier. it first mixes the aerated liquor with the rest, and then mixes the whole liquor with the flocculent. each step is achieved completely. the liquor stays in it for about one minute to get better mixing and to bring-about preflocculation to trap the air bubbles. the mixture flows out from the bottom of the mixer, which ensures the liquor only carries away minute bubbles. the larger bubbles, having a higher upward velocity, will rise to the surface of the mixer and separate from the liquor. this measure eliminates the detrimental effect on the flotation process of large bubbles, which are often formed in liquid or syrup.

7. clarifiers

clarifiers are main equipment in the flotation process and many designs have been used in the sugar industry. the major ones are: williamson's, jacobs', bulkley-dunton's, saranin's and the talo clarifier. some designs are round, and some are rectangular; most are of single layer, and one is multicell. although they have different structures, their basic principles are similar.

the aim of the flotation process is to achieve high-quality clarified liquor with high separation rate and short retention time, in conjunction with a small volume of concentrated scum. the attainment of these goals mainly depends on the previous treatment before the clarifier, but also depends on the working of the clarifier, the construction of which is also significant.

a new style of clarifier having many new improvement designed by the author has obtained satisfactory results in recent years. it has two layers, shallow but broad in cross-section, which serve for two steps of flotation. the shallowness shortens the distance the floccules are required to float through the liquor, and thus shortens the retention time, while the broad area improves scum concentration. the construction of this clarifier is illustrated in figure 4. the upper layer is used for the carbonatation-flotation process, and the lower for phosflotation sulphitation. the empty space between the two layers is provided for laying out the accessories and piping. the design of both layers is similar but, for ease of construction, the upper layer is slightly larger than the lower one. some main dimensions are as follows. 

the avai1able depth of each layer is 0.8 meter or only about one-fifth of its diameter. this ratio is much lower than that of many other designs. in this shallow clarifier, the liquor flow is mainly in the horizontal direction, giving less interference with the rising of the floccules. this is quite different from      that in some clarifiers with a deep flotation cell, in which the liquor flows mainly in a vertical direction, which exerts a direct influence on the floccules' floating. if the descending velocity of the liquor is higher than the rising velocity of the floccules, some of the smaller particles will be carried out by the liquor, make the liquid turbid.

of course, the adoption of the shallow clarifier must be based on good flotation with the system having good aeration and good flocculation, and the floccules rising quickly. moreover, the entry of liquor must be smooth without surging, and in the clarifier the liquor must be evenly distributed over the whole area and flow in a laminar state without turbulence. for this purpose, the entrance of the clarifier and the method of discharge must be carefully designed with reference to hydrodynamic principles, and adjusted through practical test running for new designs.

in our clarifier, the liquor enters the bottom at the center, through a preparing chamber, which plays the role of ensuring good flocculation and bubbles entrapment and eliminating turbulence in the feed liquor, which is then distributed over the whole area of the clarifier. the flocculum floats to the surface and then gradually concentrates into scum, which is scraped off by a low-speed rotating scraper into an annular chute and discharged. the clarifier liquor flows towards the outer circumference of the clarifier and passes through an annular pipe with many small holes, to discharge in a control tank. this controls the level of liquor in the clarifier, which is slightly lower than the overflow surface, so that the scum has a suitable time for concentration to reduce its volume.

in the process having two steps of flotation, the upper layer of the clarifier discharges more scum containing a large amount of caco₃. it flows down to the scum chute of the lower layer, mixes with the scum discharged there, and then is sent to the filtration station. this method can improve and simplify the treatment of scum.

experience has shown that this clarifier has a capacity of treating 800 to 1000 tons of raw sugar per day, depending on the quality of the raws and the working conditions of the process. the retention time of the liquor in each layer is l4-18 minutes. this double-layer clarifier also has the advantages of reducing the area required for the equipment centralizing operation and control, and reducing the heat loss. 

acknowledgements

the author expresses his thanks to professor chen shi-zhi of guangzhou cane sugar institute for his generous guidance, and to mr. li chi-sin, san shi-you, and zhi you-zhong, etc., engineers of zhongsan and meisan sugar factories, for their cooperation in this work.

summary

it has always been a goal for sugar technologists to find a way to yield high quality white sugar by a simple method at low cost. many achievements have been made, especially since the l970's. our work has shown an economical benefit and an encouraging future. through combining the traditional sulphitation or carbonatation with a newly developed flotation technique, high clarification efficiency as well as low investment and running cost has been achieved. the key factors for this system are to ensure a suitable process and its working conditions, and to adopt good methods in aeration and flocculation in conjunction with new machinery. many new improvements for this system with their mechanism and application have been discussed in detail.

reference

[1] u.s.patent 1,317,607

[2] sugar technol.rev., 1972(vol 2), 1-72

[3] bennett et al.: prtoc.14th congr.issct,1973,1569-1590

[4] bennett: isj, 1974,40-44,68-73

[5] shore et al.: sugar technol.rev., 1984(vol12), 1-99

[6] in “solid-liquid separation” ed.svarovsky, 1981,chapter 19,503-535