DE2530565C3 - Process for the production of chrome oxide green - Google Patents

Description

Because of its chemical and thermal stability chromium oxide green is a valued pigment. Due to its somewhat dull, green hue various attempts have been made to achieve a bright green. Have these attempts but so far has not led to the desired success. In addition to the hue, the color strength is an important factor Criterion for applicability. Pigments are used in practice for economic reasons with the highest possible color strength desired.

Chromium oxide green is usually produced by reducing an alkali chromate, preferably sodium dichromate, calcining the reduction product, washing, drying and grinding. Sulfur, for example, is suitable as a reducing agent. Processes have also been described in which thermal decomposition of special Cr (VI) compounds, e.g. B. CrOi or (NH ^ -CnO ?, leads to chromium oxide green. Common to all processes is the dull color of the resulting chromium oxide. In addition, the thermal decomposition of ammonium dichromate results in the CoOi formed undesirably in extremely finely divided form as a voluminous powder Small particle size, this product is unsuitable as a pigment because it has only a low color strength. Attempts to compress the voluminous decomposition product of ammonium dichromate have already been described. For example, according to USSR Patent 1,07,488, sodium dichromate is added to make a heavier product In US Pat. No. 3,278,261, densification is achieved by adding 0.1 to 5% by weight of an alkali halide during the thermal decomposition of ammonium dichromate at temperatures between 200 and 1200 ^° C. The this known process is the necessary starting compound ammonium dichromate a compound which is obtained only by a complicated process from the resulting as the primary product in the digestion of chrome ore sodium chromate or sodium dichromate we ^r the must. As is known, ammonium dichromate decomposes at elevated temperatures in an exothermic reaction with the formation of chromium oxide, water and nitrogen. Because of the risk of excessive decomposition of ammonium dichromate, this decomposition must take place under controlled conditions.

Attempts have therefore already been made to bypass the production of ammonium dichromate and instead to subject a mixture of sodium dichromate and ammonium chloride or ammonium sulfate to heating, with _{the corresponding sodium salt being formed in addition to Cr 2} Oj, which can be removed by washing.

In US Patent 14 29 912 Ammonium sulfate is mixed with an amount in excess of the stoichiometric equivalent amount of sodium dichromate, possibly reacted with addition of a small amount of water, and at 400 ⁰ C. According to the process of German Patent 7 28 233, sodium dichromate and ammonium sulfate are mixed dry in an approximately stoichiometric equivalent ratio, then calcined, whereupon the chromium oxide pigment formed is separated from the salts. The annealing takes place over several hours at 700 to 800 ° C. In contrast to the known process according to US Pat. No. 3,278,261, which starts from ammonium dichromate, in the known processes according to US Pat. No. 1,429,912 and German Pat. No. 7,28,233, which is based on sodium dichromate

} o go out, the pigment formation is carried out at relatively low temperatures, namely 400 or 700 to 800 ⁰ C, since it is known in pigment production that higher pigment formation temperatures normally lead to a deterioration in the pigment properties, since the pigment grain spectrum is broadened and the proportion of Coarse grain increases. For economic reasons, too, the aim for pigment formation is as low a pigment formation temperature as possible.

The chromium oxide pigments resulting from the known processes are, however, both in terms of the Color tone as well as with regard to the color strength and the grain distribution with deficiencies.

The aim of the present invention is therefore to produce chromium oxide pigments that have the disadvantages of No longer have chromium oxide pigments produced by known processes.

The present invention thus relates to a process for the production of chromium oxide green pigments by calcining a stoichiometrically equivalent dry mixture of sodium dichromate dihydrate and ammonium sulfate or ammonium chloride and then separating the pigment formed from the salts, which is characterized in that sodium dichromate dihydrate with particle sizes below 2 mm and ammonium sulfate or ammonium chloride with particle sizes below 0.5 mm are used and that the mixture to pigment formation temperatures of 800 to 1100 ° C within 15 minutes

ho is heated.

In the process according to the invention, care must be taken to ensure that the components are dry mixed Sodium dichromate dihydrate and ammonium salts will not be so intense that sodium dichromate dihydrate will be

(15 Water of crystallization is lost because then undesirably strong sticking and clumping effects occur. Suitable mixing units are, for example, the known drum mixers. Mixing devices such as 7. R.

Paddle screws, flight shear mixers, countercurrent high-speed mixers, Tumbler mixer. A major influence on the resulting color properties of the Chromium oxide pigments have the particle sizes of the starting substances used. It is necessary to the technical ammonium sulfate or ammonium chloride at least on particle sizes smaller To grind 0.5 mm, preferably less than 0.25 mm. The particle size of the sodium dichromate dihydrate is not so critical, however, it should be smaller than 2 mm.

Particularly saturated pigments are obtained by adding organic modifiers, the should preferably have a decomposition point above about 150 to 200 ° C. These organic Modifiers come in amounts of 1 to 10 wt .-%, preferably 2 to 4 wt .-%, based on the Mixture of sodium dichromate dihydrate and ammonium salts, to use. Organic modifiers from which the organic phase is distilled off without decomposition not suitable. Examples of suitable organic modifiers are starch, urea, medium viscosity Mineral oil, polyethylene powder, polyethylene oxide, sawdust. Wood flour, rosin, rice straw powder, activated charcoal, lignin preparations, molasses (beet waste), unsaturated Cellulose derivatives, polyester resins. For reasons of economy, cheap ones are preferred Waste or natural products such as wood flour, starch,

ίο lignin used.

The term "stoichiometrically equivalent amount" should be understood to mean that to I mol Sodium dichromate dihydrate 1 mol of ammonium sulfate or 2 mol of ammonium chloride are added to 1 mole of chromium oxide and 1 mole of sodium sulfate or 2 moles of sodium chloride to form after decomposition.

The chemical reactions of decomposition can be formulated according to the following equations:

1. Na ₂ Cr ₂ O ₇ ■ 2H ₂ O + (NH ₄ J ₂ SO ₄

2. Na ₂ Cr ₂ O ₇ · 2 H ₂ O + 2NH ₄ Cl -

- N ₂ + 6 H ₂ O + Cr ₂ O ₃ + Na ₂ SO ₄
- »N ₂ + 6H ₂ O + Cr ₂ O ₃ + 2NaCl

In the implementation according to these equations, small, z. B. 10% deviations after be tolerated on both sides. A deficit of ammonium salt makes itself through a decrease in Cr> O] yield undesirable in pigment formation noticeable. An excess of reducing agent increases the conversion, but decreases it slightly Saturation and brightness values of the resulting chromium oxide pigments and leads to condensation of Ammonium salts in downstream equipment.

For the production of chromium oxide of high color strength, according to the present invention, completely certain annealing conditions are met. It has been found that a slow heating of the Mixing up to pigment formation temperature leads to significantly worse pigments than a fast one Heating up. A slow heating is obvious from a technical point of view, since z. B. at the Using a rotary kiln, give up the starting mixture at its cold end and put the product in the Countercurrent to the combustion gases can lead through the furnace. Thereby one of the energy expenditure particularly cheap heating guaranteed.

If you instead bring the mixture directly into the hot oven, so that it is in a short time, z. B. in the area from a few seconds to about 15 minutes, is heated to the pigment formation temperature, so result the good pigments according to the invention.

The mixture according to the invention can be heated by adding the mixture directly to the hot zone of a rotary kiln, by feeding it onto the hot plate of a plate furnace or a rotary ring furnace (German Offenlegungsschrift 23 20 806), the plate of which is kept at the required temperature. For smaller quantities, muffle furnaces with sufficient heat capacity can be used, into which the starting mixture is introduced at the pigment formation temperature. Among pigment-forming temperature, a temperature Turbo Reich between about 800 and about 110o C ⁰ is understood to mean, wherein at a lower pigment-forming temperature for a longer residence time in the range of a few hours, at higher temperatures a shorter residence time in the range of several minutes is required. The pigment formation temperature between 900 and 1000 ° C. is preferred, heating times between approximately 10 minutes and 1 hour being necessary.

The pigment is worked up in a conventional manner after the calcination. This shows a another advantage of the method according to the invention. The annealing products are extremely easy to use split up, and in water they disintegrate in a very short time, the soluble sodium salts quickly dissolving. After filtration, the products are dried in a known manner and conventionally ground. the But grinding can also be carried out in suspensions, e.g. B. before filtration or before drying, made will.

As inorganic modifiers, boron compounds such. B. boric acid or borates, for example Alkali borates, well proven. With the addition of these modifiers, the hue of the obtained Move chromium oxide pigments from a yellowish green to a bluish green. See these inorganic Modifiers come in amounts of 1 to 10% by weight, preferably in amounts of 2 to 5% by weight, based on the mixture of sodium dichromate dihydrate and ammonium salt, for use.

The invention is explained in more detail below with the aid of examples. As with the pigment assessment Usually, a reference sample is used as a standard for assessing the properties of the invention Products are also used. The results are accordingly based on this standard. When A chromium oxide green produced according to the state of the art was chosen as standard. Its manufacture takes place by reducing alkali dichromates with sulfur, glowing the reduction product, washing, Drying and grinding.

Oo The color strength was determined in the rutile blend (ratio 1: 5) determined in the paint according to DIN 53 234. These values are in good agreement with the determination of the Standard color depth 1: 25 according to DIN 53 235. The color strength of the standard is 100

fts color strengths of the products prepared according to the invention standardized.

The color differences between the products and the standard were determined in accordance with DIN 6174. Included

are the following color distances in the color space according to Adams-Nickerson receive:

ΔΕαν: total color difference in MW units,

Αφ> 0 bluer than standard,
<0 yellower than standard,

AAs: saturation difference (> 0 = more saturated than standard),

AAu difference in brightness (> 0 = brighter aU standard).

0.5 ^ units correspond approximately to an eye threshold value

The measurements were made on a three-filter Elrepho photometer from Zeiss (without a glossy aperture) and on Spectropolometer DMC 25 from Zeiss (with gloss diaphragm) carried out.

Example 2

Na ₂ Cr ₂ O ₇ · 2 H ₂ O (grain size <1.0 mm) is _{mixed with finely ground (NH 4} I ₂ SO ₄ (grain size <0.25 mm) with the addition of 2.5% peripheral wood flour in a drum, annealing conditions : 1 hour; 950 ⁰ C. (Lignin preparations, methyl cellulose, molasses [beet waste], powdered unsaturated polyester resins, starch, urea, mineral oil of medium viscosity, polyethylene powder, polyethylene oxide, sawdust, wood flour, rosin, rice straw powder, activated charcoal and others gave similar results. )

Example 3
(Comparative example)

As in Example 2, only the (NH ₄ I ₂ SO _{4 used has} a grain size of less than 1 mm.

Examples

In the experiments below, 1 mol of sodium dichromate was used in each case. The percentages of the additives are based on the amount of the mixture of

Na ₂ Cr ₂ O ₇ · 2 H ₂ O

and ammonium salt. The mixtures are laid and on flat Quarzgutschalen in 1 to 2 cm height has been set (950 ⁰ C, for example) preheated muffle furnace in the on annealing temperature.

The temperature is reached in the good after about 15 minutes, then the time indicated in the examples is held at the temperature and then the bowl is removed from the hot oven. After cooling, water is washed with a total of 51 and dried at 120 ⁰ C. The conversion is determined in the 1st filtrate by titration of the Cr (VI) content still present. The results of the tests with regard to the color properties are summarized in Table 1.

Comparative example A.
(German patent 7 28 233)

70 parts by weight of Na ₂ Cr ₂ O ₇ · 2 H ₂ O with a grain size of less than 1.5 mm are _{mixed dry with 30 parts by weight of (NH 4} I ₂ SO _{4 with a} grain size of less than 1.0 mm. Annealing conditions 6 hours; 750 ° C.)

Comparative example B
(German patent 7 28 233)

Like A, only with (NH ₄ ) ₂ SO _{4 with a} grain size of less than 0.25 mm.

Comparative Example C
(US Patent 14 29 912)

100 parts by weight of Na ₂ Cr ₂ O ₇ (spray-dried, particle size less than 0.05 mm) are mixed dry _{with 50 parts (NH 4} I ₂ SO ₄ ^{and calcined at 400 °} C. for 70 minutes.

Example 1 _fto

Na ₂ Cr ₂ O ₇ · 2 H ₂ O (grain size <1.5 mm) is _{dry mixed with finely ground (NH 4} ) ₂ SO ₄ (grain size <0.25 mm). Annealing conditions: 1 hour; 950 ° C. Comparative Example D.

As Example 2, the grain size of the (NH ₄ J ₂ SO ₄ was less than 0.25 mm, the grain size of Na ₂ Cr ₂ O ₇ - 2 H ₂ O was between 1.5 and 5 mm.

Example 4

Na ₂ Cr ₂ O ₇ · 2 H ₂ O with a grain size of less than 1.5 mm is _{mixed with (NH 4} ) ₂ SO ₄ , a grain size of less than 0.25 mm and with 5% borax. Annealing conditions: 1 hour; 950 ° C.

Example 5

Na ₂ Cr ₂ O ₇ · 2 H ₂ O, grain size less than 1.5 mm, is mixed with NH 4 Cl (<0.25) in a stoichiometric ratio. Annealing conditions: 1 hour; 950 ⁰ C.

Comparative example F

As in Example 1, only the heating from 20 to 950 ° C takes place over a period of 80 minutes; then 1 hour; 950 ⁰ C.

Comparative example G

As in example 5, only the heating takes place from 20 to 950 ° C over a period of 80 minutes; then 1 hour; 950 ° C.

example Sales color strength Color differences in + 0.9 106 4.2 1.9 Pure tone 4 ,. No. 102 Ali _AS 2.5 108 3.1 -0.1 AAs A, 1.7 A. 96.5 103 5.9 difficult to filter, inhomogeneous 95 0.7 0.2 -5.7 - 1.5 B. 96.6 2.9 97.3 87 2.8 0.2 -2.2 C. 97.6 105 5.2 2.3 black 3.6 1 97.3 103 3.3 2.0 1.9 2.0 2 95.7 80 3.5 0.8 2.3 0.5 3 97.9 64 3.0 -0.9 0.5 2.0 D. 94.7 2.0 4.2 4th 98.7 2.7 2.7 5 94.0 1.7 3.1 F. 1.4 2.6 G 1.6

Claims (2)

Patent claims: 1. A process for the production of chromium oxide green pigments by calcining a stoichiometrically equivalent dry mixture of sodium dichromate dihydrate and ammonium sulfate or ammonium chloride and then separating the pigment formed from the salts, characterized in that sodium dichromate dihydrate with particle sizes below 2 mm and ammonium sulfate or ammonium chloride with particle sizes below 0 , 5 mm are used and that the mixture is heated ^{to pigment formation temperatures of 800 to 1100 0 C within 15 minutes.} 2. The method according to claim 1, characterized in that the reaction mixture up to 10 Wt .-% modifiers are added, in the case of the use of organic modifiers preferably 2 to 4% by weight and, in the case of inorganic modifiers, preferably 2 to 5 % By weight are added.