Method for stabilizing metallic mercury

Abstract

Disclosed is a method for stabilizing metallic mercury in the form of mercury sulfide. The method includes the following steps: a) dispersing metallic mercury in a polysulfide aqueous solution so as to convert the metallic mercury into mercury sulfide; and b) separating the mercury sulfide.

Claims

1. Process for stabilizing metallic mercury in the form of mercury sulphide, the process comprising the following steps: a) dispersion of metallic mercury in an aqueous polysulphide solution so as to convert metallic mercury into mercury sulphide; b) separation of the mercury sulphide, wherein step a) is performed at a temperature of at least 60° C.

2. Process according to claim 1, in which dispersion of metallic mercury is done using ultrasounds or a disperser.

3. Process according to claim 2, in which dispersion of metallic mercury is done using ultrasounds.

4. Process according to claim 2, in which dispersion of metallic mercury is done using a disperser.

5. Process according to claim 1, in which dispersion of metallic mercury is done using ultrasounds.

6. Process according to claim 1, in which dispersion of metallic mercury is done using a disperser.

7. Process according to claim 1, in which the polysulphide solution has an S/Na.sub.2S molar ratio equal to 2.5 to 4.0.

8. Process according to claim 1, in which the polysulphide solution has an S/Na.sub.2S molar ratio equal to 2.7 to 3.5.

9. Process according to claim 1, in which the polysulphide solution has an S/Na.sub.2S molar ratio equal to 3 to 3.3.

10. Process according to claim 1, in which the active sulphur content in the polysulphide solution is from 0.5 to 7 mol/kg.

11. Process according to claim 1, in which the active sulphur content in the polysulphide solution is from 0.6 to 3.5 mol/kg.

12. Process according to claim 1, in which the active sulphur content in the polysulphide solution is from 0.5 to 5 mol/kg.

13. Process according to claim 12, in which the active sulphur content in the polysulphide solution is from 2.5 to 3.5 mol/kg.

14. Process according to claim 1, in which the S.sub.active/Hg mass ratio is 1 to 3.

15. Process according to claim 1, in which the S.sub.active/Hg mass ratio is 1.2 to 2.6.

16. Process according to claim 1, in which the S.sub.active/Hg mass ratio is 1.2 to 1.5.

17. Process according to claim 1, in which the S.sub.active/Hg mass ratio is about 1.3.

18. Process according to claim 1, further comprising after step b): c) recovery of the polysulphide solution after the separation of mercury sulphide in step b); d) addition of sulphur and possibly Na.sub.2S to the polysulphide solution recovered in step c); e) repetition of steps a) and b), using the polysulphide solution derived from step d); f) possibly, one or several repetitions of steps c) to e).

Description

EXAMPLES

(1) Preparation of Polysulphide Solutions

(2) The polysulphide solutions used in the following examples have been manufactured by dissolution of flower of sulphur in a sodium sulphide solution.

(3) The sodium sulphide solution was prepared from technical quality flaky sodium sulphide. This compound contains 60% Na.sub.2S.

(4) The precise compositions of the different polysulphide solutions used are given in Table 1 below.

(5) TABLE-US-00001 TABLE 1 Pure Pure Na.sub.2S/S c(S.sub.active) Ex- Water Na.sub.2S Na.sub.2S S (moles/ S.sub.active (mol/ ample (g) (g) (moles) S (g) (moles) moles) (moles) kg) 1 68.6  1.24 0.02  1.57 0.05 3.08 0.03 0.46 2 1000 22.10 0.28 29.92 0.94 3.30 0.65 0.61 3 1000 44.20 0.57 59.84 1.87 3.30 1.30 1.15 3bis 1000 68.00 0.87 92.06 2.88 3.30 2.01 1.66 4 1000 44.40 0.57 54.65 1.71 3.00 1.14 1.01 5 300 53.28 0.68 65.58 2.05 3.00 1.37 3.01

(6) During preparation of the polysulphide solution, some of the sodium sulphide used reacts with some of the added sulphur in a secondary dismutation reaction to form thiosulfate. The remaining sulphur in the zero oxidation state is active sulphur. In the framework of this invention, it is considered that the quantity in moles of active sulphur in the polysulphide solution is equal to the quantity in moles of sulphur used minus the equivalent in moles of sulphide added into the solution.

Example 1: Classical Mechanical Stirring

(7) 2.07 g of technical Na.sub.2S (namely 1.24 g of pure Na.sub.2S), 1.57 g of sulphur and 68.6 g of water were mixed in a 250 ml beaker. The S/Na.sub.2S molar ratio of the resulting polysulphide solution was 3 and the content of active sulphur was 0.46 mol/kg.

(8) 5 mg of metallic mercury was added after the dissolution of sulphur.

(9) The mixture was stirred by a magnetised bar rotating at a speed of 300 rpm. The formation of a black deposit of mercury sulphide was quickly observed. No more metallic mercury could be distinguished in a visual examination after 4 h30 of reaction. The conversion efficiency is more than 90%. 1/17 g of sulphur was then added.

(10) The reaction rate reduces over time. Thus, after 24 hours of stirring, even with excess sulphur equal to 2.6 times stoichiometry, the conversion efficiency does not exceed 99.5%. This relatively low efficiency appears to be due to encapsulation of mercury in mercury sulphide microspheres. This mercury then becomes difficult to access under conventional stirring conditions.

Example 2: Use of an Ultrasound Generator Probe (S.SUB.active./Hg Mass Ratio of 2.6)

(11) For this example, an ultrasound generator probe (L250 mm—20 kHz—300 W) made by Sinaptec composed of a NexTgen ultrasound generator at a frequency of 20 kHz was used, with a radial effect probe for uniform diffusion of ultrasound over its entire height.

(12) A polysulphide solution with a content of active sulphur equal to 0.61 mol/kg and an S/Na.sub.2S molar ratio equal to 3.3 was prepared by adding 36.8 g of technical Na.sub.2S (namely 22.1 g of pure Na.sub.2S) and 29.9 g of sulphur in one litre of demineralised water.

(13) 17.5 g of metallic mercury was mixed in 350 mL of this polysulphide solution (S.sub.active/Hg molar ratio equal to 2.6). The mixture was placed in a 400 ml Erlenmeyer flask. The probe was held in place by a bracket and was immersed in the solution. The reaction started at ambient temperature. The temperature increased to reach 80° C. during the test. Water was added regularly to compensate for evaporation and to stabilise the temperature.

(14) 9 recycling cycles were made after the first treatment cycle. The quantity of mercury added in all recycling cycles was identical to the quantity added in the first cycle (17.5 g of Hg for 350 mL of the cycle n−1 filtrate). The results are summarised in Table 2 below.

(15) TABLE-US-00002 TABLE 2 Cycle Added Added conversion No. S (g) Na.sub.2S (g) Initial pH Time Solid % 1 0 0 na 2 h Black 99.9995 2 2.9 0 na 2 h Black 99.9999 3 2.91 0 11.9 2 h Black 99.9999 4 2.91 0 na 2 h Black 99.9165 5.sup.a 2.91 0 12 2 h Black 99.9304 6.sup.a 2.92 0 12 2 h 30 Black 99.9524 7.sup.a 2.92 2.15 11 2 h Black 99.9991 8 2.89 2.17 11.8 2 h Black 99.9999 9 2.89 2.14 12 1 h 30 Brown 99.9994 10 2.32 0 na 2 h Red 99.9999 .sup.aNaOH added to adjust pH

(16) These tests show that a dispersion of mercury by ultrasound can considerably increase the conversion ratio of metallic mercury into mercury sulphide. During recycling, the supplementary addition of sulphide improves the conversion ratio. The last cycle (cycle No. 10) was done with less sulphur than the previous cycles (80% of the theoretical value). This test showed that reducing excess sulphur relative to mercury orients the reaction towards the red α-HgS form, while a larger excess is conducive to the formation of black β-HgS.

Example 3: Use of an Ultrasound Generator Probe (S.SUB.active./Hg Mass Ratio of 1.3)

(17) The test in example 2 was repeated with a polysulphide solution obtained by mixing 73.7 g of technical Na.sub.2S (namely 44.2 g of pure Na.sub.2S) and 59.8 g of sulphur in 1 litre of demineralised water. The active sulphur content was 1.15 mol/kg and the S/Na.sub.2S molar ratio was 3.3. Excess sulphur above stoichiometry was fixed at 1.3. The quantity of mercury engaged was 70.2 g per 350 mL of polysulphide solution.

(18) 4 recycling cycles were made after the first treatment cycle. The quantity of mercury added in all recycling cycles was identical to the quantity added in the first cycle (70.2 g of Hg for 350 mL of the cycle n−1 filtrate). The results are summarised in Table 3 below.

(19) TABLE-US-00003 TABLE 3 Cycle Added Added Na.sub.2S Initial Conversion No. S (g) (g) pH Time Solid % 1 0 0 na 2 h Red 99.9999 2 11.3 0 12.5 2 h Red 99.9675 3 11.3 0 12.5 2 h 30 Red 99.8904 4 11.3 2.14 12.5 2 h Red 99.9999 5 11.1 1.32 na 2 h Brown 99.9628

(20) The results are comparable to those in example 2. The production of the red α-HgS form is confirmed for an S.sub.active/Hg ratio=1.3. Addition of complementary sulphur during recycling cycles improves the conversion ratio. The drop in the conversion efficiency for cycle No. 5 despite the addition of sulphur is due to the dismutation of sulphur (secondary reaction giving rise to the formation of thiosulfate) induced by this addition of complementary sulphur. This phenomenon that reduces the available quantity of active sulphur can be counteracted by increasing the quantity of added sulphur.

(21) Another test (Example 3bis) was performed with a polysulphide solution prepared using 113.3 g of technical Na.sub.2S (namely 68 g of pure Na.sub.2S), 92.1 g of sulphur and 1 litre of demineralised water. The active sulphur content was then 1.66M and the S/Na.sub.2S molar ratio was 3.3. The S.sub.active/Hg mass ratio was fixed at 1.3, namely 105 g of mercury for 350 mL of polysulphide solution. The treated mercury quantity was thus about 30% of the mass of the polysulphide solution. After 2 h with the ultrasound probe, the product obtained is red and the conversion efficiency is 99.999%.

Example 4: Use of a Disperser (Content of Active Sulphur 1M, S.SUB.active./Hg Mass Ratio of 1.2)

(22) For this example, an Ultra-Turrax T18 laboratory disperser made by IKA equipped with a 19G (19 mm diameter) tool was used. The rotation speed is adjustable between 3000 and 25000 rpm. Note that friction increases the temperature of the medium at high speeds higher than about 17 000 rpm.

(23) A series of 20 tests with recycling of the solution was carried out under the following conditions:

(24) The polysulphide solution was prepared with 74 g of technical Na.sub.2S (44.4 g of pure Na.sub.2S), 54.6 g of sulphur and 1 litre of demineralised water.

(25) The active sulphur content of the polysulphide solution was 1.01 mol/kg and the S/Na.sub.2S molar ratio was 3.

(26) The quantity of mercury treated per cycle was 190 g, giving an S.sub.active/Hg mass ratio=1.2

(27) Stirring speed 15 000 to 18 000 rpm

(28) Reaction time: 1 h50

(29) Start temperature: 80° C.

(30) One litre of polysulphide solution was placed in a conical flask. The disperser was held by a bracket and immersed in the mixture. At the end of the indicated reaction time, the mixture was filtered using a Büchner filter. The cake was recovered by filtration and kept without being dried for future analyses.

(31) The results are summarised in Table 4 below.

(32) TABLE-US-00004 TABLE 4 Mer- NaOH in 50% Conver- cury Na.sub.2S S solution sion (g) (g) (g) (g) (%) Cycle 1 (initial) 190 74 54.7 99.99995 for 1000 ml of water Cycle 2 190 7.5 30.3 10 99.99958 Cycle 3 190 7.5 30.3 99.99991 Cycle 4 190 7.5 30.3 99.99998 Cycle 5 190 0 36 10 99.99998 Cycle 6 190 0 30.3 99.98740 Cycle 7 190 7.5 32 99.99995 Cycle 8 190 7.5 32 99.99883 Cycle 9 190 0 30.3 99.99999 Cycle 10 190 7.5 32.6 99.99935 Cycle 11 190 0 30.3 6 99.99998 Cycle 12 190 0 30.3 6 99.99920 Cycle 13 190 3.75 31 99.99997 Cycle 14 190 3.75 31 99.99998 Cycle 15 190 3.75 31 99.99994 Cycle 16 190 3.75 31 99.99999 Cycle 17 190 3.75 31 99.99999 Cycle 18 190 3.75 31 99.99998 Cycle 19 190 3.75 31 99.99999 Cycle 20 190 3.75 32 99.99999 Total 3800 149 648.4 32 average 99.9992

(33) Not all the tests were done under the same conditions. Cycles 1 to 6 were done in a square metallic bath, the other tests were done in a 1 litre Erlenmeyer flask. For cycles 1 to 12, the addition of sulphur and the compensation for dismutation of sulphur were not systematic. The solution was initially heated to 80° C. but the temperature was not kept constant throughout the reaction. The stirring speed was constant at 18 000 rpm

(34) Starting from cycle 13, the procedure was more systematic with the constant addition of sulphur corresponding to 5% of the initial quantity and compensation for dismutation of sulphur. The disperser speed was set to 18 000 rpm for the first 15 minutes and then to 1500 rpm. A conversion from the metacinnabar form to the cinnabar form was observed after 50 minutes' reaction time. The mercury sulphide obtained is then constant quality with a conversion ratio of more than 99.9999%.

(35) A leaching test according to standard NF EN 12457-2 made with this same average sample gives a value of leachable mercury equal to 1.1 mg/kg. If the leachate is filtered to a threshold of 0.2 μm (instead of 0.45 μm according to the standard), the value is 0.1 mg/kg. This indicates that the measured mercury is not soluble mercury but is composed of fine HgS particles.

Example 5: Use of a Disperser (Content of Active Sulphur 3M, S/Na.SUB.2.S Molar Ratio=3.01, S.SUB.active./Hg Mass Ratio of 1,2)

(36) The polysulphide solution was made with 88.8 g of technical Na.sub.2S (namely 53.3 g of pure Na.sub.2S), 65.6 g of sulphur and 300 ml of demineralised water. 228.4 g of metallic mercury was added.

(37) The tests were done using the same protocol as for example 4, except for the flask volume that was 500 ml instead of one litre. The initial temperature was 50 to 60° C. because the reaction is significantly more exothermic than with a polysulphide solution with an active sulphur content equal to 1 M. The temperature reached 80 to 90° C. after about 10 minutes' reaction.

(38) An analysis of volatile mercury was made. The Mercury Tracker 3000 made by Mercury Instrument was used for this test, carrying out a “washing flask test”. For this test, a 100 g sample of the solid obtained is placed in a closed one-litre washing flask for 24 hours. The outlet tube is then connected to the gas analyser and the value is noted after allowing to stabilise for a few minutes.

(39) Three successive cycles were carried out recycling the reaction solution. The results are given in Table 5.

(40) TABLE-US-00005 TABLE 5 Volatile Leaching Hg (test in NF EN washing Mercury Na.sub.2S S Conversion 12457-2 flask) (g) (g) (g) (%) (mg/kg) (μg/m.sup.3) Cycle 1 (initial) for 228.4 88.8 65.6 99.99997 0.7* 1 300 ml of water Cycle 2 228.4 8.9 38.5 99.99994 0.07 8 Cycle 3 228.4 8.9 38.5 99.99997 0.1 1 *0.2 mg/kg with filtration to 0.2 μm

(41) The change from the metacinnabar form to the cinnabar form occurs after 10 to 20 minutes instead of 50 minutes in the case of a polysulphide solution with an active sulphur content of 1 M. For cycle 3, the HgS cake was not rinsed during filtration. It will be noted that this does not reduce the product quality. On the contrary, a lack of very fine particles during filtration was observed after the leaching test. The quality of the products obtained is equivalent to or better than that obtained previously with an efficiency of more than 99.9999%, leaching of mercury between 0.07 and 0.7 mg/kg and a concentration of volatile mercury equal to 1 to 8 μg/m3.

Example 6: Transformation of Hg to HgS According to Example 1 in U.S. Pat. No. 3,061,412

(42) Example 1 in U.S. Pat. No. 3,061,412 was repeated with the same initial product quantities and the same mixing duration using a blender made by Kenwood® equivalent to the blender made by Waring® used in the patent. An exothermic reaction is observed and a very thick black sludge is recovered. This sludge was filtered and rinsed with water.

(43) The conversion efficiency is 94.45%. The concentration of volatile mercury in the gas blanket of a one-litre flask containing 100 g of residue, measured with a Mercury Tracker 3000 analyser by placing the probe above the flask, is 800 μg. This measurement method gives lower values than the washing flask test used in Example 5.