WO1999024623A2

WO1999024623A2 - Method for purifying the raw juice resulting from sugar refining

Info

Publication number: WO1999024623A2
Application number: PCT/EP1998/006841
Authority: WO
Inventors: Carlos Martinez Reyes; José Antonio Ortiz Niembro; Raùl CHACÓN RODRIGUEZ
Original assignee: Süd-Chemie AG
Priority date: 1997-11-03
Filing date: 1998-10-28
Publication date: 1999-05-20
Also published as: AU2151599A; ZA989833B; WO1999024623A3; DE19748494A1

Abstract

The invention relates to a method for purifying the raw juice resulting from sugar refining, whereby said juice is treated with calcium hydroxide. The invention is characterised in that the raw juice is treated with a mixture of calcium hydroxide and a clay mineral selected from the smectite and kaolin groups, whereby the proportion of calcium hydroxide in the mixture is less than 80 wt. % and the clay mineral, residual calcium hydroxide and poorly soluble calcium salts thus formed are separated from the purified thin juice.

Description

Process for cleaning the raw juice from sugar refining

description

The invention relates to a method for cleaning the raw juice obtained during sugar refining.

Industrially, sugar is mainly produced from sugar cane and sugar beet. The physical composition of the raw sugar solutions made from these plants differs only slightly, so that the refining of the sugar solutions made from them is similar.

In principle, the sugar refining process is divided into the following sub-steps:

1. Raw juice extraction: Λ In the case of sugar beet, the fruits are washed, crushed and extracted with small amounts of water at an elevated temperature. The raw juice is then extracted by pressing. In the case of sugar cane usually

REPLACEMENT BUTT (RULE 26) the insoluble organic material is freed of its sugar substances by pressing and extraction with hot water.

2. Juice cleaning: The raw juice has a more or less gray-black opalescent appearance and is freed of its non-sugar substances by the so-called juice cleaning. Cleaning increases the ratio of sucrose to total dry matter. As the proportion of sucrose in the sugar solution increases, the amount of sugar to be crystallized increases.

3. Concentration of the sugar solution: The thin juice obtained from the juice cleaning is concentrated in a multi-stage process from about 15% to about 65%. Here you get the so-called thick juice.

4. Obtaining the sugar from the thick juice: In a multi-stage process, solid sugar is obtained by crystallizing and separating the mother syrup (molasses).

The yield of sugar is largely determined by the second process stage, juice cleaning. The raw juice obtained from the plants by extraction or by mechanical pressing contains, in addition to the desired sugar (sucrose), other components that have to be removed during sugar refining. These so-called non-sugar substances (NZ substances) include both organic compounds, e.g. Invert sugar, raffinos and ketoses, organic acids, proteins, polypeptides, amino acids, enzymes etc., as well as inorganic compounds, e.g. Salts of potassium, sodium, calcium and magnesium, with the anions chloride, phosphate, sulfate and nitrate. The raw juice is acidic due to an excess of anions. The low pH promotes the hydrolysis of sucrose, which is linked to a reduction in the sugar yield.

The raw juice is usually mixed with calcium hydroxide (lime milk) added to raise the pH to 10.8 to 11.9 (beet sugar) or 7.0 to 8.5 (cane sugar). Here, colloidal NZ substances are precipitated by breaking the colloids. Anions of inorganic and organic acids are bound by the formation of poorly soluble calcium salts. Organic polymers are also often added to act as flocculants.

After or at the same time as the lime treatment, the excess calcium hydroxide is precipitated by introducing CO2 or SO ₂ into the raw juice by forming poorly soluble CaC0 ₃ or CaSθ ₃ . This treatment is called carbonation or sulfitation. The precipitates formed here also serve as crystallization nuclei or adsorption surfaces for other precipitation products. The carbon dioxide or sulfur dioxide required for this stage is produced in affiliated plants by burning CaCÜ ₃ (lime burning) or by burning sulfur. The process is to be classified as harmful due to the exhaust gases formed and the release of non-adsorbed gases during sugar juice treatment.

The sludge resulting from carbonization or sulfitation must be filtered, washed and dewatered because it binds considerable amounts of sugar solution. The filter cake is usually used as fertilizer lime. In order to use the fertilizer lime without problems, the moisture must be reduced until there is good spreadability.

The thin juice is then concentrated by evaporation of the water, which may result in a brown coloration of the thick juice obtained as a result of caramelization and other reactions. Solid sugar is obtained from the thick juice through crystallization processes. However, a small proportion of the thick juice cannot be crystallized and is used as a low-quality liquid sugar. From the literature reference "Vest. Akad. Kaz. SSR" (1970), 26 (3), 54-6 (73: 46914 CA) it is known to add bentonites to carbonized sugar solutions in order to remove nitrogen compounds. There is no indication that the carbonate ion can be eliminated or that the sugar solutions are decolorized and the sugar yield increases.

In "Appl. Clay Sei." 11 (1996) 55 describes the decolorization of sugar solutions with bentonites, sepiolite and diatomaceous earth. Simultaneous treatment with calcium hydroxide is not described.

The object of the present invention was to develop an improved process for cleaning the raw juice obtained in sugar refining, which can be operated with little technical effort in existing plants and in which the proportion of calcium hydroxide added can be kept low, so that carbonation or sulfitation reduced or even completely avoided and the yield of crystalline sugar can be increased.

The invention thus relates to a process for cleaning the raw juice obtained in the refining of sugar by treating the raw juice with calcium hydroxide; the process is characterized in that the raw juice is treated with a mixture of calcium hydroxide and a clay mineral selected from the smectite and kaolin group, the proportion of calcium hydroxide being less than about 70% by weight, and the clay mineral together with the separates the remaining calcium hydroxide and the poorly soluble calcium salts formed from the purified thin juice.

The proportion of calcium hydroxide in the mixture is preferably about 10 to 60% by weight.

The clay mineral is preferably first added to the raw juice sets, whereupon the calcium hydroxide is added. However, the clay mineral can also be added to the raw juice simultaneously with or after the addition of calcium hydroxide or as a clay mineral / calcium hydroxide mixture.

The smectic or kaolin clay minerals have the property of being colloid-breaking, adsorptive and ion-exchanging due to their ion exchange ability and their large surface area. It is believed, without being limited to a theory, that these clay minerals break the colloids present and at the same time act as adsorbents for the compounds released in the process. They therefore fulfill the function of the calcium carbonate or calcium sulfite formed in the known carbonation or sulfitation. They also bind excess calcium hydroxide or the poorly soluble calcium salts formed. The amount of calcium hydroxide used can be reduced, whereby carbonation or sulfitation can be reduced or even completely avoided.

A clay mineral with a specific surface area of at least about 30 m ² / g, in particular of about 50 to 200 m ² / g and a cation exchange capacity of at least about 20 mVal / lOOg, preferably of about 30 to 100 mVal / lOOg, is preferably used .

In addition, due to the ion exchange capacity of the clay minerals mentioned, the alkali ions present in the raw juice are replaced by alkaline earth ions, which contributes to increasing the crystallization yield. If ion exchangers are used in downstream processes, the removal of the alkali ions leads to a longer service life of the ion exchangers.

As a result of the presence of the clay minerals mentioned, an easily filterable and dewatered sludge continues to be formed, which contains residual calcium hydroxide (bound to the clay mineral) and poorly soluble calcium salts and which is used as a fertilizer can be used. The clay mineral acts as a filter aid during precipitation and as a fertilizer when applied as a fertilizer, which promotes an even and steady release of the fertilizer in the soil.

The thin juices produced by the process according to the invention have a light color, and a lighter product is also obtained in the subsequent thick juice production. The crystalline sugar is also lighter in color. The overall yield of granulated sugar increases.

Bentonite, in particular calcium bentonite and / or acid-activated bentonite, is preferably used as the smectitic clay mineral and / or halloysite. Halloysite is a kaolin-like clay mineral and has the formula Al ₄ (OH) ₈ [Si ₄ 0 ₁₀ ] .H ₂ 0.

Other suitable smectitic clay minerals are hectorite, nontronite, ver iculite and ilite.

The invention is illustrated by the examples below.

According to all the examples, an untreated, filtered raw cane sugar juice from a Central American sugar refinery is treated with the clay minerals mentioned and with Ca (0H) ₂ at a temperature of 30 ° C. A large-scale refined sugar solution was also analyzed for comparison (see comparative example).

The adsorbents used, based on natural smectite clay minerals, were cleaned by conventional methods, ie the accompanying minerals (quartz, feldspar) were removed by a hydrocyclone or decanter treatment. The cleaned adsorbents were dried and ground. Acid-activated smectites were processed using conventional methods, e.g. prepared by acid treatment of raw smectites with subsequent drying and grinding, or by acid activation of raw smectites in aqueous suspensions with subsequent filtration, washing, drying and grinding. The halloysite was prepared in a similar way. The products used have a water content of approximately 8 to 12% by weight and a dry sieve residue on a 63 / m sieve of approximately 20 to 30% by weight. The specific surface was generally between about 50 and 200 m ² / g, the ion exchange capacity in general between about 30 and 100 mVal / lOOg.

The specific surface was determined using the BET method (single point method with nitrogen according to DIN 66 131).

The ion exchange ability was determined as follows:

The dried clay mineral was reacted with a large excess of aqueous NH ₄ Cl solution under reflux for one hour. After standing for 16 hours at room temperature, the mixture was filtered, the filter cake was washed, dried and ground, and the NH ₄ content in the clay mineral was determined according to the Kjeldahl method.

example 1

Neutralization and subsequent decolorization

Raw juice with a pH of 5.2 and a sucrose content of about 12% by weight was neutralized with a Ca (OH) ₂ solution (3 ° Be) to a pH of 8.0. The mixture was then mixed with various amounts of acid-activated bentonite, which was obtained by coating a Ca-bentonite (specific surface 130 m ² / g) with 3 wt.% Conc. Sulfuric acid had been prepared (SAS-1), and mixed with various amounts of Ca bentonite (S1). The specific surface areas were as follows: SAS-1: 130 m ² / g; Sl: 45 m ² / g. The cation exchange capabilities were as follows: SAS-1: 68 meq / 100g; Sl: 70 mVal / lOOg.

The pH value resulting after the filtration and the degree of transmission (transmittance) at 550 nm (thickness of the measuring cell 1 cm) were determined. The Gardner number was also determined. The Gardner number is determined spectroscopically using comparison solutions (ASTM method D1544, 10 mm measuring cell). Bright solutions have a low Gardner number and high transmittance. Ad / Ca (OH) 2 denotes the weight ratio between adsorbent and Ca (OH) 2.

The results are shown in Table I.

Table I

Gardner Transmittance pH Ad / Ca (OH) ₂

Untreated raw sugar solution filtered cannot be determined 0 5.2

Sugar solution

Refinery 8.2 5.4 7.2

SAS-1, 1% 7.3 14.6 7.7 6.3

SAS-1, 2% 6.9 21.9 7.4 18.8

SAS-1, 3% 4.7 54.6 5.9 31.3

S-1, 1% 8.2 10.9 8.0 6.3

S-1.2% 7.8 12.4 7.6 18.8

Sl, 3% 7.6 15.3 7.6 31.3 Example 2

Simultaneous neutralization and discoloration

The raw sugar solution of Example 1 was prepared by introducing 2% by weight of a mixture of Ca (0H) 2 and an acid-activated bentonite, which had been prepared by slurry activation of the Ca-bentonite (S1) from Example 1 with 35% by weight of sulfuric acid (SAS- 2), treated (specific surface area 220 m ² / g; cation exchange capacity 35 mVal / lOOg). After the filtration, the sugar solution was analyzed.

The results are shown in Table II.

Table II

Gardner Transmittance (%) pH Ad /

Ca (OH) ₂

Raw sugar solution untreated, filtered undetectable 0 5.2

Sugar solution

Refinery 8.2 5.4 7.2

SAS-2, 99%,

Ca (OH) ₂ 1% 5.6 34.0 5.6 99

SAS-2, 97%,

Ca (OH) ₂ 3% 5.9 30.2 6.0 33

SAS-2, 95%,

Ca (OH) ₂ 5% 7.9 10, 6 7.3 19 Example 3

Simultaneous neutralization and discoloration

The raw sugar solution of Example 1 was treated with various concentrations of a mixture of 95% by weight SAS-1 and 5% by weight Ca (OH) ₂ . (Weight ratio Ad / Ca (OH) ₂ = 19).

The analysis of the products after filtration is given in Table III.

Table III

Gardner transmittance (%) pH

Untreated raw sugar solution, cannot be filtered 5.2 Sugar from refinery 8.2 5.4 7.2 1% SAS-l / Ca (OH) ₂ 5.4 33.3 5.9 3% SAS-l / Ca (OH ) ₂ 6.6 25.6 7.3 5% SAS-1 / Ca (OH) ₂ 6.9 23.3 7.8

Example 4

Discoloration with subsequent neutralization

The raw sugar solution from Example 1 was mixed with various amounts of SAS-1 or with a calcium-containing halloysite (S-2) and then neutralized to pH = 7 with the Ca (OH) 2 solution (3 ° Be). The halloysite had a specific one Surface area of 153 m ² / g and a cation exchange capacity of 58 mVal / lOOg.

The results are shown in Table IV.

Table IV

Gardner Transmittance (%) pH Ad /

Ca (OH) ₂ raw sugar solution untreated, filtering cannot be determined 0 5.2 Sugar solution from refinery 8.2 5.4 7.2

SAS-1, 0.6% 7.2 12.5 7.1 6.0

SAS-1, 0.8% 7.0 13.7 7.0 6.7

SAS-1, 1.2% 6.8 16.7 7.1 8.0

S-2, 0.6% 8.1 10.0 7.1 7.5

S-2, 0.8% 7.9 10.4 7.0 8.9

18.2 7.1 10.9

Example 5

Discoloration with subsequent neutralization

A fresh raw sugar solution (sucrose content 12% by weight) was mixed with various amounts of calcium-containing halloysite (S-2) after filtration and then neutralized to pH = 7.0 with Ca (OH) 2 solution (7 ° Be). After 30 min the mixture was filtered again and the filtrate was analyzed spectrometrically. For comparison, a technically obtained fresh raw sugar solution was also examined. The results are shown in Table V.

Table V

Transmittance (%) pH Ad / Ca (OH) ₂

Raw sugar solution untreated, filtered 4.5 4.5 Sugar solution from refinery (sulfitation) 18.5 6.5 - Sugar solution treated with 0.05% by weight S-2 19.5 7.0 1.2 Sugar solution treated with 0.07% by weight % S-2 20.5 7.0 1.0 sugar solution treated with 0.1% wt. S-2 22.5 7.0 0.7

Large-scale refining trials were also carried out in a sugar refinery.

Comparative example

The following procedure was used in a typical Central American sugar refinery based on cane sugar. The stated% by weight values relate to the mass of sugar cane used. The raw sugar juice (pH = 4.8) obtained by pressing / extraction was sulfurized down to a pH of 4.2 with 0.018% by weight sulfur dioxide. The sulfur dioxide was obtained by burning sulfur. Ca (0H) 2 (7-9 ° Be) was added to the raw sugar solution until a pH of 7.0 to 7.5 was reached. This required 0.094% by weight of Ca (OH) ₂ . The temperature was 32 to 36 ° C. The mixture was heated to about 100 ° C by indirect heating and under gentle Stirring held at this temperature for 40 min. A polyacrylamide flocculant (7.3 ppb) was added to improve the precipitation reactions. After a 40 minute hold, the mixture was filtered and the filter cake was washed with a little hot water. The filtrate (purified thin juice) was concentrated under reduced pressure at a temperature of 100 ° C. The crystal sugar obtained was 10.4% by weight. The proportion of non-crystallizable material (molasses) in the total solids content of the purified thin juice was 3.9% by weight.

Example 6

Refining was carried out in the same refining plant and with sugar cane in the same growing season as in the comparative example, in accordance with the following scheme according to the invention:

A premix of calcium-containing halloysite (S-2) and Ca (0H) 2 (7-9 ° Be) was added to the raw sugar juice (pH = 4.8) obtained by pressing / extraction until a pH of 7.0 until 7.5 was reached. Until this pH was reached, 0.051% by weight of Ca halloysite (based on the mass of the processed sugar cane) was introduced. The amount of Ca (OH) 2 added corresponds to 0.054% by weight (Ad / Ca (0H) ₂ = 0.9), which is only about half the amount used in the comparative example. The temperature here was 32 to 36 ° C. The mixture was heated to about 100 ° C. by indirect heating and kept at this temperature for 40 minutes with gentle stirring. The flake formed during the precipitation was easy to filter, so that no additional precipitation aids were necessary. The filter cake was washed with a little hot water and the combined filtrates (pH = 6.8 to 7.0) were concentrated under reduced pressure at a temperature of 100 ° C. The yield of granulated sugar was 11.3% by weight, the loss in the molasses was 3.3% by weight. The results are shown in Table VI.

Table VI

S0 ₂ ⁽¹⁾ Ca (OH) ₂ ⁽¹⁾ Halloys. ^(χ) color yield ^ ⁴⁾

Zu.-Lsg ⁽²⁾ zu.-krist ⁽³⁾

see. -E.g. 0.18 0.94 8.0 40.0 10.9

Ex. 6 - 0.54 0.51 9.7 41.6 11.3

1) kg per ton of sugar cane

2) Kopke turbidimeter (Mod. 9591-K-10), (higher values: lighter solution)

3) Visually against standard (higher values: lighter crystals)

4)% by weight, based on sugar cane

Claims

PATENT CLAIMS

1. A process for cleaning the raw juice obtained in the refining of sugar by treating the raw juice with calcium hydroxide, characterized in that the raw juice is treated with a mixture of calcium hydroxide and a clay mineral selected from the group consisting of smectites and kaolins, the proportion of calcium hydroxide in the mixture is less than 70% by weight, and the clay mineral, together with the remaining (adsorbed) calcium hydroxide and the poorly soluble calcium salts formed, are separated from the purified thin juice.

2. The method according to claim 1, characterized in that one uses the calcium hydroxide in the mixture in an amount of about 10 to 60 wt.%.

3. The method according to claim 1 or 2, characterized in that the clay mineral is added to the raw juice either before, after or simultaneously with the calcium hydroxide or as a clay mineral / calcium hydroxide mixture.

4. The method according to any one of claims 1 to 3, characterized in that a clay mineral with a specific surface area of at least about 30 m ² / g, preferably from about 50 to 200 m ² / g, and a cation exchange capacity of at least about 20 mVal / 100g, preferably from about 30 to 100 meq / 100g.

5. The method according to claim 1 to 4, characterized in that bentonite, in particular calcium bentonite and / or acid-activated bentonite as a smectite clay mineral and / or halloysite or acid-activated halloysite used as a mineral from the kaolin group.

6. The method according to any one of claims 1 to 5, characterized in that the separated mixture of clay mineral, residual (adsorbed) calcium hydroxide and sparingly soluble calcium salts is used as fertilizer