1. composition of ash in white sugar

most of the cane and beet sugar factories in china manufacture plantation white sugar with the double-sulfitation or double-carbonatation process. the ash content in these products is commonly in the range of 0.03-0.l%, considerably higher than that in refined sugar. the average composition of ash in several cane sugar samples, collected from three factories with carbonatation and three with sulfitation, is summarized in table 1 (ash components were analyzed by atomic absorption spectrophotometer). 

 table 1. cation composition of ash in plantation white sugar (ppm)  

the total amount of the four main cations, i.e. potassium, sodium magnesium and calcium, in these samples is l13 to 203 ppm, among which calcium is the most with a proportion as high as 80% to 9l%. the second is potassium, although its content in these samples is less than 20 ppm.

some sugar cane, planted on saline soil near to the sea thus containing more potassium, yields a sugar with higher potassium content, as shown in table 2. 

table 2. cation content (ppm) of sugars produced in high potassium areas (saline soil)  

the potassium content in these sugars is much higher than that in other products quoted, although it is still lower than the calcium content. except for ca and k, other cation components in white sugar are negligible: na and mg only a few ppm each, cu, fe and al each less than 1 ppm or so.

of all anions, sulfate ion is the most important one with a very high percentage to the total ash in white sugar, as shown in table 3 (sulfate was analyzed by gravimetric procedure). also, in raw sugars made without sulfitation, or in refined sugars, sulfate is commonly the most important anion.

 table 3. anionic composition of ash sulfate content  

the so₄ content in these samples is over 50% of the total ash. general1y, so₄ exists as a calcium salt, so the equivalent content of caso₄will exceed 70% (so₄: caso₄ = 96: l36).

white sugar also contains small amounts of other anionic components: phosphate, sulphite, silicate and organic anions. 

2. relationship of ash content in white sugar and in syrup

it is well known that the sugar quality is closely related to the quality of the syrup or liquor from which the sugar is produced. recent research has shown that there exists a certain numerical relation between the contents of impurities in white sugar and those in the syrup (or liquor).

the author introduces a new term, "non-sugar coefficient," denoted by nsc, to indicate the relationship. this coefficient is calculated from the contents of a certain constituent present in the sugar and in the syrup (or liquor). if n₁ represents the content of a certain component in the sugar (on total solids), n₂ the content of the component in the syrup, and p₂ the purity of the syrup, then, assuming the purity of sugar to be l00:

in a word, nsc represents the percentage of the impurity/sugar ratio in the sugar to the ratio in the syrup or 1iquor. for example, if a white sugar contains potassium 0.0018% (18 ppm), and the syrup contains 0.387% potassium on solids and is of 88 purity, thus the nsc value of potassium in this example is:

0.0018 x 88 / 0.387 = 0.41

through researches on three sugar mills (c and d with carbonatation , e with sulfitation ), the nsc values of four cations are summarized in table 4. 

table 4. ash constituents (% on solids) and its non-sugar coefficient at three factories  　

these figures show that the nsc value of various ash components is quite different: k: 0.2 to 0.4, na: 2 to 3, mg: 0.2 to 0.4, ca: 5 to 7. this situation has great influence on white sugar quality and its composition. according to these figures, the calcium content in syrup is lower than potassium, but it is reversed in white sugar, where calcium content is about ten times higher than potassium. obviously, the non-sugars composition in white sugar does not simulate that in syrup. non-sugars with high nsc value enter into sugar crystals with a high proportion, but the components with low nsc value have only little influence on sugar quality.

with regard to colorants: the results of our study showed that the colorants present in cane syrup have a slight effect on white sugar color, but those present in melt liquor considerably increase the color of sugar crystals.

the nsc values for these two kinds of colorants are one to two and eight to ten respectively, for colorants in evaporator syrup, and those in dissolved raw sugar.

the crystallization process, widely used in many industries as well as in sugar, is certainly an effective method to produce high purity substances. however, sugar crystals inevitably still contain non-sugars. what is the mechanism by which non-sugars enter into sugar crystals? what are the main factors that influence this effect? these problems are very important, especially in regard to the fact that the sugar industry of the world spends millions to improve sugar quality, mainly in reducing sugar color and ash content.

the wide distribution of nsc values for various non-sugars shows that, they may enter into sugar crystal in different manners, as follow:

l.  inclusion of mother liquor droplets by the crystals.

2. adsorption of certain impurities, especially coloring matter.

3.  precipitation of inorganic salts, and blending with the crystals.

this situation mainly depends on the properties of non-sugars and the conditions of crysta1lization. the nsc values for each non-sugar measured by different factories are quite consistent for each other. however, they are influenced to some extent by boiling system and operation, crystal size and some other factors, although these conditions are not often changed.

in normal production systems, these conditions are not often changed; therefore the nsc value of any one constituent in any one factory should remain constant.

3. behavior in process of major ash constituents

based on a series of experimental results, major ash constituents throughout process are discussed below: 

3.1 potassium

potassium is a major ash component in cane juice. the potassium content is 0.l-0.3% on solids normal juice in china, but may be as high as 1-1.8% in "saline cane" in china. potassium cannot be removed during clarification and most of it goes into final molasses, except for the small part that enters into sugar crystals, giving low potassium content in sugar. potassium salts are high1y soluble in water and are highly ionized, so that they hardly adhere to, or are adsorbed on, the crystal surface. the potassium present in sugar crystals may be explained as the result of inclusion.

professor mantovani has presented excellent articles on this topic^[4], and confirmed that the presence of this impurity in sugar crystal is caused by the inclusion of mother liquor droplets by the crystals themselves during their growing. since potassium in the crystal occurs through occlusion of mother liquor, the nsc value of 0.2-0.4 could be a measurement of the extent of inclusion.

3.2 sodium

sodium content of cane juice is much less than potassium, usually 0.03-0.1% on solids. sodium content changes little during the process; and most sodium goes into molasses. white sugar content of sodium is usually below 1 ppm.

3.3 magnesium

magnesium is another major ash components in cane juice; mixed juice content range is 0.l-0.3% on solids. it is removed only a little by normal sulfitation, but removed 80-90% by carbonatation, since magnesium forms mg(oh)₂ precipitate at the alkaline phs found in carbonatation. the nsc value of magnesium is low, thus the magnesium content in white sugar is only several ppm after sulfitation and less than 1 ppm after carbonatation.

3.4 calcium

calcium is the most important cation in cane juice and in sugar. its content in mixed juice in china usually ranges 0.l7-0.25% on solids (saline cane juice contains more potassium but less calcium, probably less than 0.l%). after clarification, calcium content is increased because of the solubility of calcium salts. the content of cao % solids in clarified juice is about 0.4-0.5% for sulfitation and 0.25-0.5% for carbonatation factories in china.

calcium is a very high percentage of the cations in white sugar. calcium has a high nsc value of 5-7, much higher than that of other cations. possible reasons are that major calcium salts, such as calcium sulfate, are partly precipitated out during sugar boiling and can precipitate out with sugar crystals.

the calcium content in clarified juice is affected by the type and conditions of the clarification process. for instance, in the sulfitation process, if the ph of clarified juice is either higher than, 7.4 or lower than, 6.8, the juice would contain more calcium, as more lime has been added in the former case and soluble calcium bisulfite salts have formed in the latter case. for normal fresh cane, clarified juice contains minimum calcium at ph 7.1-7.3, but in processing deteriorated cane, ph of clarified juice may be lower. carbonatation or alkaline sulfitation are capable of removing much more color, colloid and ash constituents from juice, but if clarification is not carefully monitored, invert sugar may be destroyed, and thereby form organic acids and increase soluble calcium salts.

3.5 sulfate

of all ash components, calcium sulfate has the greatest effect on sugar manufacture. in mixed juice, so₄ content usually ranges from 0.3-0.6% on solids or more. sulfate cannot be removed by defecation or sulfitation. moreover, if commercial calcium superphosphate (it often contains high percentage of caso₄) were added to mixed juice as a phosphate source, it would increase the calcium and sulfate content in clarified juice. in the sulfitation process, sulfur is burned to produce sulfur dioxide, it dissolves in juice and forms sulfurous acid, and may be partly oxidized into sulfur trioxide (in burning gas) or sulfuric acid (in juice or syrup). in a well-conducted process, these actions are not notable, thus the increment of sulfate is not large, for example, about 0.02-0.05% on solids. but if the process is not operated well, more oxidation would occur and more sulfates would be formed and found in clarified juice. in china, many measures are applied on the equipment and operation to prevent this unfavorable situation. with the carbonatation process, about 10% calcium sulfate could be removed by formation of a large amount of caco₃ and its co-crystallization.

calcium sulfate has a quite high solubility in hot water and juice; therefore, it is difficult to remove by ordinary clarification. it is more soluble at lower temperatures. however, in the course of evaporation, as plenty of water is evaporated, calcium sulfate will become supersaturated  and settle out as scale in evaporators. "middle juice clarification," which draws out the juice  from the middle body of evaporators and clarifies it by carbonatation or sulfitation, can be effective in removing calcium sulfate by carbonation. the efficiency of this clarification increases with the rise of middle juice concentration and ph. cane syrup treatment by phosflotation has the same function.

during sugar boiling, the calcium sulfate in the syrup will further settle out as scale or suspended particles. analytical results indicate that the insoluble material in syrup and molasses, and scale of vacuum pan, all contain high percentage of calcium sulfate. table 5 gives the composition of the inorganic components (exclusive of moisture and organics) in these materials.

table 5. composition of insoluble inorganic materials in the factory

in these three materials as also in white sugar, calcium sulfate is the main component, this shows that they come from the same source. some calcium sulfate may co-precipitate and mix with the crystals and enters into the product. in the boiling house, b and c sugar contain more caso₄, if they are used as the seed of a strike, the sugar product will contains more sulfate.

another important point is that calcium sulfate, which has been separated from the process stream, is easy to disso1ve again when it is mixed with water or thin juice, for sugar recovery from scum or mud. for example, the scum discharged from syrup clarification mixes with water of a certain ratio, heated to 90ºc and filtered, analyses the calcium and sulfate contents in the filtrate. the results are shown in table 6. obviously, more caso₄ re-dissolve when more water is added. therefore, it should be avoided to add much water or thin juice to the scum.

table 6. calcium and sulfate content in the filtrate

3.6 silicate

"silicate is important in sugar manufacture because of its implication in beverage floc formation, its presence as a scale ....." as pointed out by godshall and clarke^[5]. in cane juice, the soluble si1icon compound expressed by sio₂ is a about 0.2% to 0.3% solid. it is partly removed during clarification, about 10% to 30% by sulifitation and 70% to 90% by carbonatation, since it forms insoluble matter at high ph. silicon levels of up to 20% in scale on the final evaporator have been observed. high silicate levels may increase the turbidity of clarified juice and white sugar. like calcium sulfate, silicate compounds also have the settling-dissolving characteristic and may recycle in the process.

3.7 other inorganic compounds

cane juice contains phosphate, chlorine, iron, and aluminum, in addition to the above major components. inorganic phosphates settle out for the greater part during clarification, but chloride is too soluble to precipitate. iron and aluminum, expressed by their total sesquioxides in mixed juice, usually amount to 0.1-0.25% solids; their removal by sulfitation is about 60-70%, and by carbonatation about 90%. these components are usually negligible in white sugar.　

4. measures to reduce ash content in white sugar

the following measures are recommended to reduce the ash content in white sugar, depending on the ash composition and content:

(l) monitor the calcium and sulfate contents in mixed juice, clarified juice, filtrate, and syrup, a strike, and a seed. if any material shows an unreasonable high level of sulfate, determine, find the cause, and reduce the sulfate content.

(2) poor qua1ity superphosphate should not be used as a phosphate source.

(3) optimize the c1arification process to achieve the best quality clarified juice with low calcium content. the working conditions of the process should be monitored frequently, as juice composition changes.

(4) the phosflotation process for syrup is effective in removing ash, color and colloidal substances. ash content of white sugar can be reduced to about 0.02% to 0.03%.

(5) improve the quality of a seed. an a strike made on syrup alone, with a seed, instead of using b or c sugar as footing, can greatly improve the sugar quality. superior plantation white sugar, with ash content of 0.025% and low color of be1ow 80 icumsa units, has been manufactured by a combined scheme including this method with juice sulfitation and syrup phosflotation.

(6) middle juice clarification is a good method to reduce the ash level in syrup and sugar. if this process is combined with the new flotation-separation technique, better sugars can be achieved.

(7) a or b molasses, treated by diluting (to 62-65ºbrix), heating (to 80-85"c) and filtering (by centrifugal with cloth) can yield better products and higher efficiencies.

references

1.   meade,g.p. and chen, j.c.p., eds., 1985. cane sugar handbook, 11th edition. wiley-interscience, new york.

2. honig, p., ed., 1963. principles of sugar technology vol. 1. elsevier co., amsterdam.

3. silin, p. 1958. technology of beet sugar production and refining. usda and nsf. washington, dc (trans. and published in english 1964)

4. g.mantovani et al．investigation on industrial factors decreasing sugar crystal color. zuckerind 1986. 643～650