DK143796B - PROCEDURE FOR PREPARING SOFT CHROMATE WITH A PURITY OF 99.5 PCT. - Google Patents

Description

(19) DENMARK

FWN

® | (12) PUBLICATION NOW 143796 B

DIRECTORATE OF THE PATENT AND TRADEMARKET SYSTEM

(21) Application No. 5244/73 (51) lnt.CI.3 C 01 0 37 / ή (22) Filing date 25 · sep. 1973 (24) Running day 25-Sep. 1973 (41) Aim. available 27 · tnar. 1974 (44) Submitted Act 12. 1981 (86) International Application No. - (86) International Filing Day - (85) Continuation Day - (62) Master Application No. -

(30) Priority Sep 26 1972, 7234056, FR

(71) Applicant SAFT-SOCIETE DES ACCUMULATEURS FIXES ET DE TRACTION, 93230

Romainville, FR.

(72) Inventor Andre Lee is f, FR.

(74) International Patent Bureau.

(54) Process for producing silver chromate with a purity above 99 * 5%.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a process for producing silver chromate having a purity of greater than 99.5%, particularly useful as an active material in an electrochemical generator, by reacting between a soluble chromate and a silver salt.

Silver chromate is a positive active material which is of particular interest to an electrochemical generator with a negative lithium electrode and a non-aqueous electrolyte, especially because of its very poor solubility in the electrolyte solvents used in such generators, generally cyclic ethers.

D

D elies carboxylic acid esters or mixtures of these two types of substances. This is honestly described in French Patent Specification No. 2,110,765.

t However, the silver chromate used must be very clean. Namely, elements made with commercially available silver chromate samples, containing only an amount of impurities of 0.5%, undergo very rapid self-discharge. Thus, such products cannot be used as active material in generators of the aforementioned type.

Furthermore, since silver chromate is not an industrial product, it is appropriate to use a manufacturing method adapted to industrialization, ie. requires the least investment, space, running time and operating costs.

It is an object of the present invention to provide a process for producing silver chromate sufficiently pure to be used as a positive active material in an electrochemical generator with a negative lithium electrode and a non-aqueous electrolyte, which method must satisfy the requirements of be put to an industrialization.

A known method for preparing silver chromate consists in reacting a soluble chromate, e.g. potassium chromate, with silver nitrate in an aqueous solution. The silver churnate, which is very soluble, precipitates after the following reaction: 2 Ag + + Cr0 + -> Ag2Cr04

Y

However, due to the basic nature of the chromium GrO2, the product obtained is contaminated with silver oxide and is not suitable for its intended use because the silver oxide has a very high solubility in the aforementioned electrolyte solvents. To avoid the presence of silver oxide, it is necessary to work in a very dilute solution, which is incompatible with an industrial process.

It has been found that in the presence of dichromate, the reaction medium maintains such a pH that silver oxide does not precipitate.

Accordingly, the process according to the invention is characterized in that the reaction is carried out in an aqueous environment containing sufficient of a dichromate to maintain a pH where silver oxide is not precipitated. a silver salt and of a concentrated solution of a chromate to a concentrated solution of a soluble dichromate.

According to another embodiment of the process according to the invention, the dichromate used is silver dichromate and only partially dissolved in the reaction medium, for silver dichromate is little soluble in water. Then, a chromate solution is added continuously. Since the solubility of silver chromate is poor relative to the solubility of silver dichromate, a double decomposition occurs:

Ag2Cr2O7 + CrO4

The reaction medium is preferably heated to obtain a large yield precipitate that is very easy to wash and dry. The medium can easily be brought to a boil.

Other features and advantages of the method of the invention will become apparent from the following embodiments.

Example 1

Preparation of a solution of magnesium chromate.

Since the magnesium chromate used for the precipitation of silver chromate is not a commercially available product, the solution was prepared as follows:

3.35 moles of chromium oxide, CrO 2, and 2.94 moles of magnesium oxide, MgO, are incorporated in slightly less than 1 liter of water and stirred to complete dissolution of MgO.

Water is added to 1 liter. Thus, the solution contains 2.53 mol / liter of magnesium chromate, MgCrO 2, and 0.41 mol / liter of magnesium dichromate, MgC 2 O 2. The use of an excess of CrO 2 relative to the stoichiometric amount allows a complete and rapid solution of MgO and leads to a solution whose chromate and dichromate titers can be calculated from the starting amounts. The presence of dichromate in the solution does not affect the order of operation. You could just as well start from stoichiometric amounts and filter to remove dissolved magnesium oxide. A chromatin solution would then be obtained practically free of dichromate.

Precipitation of silver chromate.

3

100 cm of a solution of 0.26 M magnesium dichromate is heated for boiling. This solution has a pH of 4.5. To this is added continuous and at the same time 3 by means of calibrated capillary tubes 80 cm 2.53 M magnesium chromate solution containing 3.202 moles of chromate and 122 cm silver nitrate solution containing 0.410 g ions of silver are added. The precipitate obtained is washed several times with boiling distilled water and then dried for 24 hours at 150 ° C.

Example 2

Preparation of silver dichromate.

A solution containing 0.250 mol of sodium dichromate and 0.125 mol of chromium oxide in 200 cm of water is brought to a boil. To this is added 100 cm of a 5 M solution of silver nitrate. A precipitate of silver dichromate is obtained, which is filtered and dried. The weight of the moist precipitate is 108.6 g.

Precipitation of silver chromate.

3

The previously obtained silver dichromate is suspended in 800 cm of water, which 3 is brought to a boil. Continuously 100 cm of the 2.53 M magnesium chromate solution used in Example 1 is added. The precipitate obtained is washed and dried as in the previous example. The dry weight is 79.5 g, 0.240 mol, silver chromate.

The method of Example 2 has the following advantages over the method of Example 1: In the first example, it is necessary to synchronize the additions of magnesium chromate and silver nitrate well to avoid an excess of chromate leading to an increase in pH and to a precipitation. of silver oxide. Such synchronization in an industrial process requires aids such as metering pumps which are costly and subject to failure. These requirements do not exist for the method of the second example which comprises 2 successive steps.

A synthesis with the advantages of both methods can be realized by modifying the first one as follows: Instead of simultaneously adding the magnesium chromatin solution and the silver nitrate solution, all silver nitrate is added first.

In that case, at the outset, one must have enough dichromate sufficient to precipitate all the silver in the form of silver dichromate, e.g. 0.205 moles of magnesium dichromate instead of 0.026 in Example 1. Next, all soluble chromate is added to transfer, as in Example 2, the silver dichromate into silver chromate. Thus, at one time, the hassle of the method of Example 1 and the intermediate manipulations of Example 2. are avoided. avoiding an increase in pH, which occurs by using the dichromate ions and by excess chromatics.

Examples 3 to 6

The method of Example 2 is used, replacing magnesium chromate with the same number of moles of:

Example 3: Lithium Chromate Example 4: Sodium Chromate Example 5: Potassium Chromate Example 6: Ammonium Chromate

The following table gives for different samples of silver chromate the weight of silver determined gravimetrically as AgCl, the weight of chromation determined volumetrically using Mohr's salt and the sum of the two values.

The following samples were analyzed:

Two commercially available samples, labeled A and B, thirteen samples prepared according to Examples 1 and 2, labeled C, for which the mean values are indicated,

Samples Ag,% CrO 2,% Ag + CrO 2,%

AggCrO 4, pure 65.03 34.97 100 A 65.0 34.5 99.5 B 61.6 33.9 95.5 C 64.88 34.93 99.81 D 65.3 34.6 99, 9 E 65.1 34.8 99.9 F 1 65.15 35.0 100.15 F 2 65.15 35.0 100.15 G 1 65.0 35.0 100.0 G 2 65.2 35 , 0 100.2 two samples prepared according to Example 3, respectively 4, labeled D and E, two samples prepared according to Example 5, labeled F 1 and F 2, two samples prepared according to Example 6, labeled G 1 and G 2.

The comparison of the values obtained for Ag + CrO 2 shows that the samples prepared according to the invention are much cleaner than samples A and B. The values slightly above 100% are explained by the analysis accuracy.

The process of the invention makes it possible to obtain a silver chromate where the weight of silver is between 64.8 and 65.2%, preferably between 64.9 and 65, Γ%, and wherein the content of chromation is at least equal to 34.6% and preferably is between 34.8 and 35.0%.

Although this fact is not explained in detail, it appears that the presence of alkali or ammonium cations in the reaction medium causes the precipitation of traces of silver oxide. Therefore, besides the silver cations, it is preferable to add only magnesium cations. On the other hand, if silver dichromate is separated from its environment of origin, the nature of the soluble dichromate used to obtain it is of no importance. Thus, in Example 2, sodium dichromate has been used.