WO2015110575A1

WO2015110575A1 - Mixture containing particles of zinc oxide having a reduced tendency of caking upon storage

Info

Publication number: WO2015110575A1
Application number: PCT/EP2015/051348
Authority: WO
Inventors: Florian WALTZ; Jürgen BEZLER
Original assignee: Grillo Zinkoxid Gmbh
Priority date: 2014-01-24
Filing date: 2015-01-23
Publication date: 2015-07-30
Also published as: EP3097153A1

Abstract

A mixture containing particles of zinc oxide and other oxide particles selected from the group consisting of oxides of silicon, aluminum, titanium, magnesium, calcium, or combinations thereof, and the use thereof for reducing the tendency of caking upon storage.

Description

Mixture Containing Particles of Zinc Oxide Having

a Reduced Tendency of Caking upon Storage

The invention relates to a mixture of particles of zinc oxide and other oxide particles, and to the use thereof for reducing the tendency of caking upon storage.

Zinc oxide is a substance that is employed in many forms in many fields, for example, utilized as a pigment under the designation of zinc white. Being a semiconductor, zinc oxide is employed as a transparent conducting layer in the production of blue light emitting diodes (LEDs), liquid crystal displays (TFT; thin- film transistor), varistors (VDR; voltage dependent resistor), and thin-film solar cells. As semiconducting nanowires, zinc oxide is employed in measuring technology because of its piezoelectric properties and UV light transparency. As a component of pharmaceutical zinc ointments, pastes, plasters (Leukoplast) and dressings, zinc oxide causes the skin surface to dry out. This is used, for example, in intertrigo, diaper dermatitis and other exuding wounds. In athlete's feet and other dermatomycoses, zinc oxide promotes healing. Zinc oxide is also employed in sunscreens and as an additive in the vulcanization of rubber. Since zinc is an essential trace element for humans and animals, zinc oxide as a source thereof is employed both in food supplements for humans and in animal nutrition. In addition, it is employed as a phosphor in fluorescent lamps.

Zinc oxide is produced industrially on the order of 1-2 megatons a year. Customers include the pharmaceutical and chemical industries, and feed producers. The biggest customer of ZnO is the tire industry, zinc oxide usually being added to the raw rubber within a range of from 0.5 to 10 phr, its main task being the activation of sulfur-cured vulcanization. Because of the high demand for and the importance of zinc oxide, some storage of supplies of zinc oxide is indispensable for undertakings. Because of storage in storehouses that are often not temperature-controlled, moisture deposition/condensation may occur. Water bridges form between the zinc oxide particles, and under the pressure of the zinc oxide's own weight, for example, in a storage silo, caking of the zinc oxide occurs. This leads to dosing problems and problems with the admixing/incorporating of the zinc oxide. At the same time, pipelines may clog as a consequence of caking, and in such a case, they must be beaten free tediously, in part manually.

Because of their small particle size of often below 100 prn, industrial oxide raw materials, such as zinc oxide, show strong interparticulate attractive interactions (e.g., van der Waals interactions), which exceed the weight force of the individual particles and thus lead to poorer free-flowing properties of the material. Zinc oxide, which is usually produced with particle sizes of below 10 prn on an industrial scale, falls under the bulk material category of highly dispersed powders, for which very poor free-flowing properties are expected [Tomas J., Leinschmidt S., Chem. Ing. Tech. 2009, 81 (6), 717-733]. In practice, the flow characteristics of zinc oxide considerably complicate the conveying and transport from silos and through pipelines, and the further processing.

When zinc oxide is stored, there may be a disadvantage in that caking of the zinc oxide bulk occurs. This may lead to deterioration of the withdrawal of the zinc oxide from the storage vessel, and to reduced flow properties.

EP 2 409 952 Al, US 2010/047590 Al, US 2006/073092 Al and Zhou et al. described zinc oxide particles doped with other elements, but not mixtures of discrete zinc oxide particles with discrete other oxide particles.

US 2012/0233929 Al discloses discrete hydrophobically modified nanoparticles, which are to be added to a wide variety of other particles to prevent absorption of moisture in a powder, whereby the powder can be stored even at an increased humidity.

Summary of the invention

An object of the invention is to provide a zinc oxide having a reduced tendency of caking upon storage. According to the invention, this problem is solved by a mixture containing particles of zinc oxide and other oxide particles selected from the group consisting of oxides of silicon, aluminum, titanium, magnesium, calcium, or combinations thereof.

In one embodiment of the invention, the particles of zinc oxide cannot be doped with an element selected from the group consisting of Sc, Y, In, Ga, Al, Ti, B, Ge, Sn, and lanthanides.

In another embodiment of the mixture according to the invention, it may contain said other oxide particles in amounts of from 0.1% by weight to 25% by weight.

In another embodiment, the size of said other oxide particles may orient itself by the particle size of the zinc oxide employed, and they may have a primary particle size that is smaller than that of the zinc oxide employed by a factor of at least 10 or by a factor of at least 50. The term primary particle is understood according to the definition of DIN 53206. Typically, the mixture according to the invention has zinc oxide particles with a particle size of from 0.03 to 500 prn. In particular, "particle size" relates to d50 values of the volume-weighted particle size distribution of an aqueous suspension of the particles as determinable by means of laser diffraction. Such values can be determined with suitable devices, for example, Malvern Mastersizer 2000.

In particular, the particles of zinc oxide are agglomerated and are not in the form of discrete particles.

The mixture according to at least one of claims 1 to 6, wherein said other oxide particles are not modified with a hydrophobic surface. Oxide particles provided with a hydrophobic surface are disclosed in US 2012/0233929 Al .

In yet another embodiment, the size of the other oxide particles in the mixture 1 to 3 according to the invention may be from 0.003 to 1 prn.

The size of the other particles of oxides of silicon in the mixture may be < 1000 nm, especially < 300 nm, especially from 1 nm to 300 nm. Because the primary particles are frequently in a strongly agglomerated/aggregated form, it may be more appropriate to state specific N₂ surface areas (BET) according to DIN 66132. The N₂ surface area of the oxides of silicon in the mixture may be > 50 m²/g, especially > 100 m²/g, especially from 150 to 350 m²/g.

The origin of the zinc oxide is of less importance. In the mixture according to the invention, the zinc oxide of the zinc oxide particles may be obtainable, in particular, as the product of a thermal or wet-chemical process.

The invention also relates to the use of the mixture according to the invention for reducing the tendency of caking upon storage.

The mixture according to the invention is advantageous because, in addition to the reduction of the tendency of caking upon storage, there is also a good flowability of the mixture.

Figure 1 : Graphical plot of the free-flowing capabilities (ffc) for different consolidation stresses (sigma 1) of pure pharma grade zinc oxide and of mixtures thereof with different silicas.

Figure 2: Graphical plot of the free-flowing capabilities (ffc) of different zinc oxides and mixtures of such zinc oxides with pyrogenic silica as determined at consolidation stresses of about 5000 Pa, and after three days of storage at such consolidation stress.

Detailed description of the invention

The present invention discloses a mixture containing particles of zinc oxide and other oxide particles selected from the group consisting of oxides of silicon, aluminum, titanium, magnesium, calcium, or combinations thereof.

In one embodiment of the mixture according to the invention, it contains said other oxide particles in amounts of from 0.1% by weight to 25% by weight. Generally, the addition of said other oxide particles leads to reduced caking upon storage, and to an increased flow property of the mixture. The described properties do not change linearly with the amount of other oxide particles added, but a plateau effect is seen. A significant improvement of caking upon storage and of the flow behavior of the mixture is obtained already when less than 2% by weight of the other oxide particles are admixed.

In another embodiment, the mixture according to the invention has zinc oxide particles with a particle size of from 0.03 to 500 prn. In this context, "particle size" relates to d50 values of the volume-weighted particle size distribution of an aqueous suspension of the particles as determinable by means of laser diffraction. Such values can be determined with suitable devices, for example, Malvern Mastersizer 2000.

Typically, volume-weighted d50 values according to measurement by laser diffraction of industrially produced zinc oxide are < 5 prn. Only so-called feed grade zinc oxide (for example, Grillo zinc oxide F72 and F80) have d50 values above 10 pm. Although such zinc oxides already have a clearly lower tendency to caking upon storage and a higher flowability of the powders as compared to other grade zinc oxides, the addition of other oxide particles, which leads to the mixture according to the invention, results in an improvement of flow property in this case too.

The selection of the size of said other oxide particles may orient itself, for example, by the particle size of the zinc oxide employed. In particular, they may have a primary particle size that is smaller by a factor of at least 10 or by a factor of at least 50. According to one embodiment of the invention, the size of the other oxide particles in the mixture according to the invention may be from 0.003 to 1 pm.

The size of the particles of oxides of silicon in the mixture is typically < 1000 nm, especially < 300 nm, especially from 1 nm to 300 nm. Because the particles are frequently in a strongly agglomerated/aggregated form, it may be more appropriate in practice to state specific N₂ surface areas (BET) according to DIN 66132. The N₂ surface area of the oxides of silicon in the mixture is > 50 m²/g, especially > 100 m²/g, especially from 150 to 350 m²/g- The origin of the zinc oxide is of less importance. In the mixture according to the invention, the zinc oxide of the zinc oxide particles may be obtainable as the product of a thermal or wet-chemical process.

According to the raw materials employed, thermal processes are classified into two basic processes, a direct one (also referred to as an American process), and an indirect one (also referred to as a French process). In the direct process, oxidic precursors, so-called white ashes, are reduced in a rotary kiln at high temperatures using coke, followed by oxidation with atmospheric oxygen to form zinc oxide, and collected by filters. The zinc oxide obtained usually has a star-shaped morphology and N₂ surface areas according to DIN 66132 of 1-2.5 m²/g - Examples of a zinc oxide produced by this process include zinc oxide 2011 from Grillo Zinkoxid GmbH, Goslar.

In the indirect process, metallic zinc precursors, for example, hard zinc, refined zinc, floats from strip galvanizing, SHG zinc, and remelted zinc are employed. These are heated above the boiling temperature of zinc by means of a furnace, such as a column, retort, muffle kiln and rotary kiln. The vaporous zinc is subsequently oxidized by atmospheric oxygen and collected in a filter. Industrial examples of zinc oxides produced by this method by means of a rotary kiln include Rotsiegel, Grunsiegel and Weisssiegel from the Grillo Zinkoxid GmbH Goslar, which are distinguished by their purity, the purity increasing towards the Weisssiegel. These zinc oxide types usually have N₂ surface areas according to DIN 66132 of 3- 7 m²/g- Examples of zinc oxides produced by the French process using a retort include the so-called pharma grades from the Grillo Zinkoxid GmbH. These are characterized by very little contamination with heavy metals, wherein the ZnO content according to DIN 55908 > 99.9%. According to DIN 66132, the N₂ surface areas of these types are usually from 3 to 20 m²/g-

Another thermal process for producing zinc oxide is the clinker process, in which so-called zinc ashes obtained from galvanizing processes are employed as raw materials. In a rotary kiln, contaminants are discharged in gaseous form at temperatures of > 1000 °C, and any metallic zinc present is oxidized. A brownish zinc oxide with zinc contents of from 70 to 80% is obtained as a product. The thus obtainable zinc oxide is usually employed in animal nutrition (also referred to as feed grade zinc oxide), where it serves as a zinc source for preventing zinc deficiency symptom. Examples of zinc oxide produced by the clinker process include the feed grades F72 and F80 from the Grillo Zinkoxid GmbH. These two grades are distinguished only by the zinc content of the oxide, F72 > 71.5% and F80 > 74.5% according to DIN 55908. Zinc oxide obtained by this production process is characterized by relatively large particle sizes of > 10 prn, corresponding to N₂ surface areas according to DIN 66132 of < 1 m²/g.

The mixture according to the invention avoids the disadvantages arising from the above described caking. At the same time, the addition of oxides of silicon to zinc oxide should not create any further problems in the tire industry since oxides of silicon are present as fillers in many rubber blends with filling rates of more than 50 phr. Therefore, no contamination occurs, but rather the amount of oxide of silicon employed as a filler can simply be reduced optionally.

The invention is further illustrated by means of the following Examples. Example 1 :

To 200 g each of Grillo Zinkoxid Pharma 4, a zinc oxide prepared in a retort by the indirect (French) method, 8 g each (corresponding to an addition of +4% by weight) of silica (an amorphous silicon oxide) with different specific N₂ surface areas (BET) according to DIN 66132 (precipitated silica A = 164 m²/g, pyrogenic silica B = 333 m²/g, and C = 199 m²/g) was added. The flowability of these mixtures was determined by means of a ring shear test according to Schulze in accordance with ASTM D6773-08. In addition, the flowability of the pure zinc oxide as mentioned above was determined as a reference. The corresponding flowabilities (ffc) at different consolidation stresses (sigma 1) can be seen from Table 1, and as a graphical plot from Figure 1. It is to be seen that the admixing of silica results in a significant increase of flowability. In particular, this increase of flowability occurs in mixtures with pyrogenic silica having a specific N₂ surface area (BET) of 199 m²/g, in which an increase of flowability of up to 80% could be observed, depending on the consolidation stress.

Table 1 : Flowabilities (ffc) at different consolidation stresses (sigma 1) of pure Grillo Pharma 4 zinc oxide and of mixtures thereof with different silicas.

Figure 1 : shows a graphical plot of the free-flowing capabilities (ffc) for different consolidation stresses (sigma 1) of pure pharma grade zinc oxide and of mixtures thereof with different silicas.

Example 2:

The free-flowing capabilities (ffc) of different zinc oxides and mixtures of such zinc oxides with +4% by weight, or +2% by weight in one case, of a pyrogenic silica with a specific (N₂) surface (BET) according to DIN 66132 of 206 m²/g were determined by ring shear tests according to Schulze in accordance with ASTM D6773-08 at consolidation stresses of about 5000 Pa. In order to evaluate the influence of the silica addition on flowability after storage (caking with time), the flowabilities of the pure zinc oxides and of the mixtures after a storage time of three days under the consolidation stress. As shown by means of the flowabilities of the zinc oxides and their mixtures with silica in Table 2 and Figure 2, the addition of silica in the zinc oxides examined here leads to an increase of the flowing property. However, this increase depends on the zinc oxide employed and is strongest for Grillo Rotsiegel zinc oxide with an increase of +125%, and weakest for Grillo F80 zinc oxide with an increase of +34%. Further, from the flowabilities of Grillo Pharma 4 zinc oxide and the mixtures of Grillo Pharma 4 zinc oxide with +2% and +4% by weight of silica, it becomes clear that the flowability is not directly linearly dependent on the quantity of the added silica. In this Example, an addition of +2% by weight silica results in an increase of flowability of +60%, and an addition of +4% by weight results in an increase of flowability of +73%, based on the pure zinc oxide. At the same time, Table 2 and Figure 2 show the changes of the caking upon storage of the individual zinc oxides and the mixtures thereof. In all cases, except for pure Grillo Rotsiegel zinc oxide, there is a reduction of flowability after three days of storage. However, the flowability of the mixtures after three days of storage was improved by the addition of pyrogenic silica in all cases as compared to the flowabilities of the pure zinc oxides upon storage. The largest difference could be seen in Grillo 2011 zinc oxide with an increase of flowability upon storage of +114%, and the smallest difference could be seen in Grillo F80 zinc oxide with an increase of flowability of +3%.

Table 2: Free-flowing capabilities (ffc) of different zinc oxides and mixtures of such zinc oxides with pyrogenic silica as determined at consolidation stresses of about 5000 Pa, and after three days of storage at such consolidation stress.

Figure 2 shows a graphical plot of the free-flowing capabilities (ffc) of different zinc oxides and mixtures of such zinc oxides with pyrogenic silica as determined at consolidation stresses of about 5000 Pa, and after three days of storage at such consolidation stress.

Claims

C L A I M S :

1. A mixture containing particles of zinc oxide and other oxide particles selected from the group consisting of oxides of silicon, aluminum, titanium, magnesium, calcium, or combinations thereof.

2. The mixture according to claim 1, wherein the particles of zinc oxide are not doped with an element selected from the group consisting of Sc, Y, In, Ga, Al, Ti, B, Ge, Sn, and lanthanides.

3. The mixture according to claim 1 or 2, wherein the mixture contains said other oxide particles in amounts of from 0.1% by weight to 25% by weight.

4. The mixture according to at least one of claims 1 to 3, wherein the particles of zinc oxide have a particle size of from 0.03 to 500 prn.

5. The mixture according to at least one of claims 1 to 4, wherein the particles of zinc oxide are in an agglomerated form.

6. The mixture according to at least one of claims 1 to 5, wherein the size of said other oxide particles orients itself by the particle size of the zinc oxide employed, having a primary particle size that is smaller than that of the zinc oxide employed by a factor of at least 10, or by a factor of at least 50.

7. The mixture according to at least one of claims 1 to 6, wherein the particles of zinc oxide are not modified with a hydrophobic surface.

8. The mixture according to at least one of claims 1 to 7, wherein the size of said other oxide particles is from 0.003 to 1 prn.

9. The mixture according to claim 8, wherein the size of the particles of oxides of silicon is < 1000 nm, especially < 300 nm.

10. The mixture according to claim 9, wherein the size of the particles of oxides of silicon is from 1 nm to 300 nm. The mixture according to at least one of claims 1 to 10, wherein the zinc oxide of the zinc oxide particles is obtainable as the product of thermal or wet-chemical processes.

Use of the mixture according to at least one of claims 1 to 11 for reducing the tendency of caking upon storage.