FI78454C - Process for the preparation of zinc gluconate trihydrate - Google Patents

Description

1 78454

Process for the preparation of zinc gluconate trihydrate

The invention relates to a process for the preparation of zinc gluconate trihydrate from an aqueous solution of zinc gluconate.

Metal gluconates, including zinc gluconate, are used as additives in the food and livestock industries. To date, it has not been possible to produce crystalline zinc gluconate products in an industrial mit-10 guarantee. For example, Melson and Pickering (Aust. J. Chem. (1968) PD 2889-3) describe a process for preparing zinc gluconate trihydrates by precipitation from organic solvents. The described method is not suitable for use on an industrial scale because the separation of small crystals from solution causes problems and also the handling of organic solvents on a large scale is always problematic.

It is already known to prepare dry zinc gluconates using spray drying of aqueous solutions. However, the spray-20 dried zinc gluconate product is impure because the drying process does not purify the product.

Roller, P.W. and Pickering, W.F. have generally studied the zinc / water / gluconic acid / hydroxy system (see Aust. J. Chem. 29, 1976, pp. 2395-2403). Their research shows that this system is very complex because numerous reactions compete with each other, and the variable parameters are temperature, pH, and concentrations of the reaction components. They failed to crystallize zinc gluconate trihydrate from aqueous solution. By raising the pH of the 30-club solution, they provided a gel-like substance that could be dried to a powdery product. The crude trihydrate was obtained by adding acetone to an aqueous solution of zinc nitrate and sodium gluconate.

However, it has now been found that crystalline metal-35 gluconates, especially zinc gluconate, can be prepared on an industrial scale by crystallization of seed crystals using a supersaturated aqueous solution, whereby the crystallization takes place in two steps.

The present invention provides an efficient method for crystallization of zinc gluconate from an aqueous solution useful on an industrial scale. The crystal yield is relatively high and the crystals are the same size and can be easily separated by centrifugation and washed and dried.

The crystallization takes place practically as an isothermal process at a relatively high temperature, at an acidic pH of 10 and using a very high degree of supersaturation and a large number of seed crystals.

The crystals obtained in the crystallization process are recovered by centrifugation, filtration or any other suitable solid / liquid separation means.

The crystallization step preferably comprises two steps: a) a step of forming new crystals; pre-crystallization from either a clear or solid solution, and b) a step of precipitating the solution from pre-existing crystals, commonly referred to as crystal growth. The present invention preferably uses a post-process.

Precipitation of a solid from solution to crystalline form occurs only if there is a compelling force imbalance. This means that the solution must be supersaturated with respect to the size of the crystals to be precipitated before the crystals can grow by precipitation from the liquid.

The degree of supersaturation at a given temperature and in the composition of the solution is calculated by the following formula: 30 S = pl_. (100 - ps)

(100 - Pl) PS

where S = degree of supersaturation 35 μl = dry matter of the solution (% by weight) ps = dry matter of the saturated solution (% by weight) 3 78454

Crystallization of zinc gluconate is particularly difficult because at high pH (pH above 6) the formation of zinc hydroxide prevents the formation of crystals, whereas at low pH (pH below 6) several forms of zinc-gluconic acid complex-5 compete. Despite these serious problems, the present invention provides an efficient method for crystallizing zinc gluconates in aqueous solution. The process describes ideal conditions for the formation of zinc gluconate trihydrate crystals, which can be effectively used on an industrial scale. According to this method, pure crystal growth occurs and the product consists of pure zinc gluconate crystals, which are ideal for most purposes.

Using the present invention, it is possible to prepare pure zinc gluconate trihydrate crystals having a length of more than 20 μπι (average length of about 50-150 μη), a width of more than 3 μη (average width of about 5-20 μη) and a thickness of more than 1 μη (average thickness of about 3 -10 μη).

The starting material for the present uses is an aqueous solution of zinc gluconate buffered to a pH of about 4.0 to 7.0 (preferably about 5.0 to 6.0). The temperature of the solution is maintained above 30 ° C (preferably about 60-70 ° C). The aqueous solution is concentrated to a ratio of zinc gluconates by evaporation to a degree of supersaturation of about 1.3 to 2.0 (preferably 1.5 to 1.8).

Seed crystals of suitable size (about 20-200 μη long, 2-10 μm wide and 1-5 μm thick) are prepared by evaporative centrifugation and are properly added and dissolved in the feed solution at-2 °. The highly supersaturated, purified solution (zinc gluconate more than 99% of dry matter) crystallizes spontaneously to form microcrystals that can be separated and used as seed crystals in laboratory-scale crystallization to prepare seed crystals in the final form.

* 3 C

for large-scale crystallization.

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According to the present invention, the subsequent crystallization of zinc gluconates takes place in two steps. The first step is evaporative crystallization under reduced pressure and at a temperature of 50-80 ° C (preferably 60-70 ° C). The supersaturation of the zinc gluate-5 conates is maintained at 1.3 to 2.0 (preferably 1.5 to 1.8) by adding the feed solution. The precrystallization step usually lasts 3-10 hours, during which the crystals grow to 5-30% of the total mass. The crystals and solution obtained in the second step are transferred to a suitable stirrer, where the growth of crystals continues. The high temperature of the previous step is maintained; the temperature drop should not be more than about 20 ° C and preferably the final crystallization temperature is about 5-10 ° C lower than the initial temperature.

The crystals are separated from the solution by centrifugation and washed with water and dried with warm air, for example in a conventional tumble dryer. At this point, the long and brittle crystals break to a final length of about 20-200 mm (average length about 50-150 mm). The crystals are chemically resistant and can be dried to obtain anhydrous zinc gluconate.

A portion of the crystal mass can be left in the crystallizer as a crystal foot for the next batch to be crystallized, i.e., "extra" crystals can be used as seed crystals.

The invention is described in more detail in the following examples.

Preparation of seed crystals 1 kg of glucono-delta-lactone was dissolved in 10 liters of water at 50 ° C. 230 g of zinc oxide as a 30% aqueous suspension were added until the pH of the solution was about 4.2. The solution was filtered and evaporated under reduced pressure at 50 ° C for 11 hours. The formed crystals were separated from the solution by centrifugation and dried for about 12 hours at a temperature of about 80-90 ° C. These crystals were used as seed crystals.

5 78454

Example 1; Crystallization of zinc gluconates 150 kg of glucono-delta-lactone (GDL) was dissolved in 450 kg of water at 60 ° C. 34 kg of zinc gluconate (Kuusankoski Oy) was added as a 30% aqueous suspension until the pH of the solution was 5.4 and the solution was filtered. The dry matter content of the filtered solution was 24.1% by weight.

The solution was evaporated at a pressure of 200 mbar until the degree of supersaturation with respect to zinc gluconate was 1.9 (dry matter 47.6% by weight). 40 g of seed crystals (Preparation 10 described above) were added slurried to 150 ml of saturated aqueous zinc gluconate solution.

Evaporation was continued at 60 ° C for three hours, during which time the crystals grew into long, needle-like crystals. The bulk of the mass was then transferred to a crystallizer where the solution was stirred for an additional 30 hours. At the end of the crystallization, the temperature was about 55 ° C and the degree of supersaturation with respect to zinc gluconates was close to 1. The total dry matter content of the pulp was 48.3% by weight.

The crystals were separated by a conventional basket spinner if a mesh was used to collect small crystals. The crystals were washed with water (about 30% by weight of the pulp) and dried with warm air at a temperature of about 90 ° C in a run-drier. During centrifugation and drying, the long, brittle crystals were broken to a final length of about 20-200 mm (average length about 50-150 mm), a width of about 3-30 mm (average width about 5-20 mm) and a thickness of , which was 1-20 mm (average thickness about 3-10 mm).

The yield was 50% of the zinc gluconate used.

30 78454

Analysis of the zinc gluconate trihydrate crystal compared to the granular zinc gluconate shows the following:

Zinc-based zinc-gluconate_gluconate_ according to the prior art of the present invention

Shape crystalline powdery

Infrared 159 0 (s), 1410 (s, sh) 1590 (m), 1410 (b), absorbance 1310 (s), 1240 (s) 1170 (s), 1010 (m, sh) (cm-1) 1090 (s), 1040 (s) 870 (m) 10 980 (m), 870 (s) s = strong, sh = shoulder, m = medium, b = broad Example 2: Crystallization of zinc gluconate 150 kg of GDL and 34.5 kg of zinc oxide were dissolved in water according to Example 1. The pH was buffered to about 6.5 and the solution was filtered. The dry matter content of the solution was 34.8% by weight.

The solution was evaporated under reduced pressure of 300 mbar and at 70 ° C until the degree of supersaturation with respect to zinc gluconates was 1.8 (dry matter 54.4% by weight). 180 g of zinc gluconate crystals in 680 ml of a saturated aqueous solution were added as seed crystals and evaporation was continued at 70 ° C for three hours. The bulk of the pulp was then transferred to a crystallizer where the crystals were allowed to grow for an additional 60 hours. At the end of the crystallization, the dry matter of the pulp was 52.8% by weight and the temperature was 60 ° C. The degree of supersaturation was close to 1.

The crystals were separated from the solution by centrifugation and washed with water according to Example 1 in a yield of 45% of zinc gluconates.

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The foregoing description and examples of preferred embodiments are intended to illustrate the invention and are not to be construed as limiting the invention. Other alternatives to process parameters are possible, as will be apparent to one skilled in the art.

Claims (4)

8 78454 A process for preparing zinc gluconate trihydrate from an aqueous zinc gluconate solution, characterized in that a supersaturated zinc gluconate aqueous solution having a pH of about 4 to about 7 and a degree of supersaturation with respect to zinc gluconate of about 1.3 to about 2.0 is prepared; adding zinc gluconate seed crystals to said supersaturated solution; removing water from the solution at a temperature of about 50 to about 80 ° C to crystallize the zinc gluconate, maintaining a degree of supersaturation of about 1.3 to about 2.0; stirring the resulting crystal mixture to further crystallize the zinc gluconate 15; and recovering the crystallized zinc gluconate trihydrate. The method of claim 1, characterized in that the degree of supersaturation is from about 1.5 to about 1.8. Process according to Claim 1, characterized in that the crystallization temperature is from about 60 to about 70 ° C. Process according to Claim 1, characterized in that the crystallized zinc gluconate titrate hydrate is recovered by centrifugation.