DK172048B1 - Process for separating heavy metals from wastewater - Google Patents

Abstract

The invention relates to a process for separating heavy metals from waste water and to precipitant for carrying out this process and to process for manufacturing the precipitant. It known to use the alkaline hydroxide precipitation and the sulphide precipitation to separate heavy metals from waste water. However, these known processes are associated with serious disadvantages which, in the final analysis, have prevented their use in practice. The invention proposes, in a process according to the generic part, to combine the intrinsic advantages of sulphide precipitation in an acid medium, namely the extremely low residual solubilities of the heavy metals, with the considerable reduction in the risks of possible overdosage, i.e. safety from formation of hydrogen sulphide. The precipitant used is designed in such a way that the sulphide and other precipitates in the treated waste water automatically flocculate out at a pH of about 7.5, the sludge becoming separated from the clear water without addition of flocculating agents. The agent for separating heavy metals advantageously consists of alkaline earth metal polysulphides or mixtures of alkaline earth metal polysulphides with other alkaline earth metal sulphur compounds.

Description

DK 172048 B1

The invention relates to a process for the separation of heavy metals from wastewater by sulfide precipitation by means of calcium polysulfide.

Heavy metals are used in many areas of modern day business. For corrosion protection and beautification, metals and plastics are coated with zinc, chromium, nickel, copper, cadmium, tin and silver.

For storage of electricity, lead, nickel and cadmium are built into larger quantities in batteries and accumulators.

10 Vanadium accumulates in soot and slag after burning of mineral oil and in process water from paper mills. Much mercury is consumed in the form of amalgam in dental practices, but also for the manufacture of thermometers and other measuring instruments, to name but a few 15 examples.

After fulfilling their duties, the heavy metals end up in the waste and in the wastewater. Unlike organic toxins, heavy metals that are all toxic cannot be destroyed by biodegradation or combustion, ie. turn into harmless substances. They can only be made insoluble in water and removed as sludge from the water by the addition of suitable substances, in the chemical language termed precipitants. At higher concentrations, heavy metals 25 occur predominantly in acidic water as these are acid soluble.

'In the neutralization, i.e. upon addition of sodium hydroxide, potassium hydroxide, ammonium chloride and other to the water, they precipitate almost completely as hydroxides; i.e. they are complexed or form alkaline salts such as chromates, zincates, stannates and others.

However, all heavy metals can form complexes under certain conditions, such as e.g. with phosphates and so-called complexing agents such as EDTA and NTA derived from detergents such as e.g. with ammonium chloride and its salts, which in the water e.g. originates from excrement from 2 DK 172048 B1 animals and humans. Also the plant world produces complexing substances such as e.g. citric acid and other fruit acids and their salts, the leaf-green chlorophyll or the siderophores of bacteria.

5 However, the alkaline hydroxide precipitate has the following weighty disadvantages.

Complex bound heavy metals do not precipitate. In case of excess precipitates, some heavy metals can dissolve again. Basic precipitated sludge brought to 10 depots may end up in the water again due to acid rain or complex formation of any kind. After the precipitation, small amounts remain dissolved in the water due to their solubility. This residual solubility, often expressed in chemistry as their solubility product, forms the natural boundary for any precipitation process. In many cases, the solubility product of the heavy metal hydroxides exceeds e.g. by cadmium, the statutory residual values. Moreover, the solubility products should be considered as theoretical values since they are determined under ideal conditions. The wastewater to be treated in practice is rarely in ideal condition. For the most part, it is loaded with soluble compounds such as chlorides, nitrates and other soluble salts of sodium, potassium or ammonium. The ballast salts 25 elevated are usually even more soluble residual solubility. In addition, freshly precipitated, hydroxidic precipitates form easily hardened colloidal solutions by ballast ions. Thus, the residual metal content of the treated water is increased to a further extent, which is not taken into account in the solubility product. Cheap precipitates, which also cause the colloidal solution, are in practice used in excess to improve the insufficient alkaline precipitation. This is further favored by government regulations that allow residual alkalis of up to pH 10 in wastewater, obviously for the purpose of acid rain. Legislative limits for heavy metal residues in wastewater, which have been tightened from year to year, and have also been more effectively controlled, can no longer be met with alkaline-hydroxidic precipitation.

Further, it is known that the sulfide precipitate, i.e. the separation of heavy metals with hydrogen sulphide or soluble sulphides such as sodium sulphide, ammonium sulphide and others is substantially more effective in acidic media than the alkaline-hydroxidic.

The difference between the two precipitation methods is shown in Table 1.

Table 1

Solubility products for some heavy metals (KL) t. 2 2 * 1 Mercury.

mol / 1 j Cadmium Lead Tobber 'Nickel Zinc i, Silver Chromium III

Hydroxit? ίο'14 ΐδ'5'6 id20 lo''4 10'- lo'30 20 Sulfide 10 "27 10" 2810 "27 10-24 10'2410" 52

The difference in solubility products is between 10 ”7 and 10-14. Calculated as an estimate, the residual metal content of hydroxides is 10,000 to 10 million times higher than that of sulfides (in mol / l). In addition, there are the following additional benefits:

Heavy metals can also be displaced from their complexes and precipitated by sulfide. They cannot re-dissolve in acidic water since they are precipitated from acidic water.

Despite its potential advantages and opportunities, the sulphide precipitation has so far failed to fit into the technique used. The following stated, likewise 35 weighty disadvantages of the method are responsible for this.

Hydrogen sulphide is gaseous and is p. toxicity and explosion hazard not practically manageable under given 4 GB 172048 B1 conditions. Soluble sulphides such as sodium, potassium or ammonium sulphide release very quickly, if not with one, hydrogen sulphide when dosed to acidic wastewater. Thus, there is a danger of a toxic gas cloud if 5 doses are administered too quickly or if overdosed. The safety distance is too small, errors cannot be allowed. Only by a large supply of equipment and / or chemically protected personnel can this danger be reduced. A requirement that cannot be fulfilled when one considers how many different places in the industry metal-containing wastewater is to be treated.

To avoid the disadvantages of the sulphide precipitation, it is known to use organic sulfur compounds such as, for example, xanthogenates and trimethyl triazine, but with the disadvantage that a relatively large organic mass must be used, which leads to an increased supply of apparatus (larger filter surfaces, sedimentation basin). ). With trimethyltriazine, heavy metals cannot be precipitated with three charges, e.g. chromium III.

It is precipitated in alkaline medium, so that the danger of the anionic redissolution (eg, zinc, chromium with sodium hydrogencarbonate) and the complexation (eg with ammonia) are not removed or merely removed.

All of these known methods, including the hydroxide precipitation and treatment with montmorillonite and other clay minerals, have the disadvantage that they may or may not precipitate complex-bound heavy metals, especially those with large stability constants such as e.g. EDTA complexes that occur 30 very often.

The same applies to anionically bound metals that are either present as such in wastewater or can be redissolved by alkaline treatment. They are either ampholytes such as zincates and stannates (lead), or they can be oxidized to higher oxidation levels such as chromate and water, thereby becoming alkaline soluble. The latter may even be present in acidic medium, ie. they are reduced.

5 DK 172048 B1

A particularly environmentally relevant case is the chromatographic poisoning in wastewater from the automotive industry. It is carried out in two essential steps: reduction with sodium sulfite and hydroxide precipitation, in sulfide precipitation in acidic medium only one step and a precipitant are used, which is an advantage when chromate is present in the wastewater alone. However, wastewater from different business sections is brought together to a greater extent to be treated in a common final step. From mutual neutralization, dilution and precipitation of water of different origins, a total wastewater is obtained containing several heavy metals in the form of cations, anions or complexes. Despite dilutions, the residual metal content can be ten to a thousand times higher than the statutory values.

As stated above, sulfide precipitating agent and heavy metal separation process from wastewater can no longer be dispensed with at the current stage of environmental protection they are needed in business, 20 but only with a system-integrated safety barrier that reduces the release of the hazardous sulfur hydrogen into the atmosphere. tolerable size. Soluble sulfides added to acidic media are converted to sulfur hydrogen in a few seconds if they are not precipitated by heavy metals present.

Hydrogen sulphide is comparable in toxicity and explosion hazard to cyano hydrogen. Unlike the latter, however, this reveals itself early on by its intense smell. Table 2 clarifies the dangers and the tolerable magnitude. The values are in ppm. stated in millions of parts sulfur hydrogen in the air.

DK 172048 B1 6

Table 2

Threshold of the human nose 0.1 to 0.2

Very sharp, almost unbearable air 3 to 5 Threshold (MAK) 10 5 Serious eye damage (gas eye) 50 to 100

Odor paralysis 150 to 250

Pulmonary edema, immediate life-threatening 300 to 500 Immediate collapse with breath- 1000 to 2000 death paralysis 10

As can be readily seen, there are no insidious effects. The safety distance between registration and serious injury is 50 / 0.2 * 250 (times), which makes it possible to assume that everyone will flee 15 beforehand. Sulfur hydrogen odor is unpleasant but not dangerous.

It is thus important to prevent the hydrogen sulphide from being released suddenly and in larger quantities, so to speak, and that between the registration and the 20 dangerous dose there is no longer time to escape.

Thus, known sulphide methods also attempt to exclude this danger, though with little success and mostly at relatively high costs.

DE-OS 32 45 314 discloses a method according to which a soluble sulfide product is used in deficit - measured by the stoichiometric amount required for the precipitation of all heavy metals - in order to guarantee the secretion of all sulphide ions and also to bind the odor. completely. For controlling and controlling process 30, a sulphide and hydrogen sulfide measuring instrument is proposed below or above the water level. Also in this method, the unpleasant air of hydrogen sulphide arises, since the sulphide precipitating agent is added below the water level. This known method requires an accurate knowledge of the heavy metal contents at any given time and thus very high analytical costs. However, in the direction of a safe prevention of a hydrogen sulfide accident, we are now a step further. You can also overdose with 2/3 of the amount of sulphide if the inlet water, for example. does not contain heavy metals p.g.a. an operating failure. This literature reference requires a number of characteristics of the acidic to slightly alkaline sulfide precipitate, such as e.g. the group precipitation in various pH ranges or the use of cheap, trivial precipitants such as sodium sulfide, ammonium sulfide, etc.

DE-OS 32 42 819 discloses a process in which the complexed heavy metal is first displaced from the complex by the addition of a non-toxic metal, e.g. iron, and then sulphidically precipitated. The precipitation takes place in an alkaline medium, in which poorly filterable sulphide precipitates may be formed. For these reasons, filtration aids such as activated charcoal and clay minerals are added, which should also serve as sulfide deposits. Soluble sulfides and polysulfides are mentioned as precipitants. However, polysulfides are very unstable in alkaline medium and can be rapidly dissolved by the addition of hydrogen sulfide.

DE-PS 34 33 618 provides an insight into the approximate supply required to carry out the sulphide precipitation controlled in a continuous process. The wastewater is first neutralized (hydroxide precipitation or partial precipitation) and then controlled soluble sulfide is added. Since no pH is made, it is not possible to determine with certainty whether the danger of a hydrogen sulfide leak is no longer relevant.

Aratani et al .; Bull. chem. Soc. Japan, 51 30 (1978), 2705-2709, and 52 (1979), 218-222 and Yano et al., Dechema Monograph 80 (1976), 179-180 disclose methods for purifying heavy metal contaminated industrial wastewater by precipitation with calciumtetrasulfid.

Finally, US-A-4 462 913 discloses a process 35 for removing heavy metals from wastewater by precipitating them in weakly basic medium with a calcium sulphide mixture, whereby, first, in slightly acidic medium, a precipitate of a portion of the metals as hydroxides, and US-A-4 329 224 discloses a similar method by which heavy metals and emulsified oil residues are removed from wastewater.

5 It has now surprisingly been found that the advantages of sulphide precipitation in acidic medium, namely an extremely low residual solubility for heavy metals, can be combined with a substantial reduced risk of the formation or even eruption of hydrogen sulphide when a precipitant is used. Specifically prepared calcium polysulfide. One of these is found to be acid-stable and yet able to deliver sulfide ions to the heavy metal precipitate.

Accordingly, the process of the invention is characterized in that a) as a calcium polysulfide precipitant is used a complex mixture of calcium polysulfides with low proportions of calcium sulfide, calcium dihydrosulfide, calcium thiosulfate and calcium hydroxide and with an atomic ratio S: Ca of ( 0.8 to 1.5): 1 prepared by reacting elemental sulfur with calcium hydroxide and / or calcium sulfide at a temperature of 60 to 80 ° C with stirring in an aqueous suspension having a solids content of 30 to 25 60% by weight, b) for carrying out the sulphide precipitation, if necessary, the heavy metal-containing wastewater acidifies to a pH of 0 to 6 and then in a to 30 precipitation of the heavy metals as heavy metal sulphides, the sulphide precipitant adds sufficient amount, and then the pH is adjusted with lime milk. the value to 7.0 to 8.0, and c) let the flocculating heavy metal sulfide settle for 15 to 60 minutes to form a 35 thin sludge which, after decanting the supernatant liquid and after filtering through quantitative filter paper or using a chamber filter press, thickens to a heavy metal sulphide-containing foam and is removed.

9 DK 172048 B1

The mixture used as a precipitant is slightly soluble in water, ca. 3-4% in terms of dry matter. The system-bound safety against the sudden release of hydrogen sulphide in acidic medium at once is given as follows: Due to the low solubility, precipitant can never be added in high concentration to acidic wastewater. Even when a larger amount of solution is mixed with a fairly strong acidic water, eg down to pH 0, 10 p.g.a. a failure, only small amounts of hydrogen sulphide can enter the atmosphere, since the polysulfide ions are acid stable.

The precipitation begins with the adjustment of the pH value of the wastewater, which is the result of the optimum precipitation conditions of the present heavy metal ion (known from analytical chemistry). For example, the pH of mercury, copper, lead can be adjusted to pH = 0-3 (sulfur hydrogen group including the chloride group) and for cadmium and zinc to pH = 5-6 20 (ammonium sulfide group).

If the wastewater is well known in terms of its composition and additional properties, then these can be taken into account from the beginning. For example, when if an acidic, heavy metal-containing water occurs in one or other place in the establishment, the precipitant of the invention may already be dosed at that site and the water may be pumped to the common neutralization site where several types of wastewater run together. This process can be particularly advantageous for the enterprise's heavy metal detoxification since the once sulfide precipitated metals cannot be practically redissolved by complexing agents, which may flow to other enterprise parts.

Thus, the setting of the optimal pH depends on the following factors; of the heavy metal species or groups, of their amount (concentration) or volume distribution, of their oxidation step (e.g., about three or six charges of chromium foams), of the presence and nature of complexing agents (e.g. large amounts of ammonium salts are present such as at power plants in the removal of unwanted nitrogen compounds 5 and desulphurisation), as well as such as phosphates, citrates and others, as well as other chemical, operational and economic conditions.

In a continuous process, the construction, i.e. the apparatus for measuring and controlling technical equipment and the optimal process management depend specifically on the above-mentioned conditions.

For pre-calculation of such plants, for small amounts of wastewater (0-10 m3) and partial precipitation, for only little known wastewater with small heavy metal concentrations, for a whole series of thin wastewater types without significant and relevant characteristics are precipitated as follows: The value is adjusted to 4-5, preferably to 4.5, advantageously with hydrochloric acid. However, other mineral acids may also be used. For uniform stirring of acid, a suitable agitator, provisional or also compressed air lancer is used, or the liquid is also stirred intensely (circulated).

In a storage container, the precipitant 25 used in the invention is diluted in suspension 1:10 with water and stirred. After a short stirring time, a homogeneous yellow solution with a small proportion of insoluble matter appears, which solution is added to the waste water at once. Next, the basin content is stirred for so long that a good mixture of the precipitate with the wastewater is obtained, usually 10-20 minutes. This is followed by neutralization, preferably with lime milk. A pH value of 7-8 is preferred, preferably 7.5.

For example, white hydrate lime is soaked in a quantity of 5-10 kg in 100 L of water in a storage vessel and pumped to the wastewater. Also a lime milk of other origin and other alkaline slurries can be used for the neutralization. After adding the required amount of neutralizing agent, stir evenly moderately for approx. 30 to 60 minutes. At a pH of 7-8, preferably 7.5 flocculate the precipitate and the 5 precipitate. The supernatural water is clear.

For control, take 5-10 ml of the clear aqueous phase in test tubes or the like and drop 1-2 drops of methyl orange indicator solution. Then 3.5% hydrochloric acid is added until the color has changed from yellow 10 to red (pH 4-4.5). Next, 1-2 drops of said precipitant are added. If heavy metals are still dissolved in the clear water, a clear cloudiness will immediately appear.

The required amount of chemical is determined in a similar manner. 100 ml of the wastewater to be treated is filled into a beaker and 3-5 drops of methyl orange are added. Then make the solution acidic with 3.5% hydrochloric acid until the color change (10-30 drops of hydrochloric acid depending on the pH of the wastewater). Thereafter, the titration of the water is effected with the precipitating agent, i.e. the water is dropped into precipitant from a divided pipette and stirred until pH 6 is reached (measurement with Pocket pH meter or indicator paper). Then, neutralize with 0.05-0.1 g of hydrate lime (1 pinch-25 full).

Also in this case, stop at pH 7-8, preferably 7.5, should be stopped. This is easily recognizable by the sudden flocculation and clarification of the previously constantly flooded water.

30 The need is calculated as follows:

Need = spent precipitant in ml x 35/250 - precipitant according to the invention in liters per liter. m3 wastewater.

Example: consumed 0.7 ml of the precipitant according to the invention 35 12 DK 172048 B1 0.7 x 35/250 = 0.1 l / m3

Acid requirement * Consumption drop rate 3.5% acid / 100 «liter 35% hydrochloric acid per day. m3 wastewater 5 or in numbers: 20 drops of 3.5% hydrochloric acid consumed, 20/100 = 0.2 1 35% 1s hydrochloric acid per day. m3 wastewater

Just as easily, the need for hydrate lime is calculated: 10 Need = need for precipitant according to the invention in 1 / m3 x 1.5 or in numbers: 0.1 x 1.5 = 0.15 kg hydrate lime per day. m3 wastewater.

The above general example shows how, by using the method according to the invention, using the stated precipitating agent, even in the simplest case, it is possible to effectively separate heavy metals even under the statutory requirements. For a large number of traders and other small to medium-sized game-20 dev water producers, the year by year stricter statutory injunctions are otherwise not fulfilled.

Example I

In a waste incineration plant, in which large quantities of special waste (eg clinic waste), but also wastewater sludge are incinerated, the boiler, absorber etc. had to be cleaned on the flue gas side. Thick earthy crusts of the boiler walls, heat exchanger pipes, etc. were washed with water. Crusts were dissolved predominantly in the water by adjusting a pH of 3-5.

There were approx. 80 m3 of acidic wastewater with alternating composition and pH values, depending on whether it was wash or rinse water, container water or absorber. The water was collected in 7 35 makeshift basins of approx. 15 m3 per basin.

The following representative analyzes were made, however, which were representative only of the species and the distribution of the constituents, but not of their concentration.

The values are given in ppm = mg / 1 in Table 3 below.

5

Table 3 pH / Density kg / l 4.0 / 1.06 4.8 / i.b. I.B.

Conductivity ^ S / cm 31600 Lead (Pb) 2.5 2.0 2.0

Cadmium (Cd) 250.0 278.0 161.0

Zinc (Zn) 10143.0 11300.0 7600.0

Mercury (Hg) below 0.001 i.b. I.B.

Copper (Cu) 213.0 51.0 26.0 Nickel (Ni) 8.3 4.0 3.0

Chromium (Cr-) 0.5 i.b. I.B.

Sodium (Na) i.b. 14780.0 7890.0

Potassium (K) i.b. 6000.0 2700.0

Calcium / magnesium (Ca / Mg) i.b. 680/1510 430/1070 20 Ammonium (NH4) i.b. 450.0 180.0

Sulfate (SO4) i.b. 85620.0 35740.0

Chloride (Cl) i.b. 3850.0 1650.0

Phosphate (PO4) i.b. 610.0 320.0 i.b. = not determined

For each basin, the need for the precipitate in question was determined according to the quick method as previously described and the water volume was measured (11-14 m

30 basin).

The required amount of said precipitant (between 1.5-11.5 l / m3 wastewater depending on basin) was then dissolved in 30-100 l of water and the solution was pumped into the large volume of water with stirring of the total basin content.

Then the mixture was stirred vigorously for one hour, commuting to a pH of 5. When the pH did not change any more, neutralized with lime milk (pH 7.5 is sought but mostly exceeded). Hydrate lime consumption was between 30 and 100 kg per day. basin (2.5-8.5 kg / m3).

After further stirring for 1.5-2 hours, stirring could be stopped (precipitation was successfully followed and monitored by the aforementioned rapid detection and with zinc and cadmium rods).

The sludge was already settled within 30 minutes and densified so much over 1 hour that the super clear water could be pumped off (decanted) and the thin sludge filtered over a mobile chamber filter press.

From each basin's decantate and filtrate, 15 samples were taken, residual heavy metal content was determined by AAS (Atomic Absorption Spectroscopy), and then released for public sewage discharge.

Table 4 shows the analysis results in mg / 1: 20 Table 4

Boiler / Container pH Pb Cd Zn Cu Ni Cr 1 9.0 0.2 0.1 0.1 0.1 0.6 i.p.

2 8.9 0.9 0.1 0.6 0.1 0.7 i.p.

3 10.6 1.2 i.p. 0.1 i.p. 0.2 i.p.

254 10.3 0.7 i.p. 0.1 i.p. 0.2 i.p.

Absorb 1 7.3 0.9 0.1 0.2 0.1 0.8 i.p.

2 8.9 0.3 i.p. 0.2 i.p. 0.5 0.1 30 Limit values: (statutory) 6-ll, 0 2.0 0.5 2.0 1.0 2.0 2.0 i.p .: not detectable by AAS.

DK 172048 B1 15

Example II

In the Passivation Department of automobile factories, chromium-containing wastewater is constantly present, containing chromium with six charges at different concentrations 5 (chromating). As is known, chromate cannot be precipitated with the usual precipitating agents and the water must be subjected to special treatment (chromate detoxification by sulphite treatment). With the present precipitant, the chromium is separated by six charges in one 10 steps. Three portions of such wastewater previously purified with conventional precipitates, neutralized and filtered, were post-treated with said precipitant as follows: The pH was adjusted to approx. 3 with 15 hydrochloric acid. The amount of precipitant determined in the above method was dosed and stirred for 5-10 minutes. The pH was adjusted to approx. 7.5 again by adding lime milk by stirring for approx. 15 minutes. After extinguishing the agitator 20, the sediment of approx. 10 minutes. After a total of 30 minutes standing, the clear water was decanted off and the residual sludge filtered without difficulty.

Table 5 shows the dosages and chromium containing before and after treatment: 25

Table 5

Wastewater # 1 23

Chromium before treatment 2 6.8 51.8 mg / 1 after treatment <0.02 <0.02 <0.02 30 Precipitate kg / m3 0.1 0.4 1.0

Hydrochloric acid 33% kg / m3 1.7 2.0 2.5

White hydrate lime kg / m3 2.0 2.5 3.0

The chromium determination was performed photometrically with diphenylcarbazide by a routine analytical laboratory. The detection limit of this method is at 0.02 mg / 1 chromium. The samples were then also subjected to AAS determination. Results for all three treated samples: not detectable.

Example III

5 A large armor repair and maintenance company had cadmium problems in the wastewater. The armor plates are cadmied to prevent corrosion and for this purpose the company uses a galvanizing. In addition, the armor plates are routinely washed with 10 alkaline detergents in a wash area. Thus, even though it is treated in the company's own wastewater, the wastewater constantly contains small amounts of cadmium that accumulate in the sludge from the municipal authorities' wastewater treatment plants.

15 The authorities' water people set up a long-term study (2 months); numerous samples were taken and analyzed in the microgram range in a highly efficient AAS apparatus. The values given in the following Table 6 were determined: 20

Table 6

Cadmium in micrograms / 1

Galvanization after cyanide detoxification 2100

Armor wash space 50 to 160 25 Own purification plant (outlet) (only a partial flow was purified) 5 to 64

Total inflow to municipal wastewater treatment plant 74 single measurements (approx. 56%) 1 to 10 32 "11 to 50 30 17" 51 to 100 9 "101 to 850 Raw sludge in municipal collector 44 Single measurements gave 310 to 340 mg Cd / kg in dry sludge.

35 The authorities assume that only part of the cadmium really dissolves in water and that the other part ends up as floating sludge (residual clarity) in the municipal 17 DK 172048 B1. .This is consistent with the experience that hydroxide sludge - unlike sulfide sludge - is difficult to trap or filter. The factory's own treatment plant works according to the following schedule: 5 H-SO. Ca (OH) 2

Dirty vana * η I

• - ~ -1_ _i- -i- - -

Collection- Forneutra- Neutral- Thickener Sludge-

NaOH 10% lysis ring - * -> - * 3 - * 3 - · 1 Q approx. 120 nr 3 m 3 m H, PO. 12%

3 4 1 __i -1 i * i H

altar- ...

pH 12.5 pH 4 pH 8 -

tively I

clear water-thick sludge 15

From the flow chart, it is seen that one endeavors to maintain a pH of 12.5 by dosing either sodium hydroxide or phosphoric acid and immediately adjust to pH 4 by addition of sulfuric acid. With regard to heavy metal precipitation, this equals a useless chemical consumption and an increase in the water's salt ballast.

Samples from the collection pool, pre-neutralization pool and thickener were examined according to the method described previously.

After calculation, a precipitating need of 0.28 l / m3 of wastewater was found. The dosing site is the neutralization pool. Without apparatus change, however, by the failure of sodium hydroxide and phosphoric acid, 30 cadmium peaks and shocks fail to appear, the cadmium residue content is lowered below the detection limit of the standard AAS (about 50 micrograms / l), and when a compact precipitant precipitate is obtained, no more is transferred. some cadmium with the help of substances.

35 The above examples are not based on constructed laboratory experiments, but they are derived from practical examples. The wastewater streams indicated in the Examples contained not only widely varied heavy metal types and amounts but also ballast salts such as sodium, potassium, ammonium, chloride, sulfate and nitrate and detergent residues with anionic and cationic surfactants, with phosphates, citrates, gluconates and - in particular - with EDTA / NTA complexes.

. All of these substances and others prevent (inhibit) the hydroxide precipitation. On the other hand, EDTA forms very stable complexes, so that the complexation is in a position to compete with the sulfide precipitation reaction due to soluble sulfides such as sodium sulfide. Neither at the aforementioned nor at others, at the site or only in the laboratory with the said precipitation agent treated effluent streams from companies, there was evidence that this separation of the various heavy metals was inhibited in any way. This indirect evidence was further emphasized by the foreign control. The treated wastewater streams were first released for drainage disposal after the authorities' measurement of residual contents.

The reasons for the slight influence of EDTA on the precipitation with the precipitant at pH 4 may lie in the fact that the extremely rapid sulfide precipitation constantly and effectively disrupts the equilibrium between EDTA-25 complex and heavy metals, that the calcium ions added with the precipitant absorb at least partially the EDTA The sites that, when neutralized with lime milk, largely offer calcium ions, complete the displacement of the heavy metals from the EDTA complexes.

30 Although today, one must generally count on the absence of EDTA in corporate and municipal wastewater flows, in most cases EDTA is present in low concentration (eg below 100 mg / l). On the other hand, the water solubility of EDTA decreases greatly by lowering the pH to 3, which is a sign that the protonation of the ligand has already progressed substantially at pH 4-3 and that the formation of the heavy metal complex is heavy. with two charges is significantly disturbed and hindered. For this fundamental change of the EDTA ratio in the above pH range, the strong pH dependence of the displacement series also speaks: pH 3 r 5 Fe (III) - Cr - Cu - Ni - Zn - Co pH 5 - 7 Ni - Cu - Fe (III) - Co - Zn - Cd - Ca - 10 The solubility of the sulphides is the opposite (examples of zinc and nickel approx. Values in mg / l).

pH 3 pH 5 pH 7 zn 0.7 0.008 0.001 Ni 20.0 0.3 0.004

The example of the particularly stable and feared nickel-EDTA complex shows that the process of the present invention with the stated precipitant offers at least 20 potential, expandable possibilities to find solutions in the most difficult cases of heavy metal separation in practice.

The Ni-EDTA complex loses its stability at low pH values; Ni ions are released.

25 By increasing the pH to 5-7 steps

Ni-EDTA stability again, however, under the strong competition of the sulfide precipitation. At the same time, the calcium ion is widely offered. In the pH range of 5 to 7, calcium is extensively capable of complexing with EDTA (calcium uptake of 20-50 mg Ca 2+ / g EDTA). In the system Ni2 + - S2- - Ca2 + -EDTA, pseudobiliary weights arise at the disadvantage of the Ni-EDTA complex.

With the process according to the invention with the stated precipitant, not only cations with also a number of anionic contaminants in the water are removed, although the latter are not removed in the prescribed amount. The treatment comes at the same level as a general purification of the water or - depending on the species - an intensive purification.

Cyanide and sulfite are converted into the substantially more harmless substances rodanide or thiosulfate.

5 H S + (n-1) CN- ^ H S + (n-1) SCN "2 n 2_ 2 2- H S + (n-1) SO - ^ H S + (n-1) S O 2 n 3 2 2 3

Nitrite, nitrate, chromate-bichromate, chlorate / perchlorate, iodate, bromate (also iodide), peroxide, perborate, water date, tungsten, molybdate, persulfate and others are reduced (and also precipitated).

Fluoride, carbonate, silicate, tetraborate, arsenite and arsenate, phosphate, sulfite, sulfate, selenate, tellurate, citrate, tartrate, oxalate and others are precipitated.

15 Soap residues, anionic surfactants, fatty acids and other substances are also rendered water insoluble, thereby at least partially decomposing an emulsion. Thus, oil and colloidal solutes are displaced from the water phase.

20 The release of water for dissolved and emulsified state ballast substances has great advantages, especially for the heavy metal precipitation.

By decreasing the ionic strength, the sulfide precipitation becomes more effective; it also comes closer to the 25 ideal conditions under which literature values of the residual solubilities are given.

By the substantially degraded ion concentration and the exclusion of protective colloids, the flocculation of the precipitated sulphides is facilitated, resulting in a clear aqueous phase and an easily filterable precipitate in the pH range of 6.5-8.5, especially between 7 and 8.

The wastewater acidification (by means of hydrochloric acid), which in most cases forms a complex system of solutes, and the treatment with the reaction-friendly calcium polysulfide in the precipitant according to the invention triggers a number of reactions and other processes, whereby a clear water phase occurs with the prescribed residual metal content and a defined sludge.

Not all of these courses can be p.t. is explained or described p.g.a. the diversity of wastewater. The 5 largest part of these processes occur through the use of sulfide ions.

After the addition of the present precipitate to moderate hydrochloric acid solutions, polysulfide is not suddenly decomposed into hydrogen sulphide and sulfur. From the polysulfide chemistry, it is known that polysulfide (sulphane) forms metastable emulsions in hydrochloric acid. The less the hydrochloric acid concentrations, the less the stability. Rather, the colloidal metastable solutions are stabilized in the presence of CaCl2 · Heavy metal ions, on the one hand, act as catalysts for the removal of the metastable equilibrium, on the other hand as a sulfide consumer in their precipitation. Other sulphide consumers also initiate the cleavage as indicated above. These reactions are developed for analytical determinations as is the reaction between mercury and trisulfan, which, however, proceeds quantitatively in dry CCl4.

H2 S3 + h9 - * H2 S +2 HgS

25 A possible pathway for the slow, controlled cleavage is known from the aging of sulfide / polysulfide solutions, which, essentially by disproportionation, turns into increasingly higher homologues and eventually over weeks and months to elemental sulfur: 30 2 H2 S3 - * H2 Sj + H2 S4 H2 «2 - *» 2 S + S „asc.

»2 S4 - 2 W *% s6 35 The disproportionation of sulfur in hydrate lime solution proceeds at 60-80 ° C by breaking up the Sg ring according to 22 DK 172048 B1 2-6f 2- 2-8 S ° - * 6 S + 2 S - * 4 S 2 S2 0 j

It is also known that heavy metal polysulfides are formed only by excess sulphanes and under special conditions such as, for example, NiS3 and CuS3 from acetic acid, 5 znS3 in CS2 solution of Zn-ethylate or PbS3 from Pb-butyl mercaptide dissolved in benzene.

It is also known that the thiosulfate ion is broken down over polysulfide to sulfide and sulfate or to sulfur and sulfate.

10 By NMR analyzes, it has been established that H2S4 always dominates and H2S2 practically does not occur in the protonation of Na2Sx (x = 1 to 5). For example, Na 2 S 4 gives the following composition of the sulphanes: 15 H S 0%, H S 18%, H S 33%, H S 17%, H S (n = 6) 32%.

22 23 24 25 2n

Also with calcium polysulfides, the relatively stable tetrasulfide plays a dominant role, more or less independent of the ratio S / Ca = 1 to 5, as is known from previous experience.

Moreover, the distribution of the sulphans is dependent on the acid species (HCl, H2SO4, H3PO4), the temperature, the cations (Na + or K +) and other circumstances of the protonation. All solutions contain OH ″, HS ″, S42 and S52 ″, while lower sulphans (H2S and H2S2) also remain in ionic form to a very small extent.

Despite low concentration of the lower sulphans or sulfide ions, the precipitant-salt-acid mixture in the precipitation reaction was composed of soluble monosulphide as shown in the following experiment: 12.5 g of Zn-Cl2 (anhydrous) dissolved in 100 ml of distilled water (pH = 6). Of this, 20 ml was taken and 80 ml was added thereto. The pH of this solution was then adjusted to 4 with 3% hydrochloric acid.

23 DK 172048 B1

The zinc solution was then titrated with the precipitant (3% solution in water, atomic ratio Ca: S = 1: 1, content equivalent to 2.8% CaS). The titration was carried out by stirring and heating to 50 ° C and measuring the pH.

The precipitate-measuring solution was added portionwise until no more precipitation was detected at pH 6.5. Then, the pH was adjusted to 7.5 by the addition of 2% NaOH and the precipitate was allowed to settle. In the above solution, neither zinc (dithizone) nor sulfide (silver nitrate) could be detected. Consumption was 55 ml of the precipitant of the invention.

Calculation: 15

Mol of ZnCl 2 - = 12.5 x 0.2 / 136.3 = 0.0183

Mol of CaS - = 55 X 0.028 / 72 = 0.0214 20

Over 85% of the dosed sulfur was present as zinc monosulfide.

, The metastable polysulphane system in acidic, colloidal solution is decomposed relatively slowly. It is capable of many reactions, and its reactions are in accordance with the Law of Mass Effect according to the type and amount of the individual reaction partners in the present wastewater. If sulphide is used and consumed, the equilibrium pushes towards sulphide production.

If no sulphide consumer is present, the reaction direction changes to form still higher sulphane homologues and eventually elemental sulfur which coincides with the other precipitate in the wastewater. The deposition effect and excretion as water-insoluble bottom deposit reduces the risk of overdose, thus the release of toxic sulfur hydrogen as well as the retention of sulfide ions in the water. This risk must first be mitigated when precipitating with soluble sulphides by means of substantial supply of apparatus, control and regulation instruments or other aids.

The preparation of the precipitating agent 5 can, for example, be carried out as follows:

Example IV

23 kg of white hydrate lime and 10 kg of ground sulfur were mixed with approx. 90 l of water and the mixture was stirred with a fast-running agitator while heating to 80 ° C. After a thawing hour, as the temperature had risen to 50 ° C, a homogeneous, lemon-yellow suspension occurred, which only precipitated slowly. At about. At 65 ° C, a strong reaction, recognizable by the eclipse of the dye to deep orange, and the increase in heat generation began. After a stirring time of a total of 1 hour, the temperature had risen to 80 ° C with further deepening of the color to dark cinnobar. Next, stirring and heating were interrupted.

20 Approx. 115 kg of product as a suspension with the specific gravity 1.15 kg / l, a pH of approx. 13.0 and an active substance content of approx. 23%.

Example V

25 70 kg of white hydrate lime and 35 kg of sulfur powder were mixed in a kneading machine and added 125 l of warm water (65 ° C). Already 5 minutes after starting the kneading machine (slow stirring and kneading) a yellow suspension appeared with mushy and creamy consistency. Next, 30 additional 65 kg of white hydrate lime and 20 kg of sulfur powder are added and stirred and kneaded for an additional 3 hours with heating at 60-50 ° C. 250 kg of orange-colored suspension, which gradually settled, was filled in a still homogeneous state in barrels with a lid of PE and was stored without closing the lids. After a storage period of 7 months, the precipitated sludge could be easily stirred with the orange liquid and made pumpable. However, red, needle-shaped crystals were formed which dissolved in water to form an opalescent solution.

Example VI

5 250 kg of powdered building lime in powder form was mixed with 900 l of water and the whole was heated to 40 ° C. With stirring, 100 kg of powdered sulfur was dosed, which had already been substantially lumped together. When the dosing was completed after one hour or so, stirring was continued for approx. 4 hours until the temperature had reached 65 ° C. A leaf stirrer ran with approx. 40 rpm Emptying was done with a small pump with the performance approx. 2.0 m3 / h over a NW35 cable.

For example, 26.5 kg of white hydrate lime and 16.5 kg of ground sulfur were added to 110 l of water and the mixture was stirred at 90-95 ° C and with approx. 150 rpm After 15 minutes, the mixture became completely homogeneous and gave a vigorous 20 orange color. The mixture was then stirred for another 30 minutes at 80-85 ° C. After cooling, 125 kg of a brown solid appeared, however, which proved to be thixotropic by suckling and stirring, and thus can be stirred and pumped.

25

Example VIII

Polysulfide mixture with barium as cation.

4.2 kg of barium hydroxide monohydrate was dissolved in 11 liters of hot water and 1 kg of ground sulfur was added.

After 3-4 minutes of stirring at approx. 150 rpm and at 70 to 75 ° C the mixture became homogeneous and gave a deeper brown-orange color. The temperature was further raised to 90 to 95 ° C and stirred for a further 20 minutes, then quenched. 13-4 35 minutes, a little sludge was precipitated with a cherry red, clear supernatant liquid. 13.5 kg of precipitate was obtained which, upon cooling, solidified to a brown mass. The mass also proved to be thixotropic.

Claims (3)

26 DK 172048 B1 A process for separating heavy metals from wastewater by sulfide precipitation by calcium polysulfide, characterized in that a) as a calcium polysulfide precipitant, a complex mixture of calcium polysulfides with low proportions of calcium sulfide, calcium dihydrosulfide, calcium thiosulfate and calcium hydroxide and calcium hydroxide is used. atomic ratio S: Ca (0.8 to 1.5): 1, prepared by reacting elemental sulfur with 10 calcium hydroxide and / or calcium sulfide at a temperature of 60 to 80 ° C with stirring in an aqueous suspension with a solids content of 30 to 60% by weight; b) for carrying out the sulphide precipitate if necessary, the heavy metal-containing wastewater acidifies to a pH of 0 to 6 and then adding the sulphide precipitant of step (a) to one of precipitating the heavy metals as heavy metal sulphides. sufficient quantity, after which the pH value of 7.0 to 8.0 is adjusted, and c) let the flocculating heavy metal ulfid settle for 15 to 60 minutes to form a thin slurry which, after decanting the supernatant and after filtering through quantitative filter paper or using a chamber filter press, thickens to a heavy metal sulfide-containing thick slurry and is removed. Process according to Claim 1, characterized in that before the operation of step (b), the calcium polysulfide precipitant with water in the ratio of 1:10 is diluted as a suspension and stirred. Process according to claim 1 or 2, characterized in that, in step 35 (a) of the calcium polysulfide precipitant as a sulfur source, ground sulfur, precipitated or by other chemical processes obtained sulfur, sodium and potassium polysulphur are used. fider and aged sulfide compounds.