Removing mercury from concentrated sulfuric acid using iodides
United States Patent [191 Kurikami 1 Apr. 1, 1975 REMOVING MERCURY FROM CONCENTRATED SULFURIC ACID USING IODIDES [75] Inventor:  
 [73] Assignee: Toho Aen Kabushiki Kaisha,  
 Tokyo-to, Japan [22] Filed: Aug. 7, 1973 [21] Appl. No.: 386,433  
 Toshio Kurikami, Annaka, Japan [30] Foreign Application Priority Data Aug. 15, 1972 Japan 47-81556 Jan. 29, 1973 Japan 48-11692 [52] U.S. Cl 423/101, 423/491, 423/531 [51] Int. Cl C0lg 13/04, C0lb 17/90 [58] Field of Search 423/101, 102, 491, 531  
 [56] References Cited FOREIGN PATENTS OR APPLICATIONS 1,054,972 4/1959 Germany 423/531 1,216,263 5/1966 Germany 423/531 OTHER PUBLICATIONS Mellov: A Treatise On Inorganic and Theoretical Chemistry, Vol. 4 (1923), p. 902 and p. 915.  
 Pascal: Nouveau Trait De Chimie Minrale, Vol. V (1962), p.542.  
 Primary Examiner-Oscar R. Vertiz Assistant Examiner-Brian E. I-learn Attorney, Agent, or Firm-Holman &amp; Stern [57] ABSTRACT 16 Claims, N0 Drawings BACKGROUND OF THE INVENTION The present invention relates to a process of removing mercury from a highly concentrated sulfuric acid having a concentration of not less than 70 percent.  
  In general, industrial sulfuric acid contains a trace of mercury which may be estimated to amount to 0.1 to 30 mg/kg. It is well known that among heavy metals, mercury is especially harmful to the human body, and thus sulfuric acid containing a trace of mercury is, in itself, forced to have limited uses. In addition, it is desirable from an environmental pollution point of view that mercury contained in sulfuric acid is in an. amount of as little as possible. Most of mercury is introduced into sulfuric acid from the starting meterials thereof. For example, many sulfide ores of metals contain mercury though it is in a very small amount. When these ores are used as the first raw material in the production of sulfuric acid, there is produced sulfur dioxide which is, in turn, used as the second raw material to produce sulfuric acid. A major portion of mercury contained in the raw ores is entrained in the sulfur dioxide, and the entrained mercury is incorporated into sulfuric acid produced therefrom.  
  In order to produce sulfuric acid with a low content of mercury, choosing of the raw ores with a low content of mercury is desirable, but it is not practicable from an industrial point of view. Several methods have been known which include removing of mercury at the refining process of sulfur dioxide which is used as a direct material for producing sulfuric acid. For example, one method is one that comprises washing sulfur dioxide with concentrated sulfuric acid at a high temperature to remove mercury contained therein and another method is one that comprises rapidly cooling and condensing mercury in a vapor phase contained in sulfur dioxide to recover it.  
  The first method is not always practicable because it requires a relatively large scale installation wherein a large amount of sulfuric acid should be circulated therethrough and the operations should be carried out under the limited conditions of the gas. On the other hand, the second method has no reliability in respect of high removal efficiency. Further, a variety of methods have been proposed which are intended to remove mercury contained in the produced sulfuric acid. For example, there is known a process of removing mercury contained in sulfuric acid which comprises reacting the sulfuric acid with hydrogen sulfide in the presence of activated carbon to convert the mercury contained therein to sulfides. There is also known another process of removing mercury contained in sulfuric acid which comprises bringing the sulfuric acid into contact with catalysts containing platinum group metals.  
  However, all these methods are applicable only when the sulfuric acid is low in concentration, and there has not been found any useful method which may be applied to concentrated sulfuric acid or the similar sulfuric acid frequently used at the present day.  
 SUMMARY OF THE INVENTION I It is an object of the invention to provide a method of removing mercury contained in a highly concentrated sulfuric acid.  
  The above-stated object can be accomplished by the method of the present invention which comprises adding iodides to a highly concentrated sulfuricacid having a concentration of not less than 70 percent and containing a trace of mercury to precipitate the mercury contained in said sulfuric acidas mercuric iodides, and removing the precipitate from said sulfuric acid.  
 DETAILED DESCRIPTION OF THE INVENTION Mercury is present in the form of both metal and ion in sulfuric acid, and if iodine is added to the sulfuric acid, it is expected that mercuric iodide is precipitated according to the following equations:  
  However, the reaction rate of these equations (1) and (2) in the sulfuric acid is practically so slow that the reactions scarcely proceed.  
  In view of the fact that the mercuric iodide itself is very difficult to dissolve in the sulfuric acid with a high concentration, the applicants have made studies to convert the mercury contained in sulfuric acid to the mercuric iodide with a high efficiency. As a result, they discovered that such object could be achieved by using iodides, including potassium iodide. If the iodides such as potassium iodide are added to the sulfuric acid at room temperature and the mixture is stirred, the mercury contained therein is rapidly precipitated as the mercuric iodide.  
  It is assumed that the reaction is carried out according to the following reaction equations:  
  The equation (3) is followed by the equation (4) in a very short period of time. That is, the potassium iodide in the concentrated sulfuric acid decomposes into iodine and potassium, and the produced iodine in a nascent state combine rapidly with the mercury contained in the concentrated sulfuric acid, whether it is present in a metallic or ionic state. Therefore, it is one important requirement that the iodine is in a chemically active state. In other words, it appears that the higher the concentration of the sulfuric acid, the greater the effect of the addition of the iodides, and that with sulfuric acid having a lower concentration iodides are of little usefulness. From the fact that the addition of iodates gives no effect, it may be confirmed that it depends on the action of active iodine to give the formation of the mercuric iodide. The critical concentration of sulfuric acid for which the iodides to be added is useful may be expected to be in the range of 50 to percent. However, the applicants adopted the sulfuric acid in concentration range of not less than 70 percent as a preferred range in the present invention.  
  Although the reaction between the nascent iodine from the decomposition of the iodides and the mercury is rapidly effected, the iodides are preferably added to the sulfuric acid with stirring taking into consideration With addition of K1 no Sign of curic iodide is very&#39;rapid and is little dependent on the mercury concentration, a large scale reactioninstallation is not required which results in simplicity and convenience from an industrial point of view, the  
 the diffusion of the iodides to be added and the decom- 5 recovery of mercury is possible and the present method posed products thereof into the solution of the sulfuric is applicable to a highly concentrated sulfuric acid acid. However, stirring requires no long period of time. without exerting any effect on the concentration of sul- The reaction is selective, and it proceeds without being f ric acid in the course of the removal of mercury.  
 affected by any other metal impurities present in sulfu- EXAMPLE ric acid. With regard to the removal of these metal im- I purities, many kinds of methods have been hitherto 0.2 g of potassium iodide per 1 kg of sulfuric acid was taken and there is no particular problem. added to each 31 of industrial sulfuric acid having a The formation of the mercuric iodide by the iodides concentration of 98 percent and containing mercury in occurs almost absolutely without depending upon the an amount of5 mg per kg at a temperature of 25C, and  
 mercury content of sulfuric acid. The iodides may be the mixture was stirred and filtered. The present examadded in an amount of at least chemical equivalent, ple was directed to an examination of the effects of stirpreferably 1 to 2 times chemical equivalent required to ring time and the state of potassium iodide on the reconvert mercury to mercuric iodide to provide satisfacmoval of mercury. The results are shown in Table 1.  
 tory results. The amount above several times chemical 4 TABLE 1 equivalent may be, of course, added, but further advantage cannot be obtained with such excessive amounts Reaction condition Hg content Hg content because of the selectwlty of the reacnon Stated Kl state Stirring time before treatment after treatment above. i  
  While the mercuric iodide is stable in the highly con lfi 2 8 mglkg 81;; mg/kg centrated sulfuric acid at a temperature below room 15 minute 5.0 0.92  
  aqueous 7 temperature, it tends to be dissolved m the highly con solution I minute 50 I 0.88 centrated... sulfuric acid at an elevated temperature. Thus, the addition of iodides may be advantageously effected 3 9 temperatlolrel Preferably, at a 3 Table l&#39;shows that the reaction rapidly takes place P r &#34;Q hlghfi than C, t Preferably at a 0 without being affected to an&#39;appreciable extent by the temperature not higher than 15C, still more preferably State f i did d h i i i not higher than 10C. In this respect, consideration should be given to the fact that the dissolution and the I EXAMPLE 2 precipitation process of mercuric iodide in sulfuric acid ()4 g f potassium i did r &#39;1 k of&#39;sulfuric acid was is somewhat reversible relative to heat. added to each 31 of sulfuric acid having a wide concen- -Theliodides used in the present invention may intration range of 10 to 90 percent produced from indusclude any kinds of iodides, including potassium iodide, trial sulfuric acid having a concentration of 98 percent and in practice, they may be advantageously selected and containing a trace of mercury, and the mixture was on the basis of the reasons of convenience, economy 40 stirred for 1 minute and thereafter filtered. The results and the like. are shown in Table 2.  
 .T able 2 Concentration of Hg content of Addition amount (Multiple of Hg content sulfuric acid sulfuric acid of K1 equivalent) after treatment 10% 18.9 mg/kg 0.4 g/kg (12.x) 30 16.5 0.4 (14.7) 13.0 mg/kg 50.&#34; 14.4 0.4 (16.8) 11.7 70 12.5 i 0.4 (19.4) 2.0 70 3.6 0.4 (67.3) 1.20 1 80 4.1 0.4 (57.6) 1.13 90 4.6 0.4 (53.7) 0.68  
 the formation of mercuric iodide was observed, and-thus no analysis was carried The method of the present invention for removal of Table 2 shows that with only sulfuric acid having a mercury&#39;from highly concentrated sulfuric acid by the addition thereto of iodides is characterized in that it is &#39;specific to iodides, i.e., when other halides suchas chlorides or bromides are added to concentrated sulfuric acid-containing mercury, precipitation of mercury compounds does not occur. a  
  The advantages obtained according to the present invention are as follows: the formation-reaction of merconcentration not less than percent the addition of K1 results in remarkable effects.  
  EXAMPLE 3 0.01 to 0.4 g of potassium iodide per&#39; 1 kg of sulfuric acid was added to each. 31 of industrial&#39;sulfuric acid having a concentration of 98 percent and containing 5 mg of mercury per 1 kg sulfuric acid and sulfuric acid containing the mercury content of 30 mg per kg prepared by adding a solution of mercuric sulfate to said industrial sulfuric acid, respectively, at a temperature of 25C, and these mixtures were stirred for 1 minute.  
 and thereafter filtered. The results are shown in Table 3.  
 Table 3 Hg content Addition amount (Multi 1c of Hg content before treatment of K1 equiva ent) after treatment 5 m /k 0.01 /k (1.2) 1.21 m /k 5 g 0.02 g (2.4) 0.92: g 5 0.04 v (4.8) 0.82 5 0.10 (l2) 3 0.79 5 0.20 &#39;(24) 0.71 5 0.40 (48) 0.87 30 0.05 (1.0) I 12.2 30 0.075 (1.5) 2.5 30 0.10 (2.0) 7 0.65  
 ln view of Table 3. it is apparent that when potassium EXAMPLES iodide is added in an. amount of l to 2 equivalent, the Various kinds&#39;of iodides were added to each 11 of inremoval efficiency of mercury sharply increases-and dustrial concentrated sulfuric acid having a concentrapotassium iodide has selectivity to mercury. Further, an &#39;tion of&#39;98.5 percent and containing a trace of mercury analytical experiment on the form of mercury in sulfuat a temperature of 30C. The mixture was then stirred ric acid with the mercury content of 5 mg per kg indifor 1 minute and filtered. The results are shown in cated that the portion corresponding to 2.8 mg per kg Table 5.  
 Table 5 Hg conten&#39;t Addition Hg content lodide before treatment amount (equivalent) after treatment 2;: sta as: 1:22 we Y 0.05 Y (1.01) 2.28 1 30 0.10 (2.02) 1.25 &#39;NH l 6.7 0.10 (10.3) 2.11 Cal .6H=O 6.7 0.10 (7.5) 1.74 H] 6.7 0.10 (11.7) 1.20  
 was in the formof ions and the remaining porfiOn&#39;coF Table 5 shows that all iodides are useful for removing responding to 2.2 mg per kg was in the form of metals. mercury from Sulfunc From the above facts, it will be apparent that the pres- EXAMPLE 6 g fi i g gg p i useful for both lomc mercury and me Each 500 g of two kinds of sulfuric acids with the I mercury content of 30.9 and 95.7 mg per 1 kg of&#39;sulfu- In addmon f by t j t fj f f ric acid were-prepared by adding a solution of mercuric demonsfrziteci ihdta I mercury 10 l es were entlca to 40 sulfate to industrial sulfuric acid having a concentramercunc lodlde v tion of 98 percent. These sulfuric acids were allowed to cool to&#39;a temperature of 5C. 50 m of otassium io- EXAMPLE 4 t t dide was added to one cooled sulfuric acid, while 160 fi l f lodlflle was added each 31 of mdusmal mg of potassium iodide was added to the other sulfuric Sulfuric acid havmg a concenttaflon of 8 percent and acids and they were respectively reacted. After standthe mercury content of 5 mg P i kg at Various tempera ing for an hour, these reaction mixtures were filtered turesover a glass filter pro-coated with diatomaceous earth. Potassium iodide was added in an amount 0102 g P The amo&#39;unt of potassium iodide added corresponded 1 kg of Sulfur c aci equivalent) and the resulting to twice the theoretical amount required for removing mixture was stirred for 1 minute andthereafter filtered. the f i ri miscarried out hil i i e results areihown in Table 4- ing the reaction mixtures at a temperature of 5C.  
  Analysis of the filtered acids by atom absorptiometric Table 4 method indicated&#39;that the contents of mercury were 0.22 and 0.2 mg per 1 kg of sulfuric acid, respectively. Reaction temperature Hg content Hg content EXAMPLE 7 of sulfuric acid before treatment after treatment 500g of sulfuric acid with the mercury content of 2 7? 2:8 8:; mgjkg 95.7 mg per kg was prepared by adding a solution of 25 0&#39;81 v mercuric sulfate to industrial sulfuric acid having a v concentration of 98&#39;percent. Theresultant sulfuric acid 45 5.0 2.73 I a 5&#39;0 .130 was allowed to cool to a temperature of 10C and to this acid&#39;was added mg of potassium iodide corresponding to twice the theoretical amount required for In view of Table 4, it is evident that the formation of removing the mercury to react them. After standing for mercuric iodide depends upon the temperature of. sul-: 65 an hour, the reaction mixture was filtered over a glass furic acid even if some amount of scatter in temperature during filtering, volatalization of some iodine or the amount of scatter in measured values by analytical errors are taken into consideration.  
 filter pre-coated with diatomaceous earth while maintaining it-at &#39;a temperature of 10C.  
  The filtered acid had the mercury content of 0.26 mg per kg.  
 EXAMPLE 8 500 g of sulfuric acid with the mercury content of 30.9 mg per 1 kg sulfuric acid was prepared by adding a solution of mercuric sulfate to industrial sulfuric acid having a concentration of 98 percent. The resultant sulfuric acid was allowed to cool to a temperature of 15C and to this acid was added 50 mg of potassium iodide. The reaction mixture was filtered and analyzed in the same manner as that indicated in Examples 6 and 7. The reaction mixture was maintained at a temperature of 15C until filtering was complete.  
  The mercury content of the filtered acid was found to be 0.45 mg per kg.  
 COMPARATIVE EXAMPLE 1 500 g of sulfuric acid with the mercury content of 6.3 mg per kg was prepared by adding a solution of mercuric sulfate to industrial sulfuric acid having a concentration of 98 percent. To the resultant sulfuric acid which showed a temperature of 22C was added 50 mg of potassium iodide and they were reacted. After standing for an hour the reaction mixture was filtered over a glass filter pre-coated with diatomaceous earth. The amount of the potassium iodide added corresponded to about times the theoretical amount. The filtration after reaction was carried out withoutmaintaining the reaction mixture at constant temperature. so that the filtered acid showed a temperature of 26C.  
  The mercury content of the filtered acid was analyzed to be 0.70 mg per kg.  
 COMPARATlVE EXAMPLE 2 500 g of sulfuric acid with the mercury content of 30.9 mg per kg was prepared by adding a solution of mercuric iodide to industrial sulfuric acid having a concentration of 98 percent. The resultant sulfuric acid was allowed to cool to a temperature ofC and to the cooled acid was added 50 mg of potassium iodide to react them. After reaction, the reaction mixture was.  
 not specially maintained at constant temperature, so that the filtered acid showed a temperature of 29C after filtering was complete.  
  The mercury content of the filtered acid was analyzed to be 0.77 mg per kg. 4  
  The above-mentioned examples and comparative examples clearly indicate that the temperature of the reaction mixture is desirably maintained at temperatures not higher than 15C. preferably not higher than 10C throughout the reaction step to the filtration step, and that the maximum temperature of the reaction mixture should be desirably limited to 15C even if it tends to increase in the course of the process.  
 EXAMPLE 9 500 g of industrial sulfuric acid having a concentration of 98 percent and containing 5.0 mg, of mercury per 1 kg sulfuric acid was allowed to cool to a temperature of 10C and to this sulfuric acid was added 0.1 ml of reagent grade hydroiodic acid having a specific gravity of 1.7 and a hydrogen iodide concentration of 57 percent and they were reacted. After standing for an.  
 hour while maintaining the reaction mixture at a&#39;tem the reaction mixture was also maintained at a temperature of 10C. After removing the residual iodine the filtered acid was analyzed by atom absorptiometric method and the mercury content thereof was found to be 0.21 mg per kg.  
 EXAMPLE 10 500 g of industrial sulfuric acid having a concentration of 98 percent and containing 5.0 mg of mercury per 1 kg of sulfuric acid was allowed to cool to a temperature. of 10C and-to this cooled sulfuric acid was added 25 mg of reagent grade sodium iodide and they were reacted. The amount of the sodium iodide added corresponded to about 3 times the theoretical amount required for removing the mercury. After standing for an hour the reaction mixture was filtered in the same manner as that in the preceding examples 6 and 7. During filtering the reaction mixture was kept at a temperature of 10C. After removing the residual iodine the filtered acid was analyzed by atom absorptiometric method and the mercury content thereof was found to be 0.25 mg per kg.  
 What we claim is:  
  l. A method of removing mercury from a highly concentrated sulfuric acid having a concentration of not less than percent and containing a trace of mercury which comprises adding iodides to said sulfuric acid at a temperature up to 45C to precipitate the mercury contained therein as mercuric iodide, and removing the precipitate from said sulfuric acid.  
  2. A method according to claim 1 wherein the amount ofthe iodides added equals to at least chemical equivalent required to convert the mercury to mercuric iodide.  
  3. The method according to claim 2 wherein the amount of the iodides added equals to l to 2 times chemical equivalent required to convert the mercury to the mercuric iodide.  
  4. The method according to claim 1 wherein said sulfuric acid is maintained at a temperature of not higher than 30C.  
  5. The method according to claim 1 wherein said sulfuric acid is maintained at a temperature of not higher than 15C.  
  6. The method according to claim 1 wherein said sulfuric acid is maintained at a temperature of not higher than 10C.  
  7. The methodaccording to claim 1 wherein said removing of the precipitate is carried out by filtering.  
  8.The method according to claim 1 wherein said iodide is potassium iodide.  
  9. The method according to claim 1 wherein said iodide is hydrogen iodide.  
  10. The method according to claim&#39;l wherein said iodide issodium iodide.  
 11. The method according to claim 1 wherein said iodide is selected from the group consisting of ammo-&#39; nium iodide and calcium iodide.  
  12. The method according to claim 1 wherein the mercury contained in the sulfuric acid is in the form of ions.  
  13. The method according to claim 1 wherein the mercury contained in the sulfuric acid is in the form of metal.  
  14. The method according to claim 1 wherein the mercury contained in the sulfuric acid is a mixture of ionic and metallic mercury..  
  15.&#39;The method according to&#39;claim 1, wherein said sulfuric acid has a concentration of higher than percent.  
  16. The method as claimed in claim 1, wherein said sulfuric acid has a concentration of not less than