FR2496083A1 - PROCESS FOR REMOVING MERCURY FROM INDUSTRIAL WASTEWATER - Google Patents

Abstract

THIS PROCESS FOR REMOVING MERCURY FROM INDUSTRIAL WASTEWATER CONTAINING CHEMICALLY BOUND MERCURY, WHICH USES THE KNOWN REACTION OF PRECIPITATION, IN THE FORM OF SULFIDE, OF MERCURY WHICH IS PRESENT, BY TREATMENT, IN THE PRESENCE OF FLOCCULANT, WITH SODIUM SODIUMULFIDE , IS CHARACTERIZED BY THE FACT THAT IT IS CARRIED OUT IN ALKALINE CONDITIONS, WITH A PH RANGING FROM 7 TO 12, TREATING THE ALKALINE SLUDGE FORMED IN THIS WAY, AFTER DECANTATION, WITH A PART RANGING FROM 10 TO 20 OF THE TOTAL, OF THE SAME WATER RESIDUE WHICH MUST BE FREE OF MERCURY, THE SAID PART BEING THEN RECYCLED, AFTER FILTRATION, AT THE PRECIPITATION STEP BY SODIUM SULPHIDE, ALL THE COMPONENTS PRESENT AS WELL IN ORGANIC FORM AS IN MINERAL FORM AND AS COLOUS, THUS A COLOUS FORM CRYSTALLIZED PRECIPITE OF MERCURY SULFIDE BEING THUS SHAPED, WHICH CAN BE EASILY COLLECTED ON A FILTER.

Description

"Process for the removal of mercury from industrial wastewater." The

  The present invention relates to a process for removing mercury from industrial effluents containing chemically bonded mercury, which process utilizes the known reaction of precipitating mercury present as a sulfide by treating, in the presence of a flocculant, with sodium sulfide. The process is characterized by the fact that alkaline conditions with a pH of between 7 and 12 are used, and that the alkaline sludges which are formed, after settling, with some of the same water are treated. residuals that are still subject to mercury removal, this part being

  then recycled, after filtration, to the precipitation stage

sodium sulphide.

  We know how important the problem of mercury pollution is, especially after having found very serious cases of poisoning, some of which were even fatal, and after having found the presence of large quantities of mercury in the flesh of the fish. caught in areas that were heavily polluted by

  waste waters containing mercury.

  The overwhelming impact of the problem has led governments to

  the most important industrial countries to establish extremely rigid standards for reducing mercury levels, both in the metal state and in the form of its compounds, in industrial effluents, especially

  those in liquid form.

  In Italy, the maximum limit allowed is 5 micro-

grams per liter.

  One of the most important sources of water pollution by mercury is estimated to be the process for the electrolytic manufacture of chlorine, which uses mercury cells, followed by catalytic processes that use mercury salts as catalysts for

  organic syntheses, such as, for example, the manufacturer

  cation of acetaldehyde and vinyl chloride from

acetylene.

  The industries involved have made intensive efforts to solve the problem and several processes have been suggested to reduce mercury levels in industrial wastewater to the values established by the new laws. Thus, for metallic mercury, special filtration steps have been used, for example with

  activated carbon or the formation of metallic amalgam

  whereas, for combined mercury, precipitation, adsorption on ion exchange resins, reduction to the elemental mercury

  other chemical and electrochemical processes.

  Among the precipitation processes, the process which

  is mainly used is the one that leads to

  mercuric sulphide. This process is generally carried out by treating polluted water with compounds of the

  mercury, with solutions of sodium sulphide or hy-

  sodium sulphite at a pH of about 8,

  cull the; colloidal precipitate thus formed with ferric chloride, for example, or with any other suitable flocculant, then filtering, after decantation, the sludge that has formed. A method of the type described above, however, has some disadvantages. In fact, in order to obtain a quantitative precipitation of mercury as complete as possible, it is necessary to use a large excess of sodium sulphide; sodium sulphide is prone to hydrolysis giving a release of hydrogen sulphide which

  is highly toxic by itself.

  In addition, the use of ferric chloride as

  culant causes the problem of the formation of large

  iron hydroxide masses, an extremely large precipitate

  which is collected on the filters with a lot of

  difficulty and which, in addition to

  problems, for example the consumption of iron salts, poses the additional and more serious problem of treating large masses of sludge with a low mercury content and, simultaneously, an extremely high percentage

  humidity (up to 60 X or even 70 X).

  Even the use of non-ionic flocculants, although it does not pose the problem of iron consumption, does not solve that of sludge masses, nor the difficulties that arise from collecting them on

  filters, for filtration and subsequent treatments.

  Nowadays, the filtration of colloidal precipitates is done with rotary filters equipped with a dicalite coating which is at the same time a support element and a filtration aid, from which the precipitate is removed by

  scraping with a scraper blade. The coating of dicali-

  you avoid damages on the filter screen during

  that the precipitate is withdrawn, Mis it always happens a tear-off

  of dicalite, so that it occurs in the filament

  continuous consumption of dicalite.

  The authors of the present invention have now discovered that the disadvantages enumerated above can be removed to a certain extent if the method according to the invention is followed, which will be described in more detail later and which comprises the step of submitting,

  after decantation, the alkaline sludge originating from the

  pity with sodium sulphide, to the action of a small

  fraction (from 10% to 20% of the total) of the water to be treated.

  These authors discovered that this treatment

  dissolve the amorphous substances that flocculate and leave, as a residue, the mercuric sulphide precipitate which is

  can easily collect on a conventional filter press

  or, in any case, without a filter aid

  such as dicalite and with residual moisture

minimum.

  The mercuric sulphide thus obtained is, moreover,

  tically free of coarse impurities, so that it can be directly used in the calcination step

  for obtaining elemental mercury.

  In order to better explain the process, it will be described hereinafter the precipitation tests of the mercury compounds contained in the waste water from the catalytic synthesis units of vinyl chloride from acetylene. The tests reported here have been

  at the beginning, in a laboratory apparatus, and, subsequently,

  later, in units on an industrial scale.

  The single figure of the attached drawing is an overall diagram of the process in question.

  In the drawing, 1 is the flow of the hydro-

  Calcium oxide, 2 is the aqueous solution of flocculant, 3 is the flow of water which is discharged after removal of mercury, 4 is the aqueous solution of sodium sulphide,

  5 is the compensator, i.e. the laboratory container

  equipped with an agitator, and in which a pH greater than 7-9 is to be maintained, - 6 is the sedimentation tank, 7 the silica filters, 8 the activated carbon filters through which the water is supplied. still contains traces of mercuric sulphide to flow, 9 is the flow tube of mercury-free water, 10 is

  the flow of alkaline sludge, 11 is the dissolving vessel

  where a fraction of the stream 3 of the water to be free of mercury arrives, 14 is a filter press or

  any other direct filtration system, 12 is the flow re-

  cycled water that must be cleared of mercury, and

  13 are acidic and wet sludge obtained by filtration.

EXAMPLE

  The waste water from the synthesis plant

  vinyl chloride is acidic, at a pH of about 1.8, and contains mercury compounds in a quantity

  is close to 10 milligrams per liter. In order to be able to

  to comply with the legal standards, the mercury content must be reduced to a level which exceeds 5 micrograms per

liter.

  In laboratory tests, this water is treated by stirring with lime milk (an aqueous slurry

  lime) to make it alkaline until it has

  ringworm a pH of about 10; simultaneously, 5 ml per liter of an aqueous solution of a nonionic flocculant is added

  polyacrylamide ("Prodefloc N2M"), a trade mark

  from the PRODECO Company) at a concentration of 1 gram

per liter.

È496083

  Then, an aqueous solution of sodium sulphide, at a concentration of 5 grams per liter, is added in one hour.

  amount which is equal to three times the stoichiometric quantity

  of mercury, while continuing to agitate for about an hour. Then, we stop the agitation to allow

to the precipitate of being deposited.

  After letting stand for one hour, the clear supernatant water is withdrawn and sent to the filtration stage.

  on activated charcoal, where the sulphur

residual mercury.

  Mercuric sulphide, which was present in the sedimentation waters in a slightly smaller amount than

  micrograms per liter, is thus reduced, after filtering

  at values ranging from 2 to 5 micrograms per liter.

  Meanwhile, the alkaline mud from the reservoir

  sedimentation is transferred to a dissolving vessel

  The solution is added as such (ie at a pH of about 1.8) to a portion of the waste water which is to be freed from mercury until it reaches a pH value. between 2.2 and 2.5. At this stage, about 80% of the sludge is dissolved, while it remains, undissolved, in the form of a heavy precipitate, only

  mercury sulphides and metal sulphides of the first

  first and second group that may be present in the water. The residual precipitate is suitably collected on a vacuum filter, on a filter cloth, without any

  Dicalite filtration aid is required.

  The treatment with sulfur sulphide is recycled

  dium, the aqueous filtrate which still contains compounds

mercury.

  The precipitate, collected on the filter, which is

  0.1 gram per liter of treated water, contains about

  % mercury, expressed as elemental metal.

  The tests are carried out on an industrial scale in

  following the same sequence of steps as that adopted in the

  tory. We only use metal devices instead

  glass equipment of larger size.

  The dimensions of the industrial equipment were compensator 5 (container in which the liquor is made

  alkaline) approximately 300 cubic meters, container 6 of flocculated

  About 120 cubic meters, activated carbon filter 8 3 cubic meters, sludge dissolving tank

alkaline: 3 cubic meters.

  Also in the industrial process, the lime milk is neutralized by a metering device up to

  a pH of 8 to 9 is reached.

  made alkaline in the flocculated container.

  tion to which sodium sulphide is also introduced (in one

  amount equal to 3 times the stoichiometric quantity required

  to precipitate mercuric sulphide, so as to

  to precipitate also the other metals possibly

  sents), and to which the non-flocculant

  ionic. It is established that the flocculant consumption in the industrial test is half that found in the

laboratory process.

  The possible residual excess of sodium sulphide is

  adsorbed by the activated carbon filter.

  After sedimentation, the flocculated sludge is

  sent to the dissolving vessel while the water

  clear is sent to the activated carbon filter.

  Water leaving the flocculation and sediment container

  has a mercury content between 50 and 200

  micrograms per liter. After adsorption on the activated charcoal

  tif, the average mercury concentration in the outgoing water is thus reduced to values between 2 and 5

micrograms per liter.

  To prevent supernatant flakes from clogging the activated carbon filter, it is connected upstream to

a silica filter.

  The sludge fed to the dissolving vessel is treated with the water to be freed of mercury. The ratio of acidic alkaline water to alkaline is a function of the pH of water and the concentration of

  solids dissolved in the sludge, and it is

  be a minimum of 10 and a maximum of 20.

  After each dissolution step, let it settle

  acidified sludge to be able to draw water by siphon-

  ment. The water was recycled to the compensating container. The acidic sludge from the dissolution vessel is sent to the filtration stage in a filter press, a compact cake being thus obtained, which can be easily removed.

filter cloth.

  The mercury removal tests are also done to compare; on the same waste water but without

  treat alkaline sludge with the water that has to be

full of mercury.

  The most remarkable difference is the need to

  empty the sedimentation and filtration tank every day

  alkaline sludge when they form a mass

  excessive, while in the case of tests involving

  the dissolution stage, it is sufficient, although not abso-

  it is obligatory to carry out filtration once

per week.

  The regeneration of the active charcoal filter is carried out after more than 45 days of operation by washing it with a 3% solution of hydrochloric acid.

by a corrosion inhibitor.

  In the following tables, the following data are collected:

  related to a number of tests according to the method described above, and, by way of comparison, the tests made without

  sludge: the quantities of mercury contained in the water

  ter and the water coming out of the active charcoal filter are

  in both examples, as well as the quantities of sludge at the outlet in both cases, the

  mercury and other data.

TABLE 1

  Mercury levels in water entering and leaving the facility

elimination of mercury.

  Test Mercury levels in water in micrograms

N liter.

Entering the ins - ortant of the -

processing of the filter

  "Container of f-oanduflo mentement culation be charcoal ctif

1 5.900 65 4.0

2 8,400 80 4.6

X (3 6,500 40 3.6

X X4 10.600 35 2.2

4.000 83 6.0

6 1,600 20 3.0

O> 7 2.100 39 3.6

8 1,100 78,1,1

o 9 3,400 60 3,7

3,600 37 3,2

X 11 3.400 83 2.9

12 8,000 95 4.0

Sludge properties obtained

TABLE 2

  by precipitation of mercury from waste water.

  Composition of sludge after filtration (% by weight) Moisture Analysis on wet samples Mercury 8, 90 9.5 8,85, 5 3,33 1,75 2,5 3,2 Iron 4,56 3,85 4,2 3.6 3.26 2.85 3.3 2.80 (% by weight) Calcium, 28 4.85, 3, 0 6.24, 8 4.2, 0 Test No Quantity (kg / m 0.05 0.06 0.048

0, 051

  0.38 0.311 0.4 0.35 p aH sIl o on t, t, (13 M ri M 13 (o) 14 () () 16 (O ko tu% $ Ob c o w TABLE 2 (continued)

  Properties of sludges obtained by precipitation of mercury from waste water.

  Test Quantity Composition of sludge after filtration (% by weight) fln Moisture Analysis on wet samples (% by weight) Mercury Iron Calcium

0.056 52 9.2 4.25 5.35

6 0.04 48 10.1 4.00 4.95

7 0.048 50 8.9 3.25 5.00

8 0.060 53 9.9 3.80 5.20

9 0.050 50 8.95 3.90 5.40

0.042 52 8.4 4.15 4.8

11 0.055 51 9.2 3.95 5.3

12 0.051 49 10.3 4.30 4.9

1 7 () 0311 71 2.2 2.9 5.5

18 (0) 0.330 70 1.8 3.2 4.9

19 () 0.295 68 2.3 2.85 5.8

() 0.300 65 3.2 2.7 6.2

21 () 0.350 69 3.5 3.5 4.5

22 (0) 0.310 64 2.9 3.3 5.9

23 () 0.298 60 3.35 2.95 5.0

24 (0) 0.308 65 2.85 3.00 4.8

  () The tests thus marked relate to the precipitation of mercury in the form of sulphide without subsequent treatment of the sludge with a fraction of the water to be removed.

mercury.

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Claims (1)

CLAIM   A process for removing mercury from waste waters containing chemically bound mercury which utilizes the known reaction of sulphide precipitation of mercury which is present by treating, in the presence of a flocculant, with sodium sulphide, this process   characterized by the fact that one puts oneself in con-   alkaline sludge, with a pH of between 7 and 12, and that the alkaline sludge which is formed, after sedimentation, with a part between 10 and 20% of the total, of the same residual water which has yet to be removed, is treated. mercury, this part being then recycled, after filtration, to the sulphide precipitation step sodium.