AU732646B3 - Treatment process for contaminated waste - Google Patents

Description

Regulation 3.2

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A PETTY PATENT (original) Name of Applicant: Actual Inventor: Address for Service: SOLUCORP INDUSTRIES LTD William C. Webster MADDERNS, 1st Floor, 64 Hindmarsh Square, Adelaide, South Australia, Australia "TREATMENT PROCESS FOR CONTAMINATED

WASTE"

Invention Title: The following statement is a full description of this invention, including the best method of performing it known to us.

la TREATMENT PROCESS FOR CONTAMINATED WASTE FIELD OF THE INVENTION This invention provides a process for treating material containing soluble metals to form metal sulfides, rendering the metals less soluble, in a manner that also ameliorates hydrogen sulfide production during the process.

BACKGROUND

Processes for effectively treating wastes such as soil, ash, sludge, baghouse dust and sediments to stabilize soluble metals are of increasing importance. If left untreated, soluble metals that become mobile may cause environmental damage, for example by entering ground water. Environmental regulations have been enacted in many jurisdictions proscribing unacceptably high levels of soluble metals in waste materials, and describing standard methods for evaluating the solubility of metals in solid wastes.

A variety of methods are known for stabilizing soluble metals in relatively waterinsoluble forms. These methods may be used to reduce the solubility of the metals to levels that are environmentally relatively benign and acceptable under the relevant regulations. For example, a variety of methods are known for fixing metals as sulfides.

Some prior art metal fixation processes for producing metal sulfides may produce hydrogen sulfide, a potentially toxic and foul smelling gas. For commercial applications of such processes hydrogen sulfide emissions may be controlled by collection and treatment of the gas. Even when it is controlled, the production of hydrogen sulfide may therefore make such metal fixation processes more difficult by complicating the operating conditions and increasing the costs of implementing the process.

SUMMARY OF THE INVENTION The invention provides a process for treating solid-containing material that includes a soluble metal to form a sulfide of the metal. The process includes the step of contacting the material with a mixture, preferably in the presence of water, and preferably at a pH of greater than about 8. The mixture includes a reactive sulfide, preferably selected from the group consisting of calcium sulfide, calcium polysulphide, sodium sulfide and sodium hydrosulfide. The sulfide may be in an amount between about 0.5% to about 8% of the weight of the material. The mixture may also include an inorganic phosphate, preferably containing calcium phosphate. The inorganic phosphate may be present in an amount of between about 1% to about 8% by weight of the material. The mixture further includes calcium hydroxide, which may advantageously be present in an amount of at least 0.25% by weight of the material. The process may also include the step of adding water to facilitate the reaction between the mixture and the solid material.

A wide variety of soluble metals (which may be in the form of soluble metal compounds) may be treated in accordance with the invention. In various embodiments of the invention, the metal may be selected from the group consisting of arsenic (As), antimony barium beryllium cadmium chromium copper (Cu), lead mercury nickel selenium silver thallium thorium vanadium and zinc (Zn).

Although sulfide alone may be used to stabilize soluble metals, the addition of phosphate may reduce the amount of sulfide required or further reduce the solubility of the fixed metal over time. In some embodiments, the phosphate may increase the longterm stability of the fixed metal by reducing the extent of long term oxidation of the fixed metal sulfides. The phosphate may accordingly be added in an amount sufficient to ameliorate remobilization of the metal sulfide. The phosphate may be usedto precipitate ferric iron in the treated material, so that the redox potential of the material is insufficient to oxidize the fixed metal sulfide. Orthophosphates may be preferred phosphates in some embodiments. In alternative embodiments, specific phosphates for use may be calcium phosphates (such as monobasic, dibasic, tribasic calcium phosphates or rock phosphate), phosphorus anhydride, P 2 0 5 salts, monoammonium phosphate or diammonium phosphate. Some embodiments may use commercially available triple super phosphate (calcium phosphate monobasic, Ca(H 2

PO)

2

H

2 0O).

The base, which includes calcium hydroxide, may include in alternative embodiments calcium carbonate or calcium oxide or both. In some embodiments, the base may be used in an amount sufficient to provide approximately 2 or more times the stoichiometric amount of neutralization capability as there is acid generation potential in the added sulfide. Preferably, sufficient base is added to rapidly bring the initial pH of the mixture above 8.0. In some embodiments, the total base added may be 0.25% to 12% by weight of the waste.

The waste may be acidic or basic and the amount of base added will thus vary, more for an acidic waste, less for a basic waste. Water may be added to adjust the consistency of the waste, which may facilitate the fixation reaction. The waste may be pretreated, prior to stabilizing the metals as sulfides, by techniques such as oxidation or reduction, to convert the metal compounds in the waste into forms that are more easily treated by the present invention. The phosphate, sulfide or base may be added in dry form, and may be mixed prior to addition to the waste.

DETAILED

DESCRIPTION

The process of the invention may be useful for reducing the leachability of a wide variety of soluble metal compounds, including soluble compounds of metals such as arsenic antimony barium beryllium cadmium chromium (Cr), copper lead mercury nickel selenium silver thallium thorium vanadium and zinc The soluble metals for treatment in accordance with the invention may be contained in soils, sludges, sediments or other solid-containing materials. The invention provides processes for converting such metals from soluble forms, such as hydroxides, carbonates and oxides, to less soluble sulfides, while ameliorating the release of hydrogen sulfide gas from the treated material.

-4- U.S. Environmental Protection Agency (EPA) Superfund feasibility studies of the processes of the invention have been conducted on 500 tons each of three different hazardous wastes/soils at the Midvale Slag Superfund Site in Midvale, Utah, USA. The processes of the invention demonstrated an ability to reduce the leachable concentrations of As, Cd and Pb to less than the EPA Toxicity Characteristic Leaching Procedure (TCLP) regulatory limts of, respectively, 5.0, 1.0 and 5.0 mg/i for these metals.

Hydrated lime (calcium hydroxide, Ca(OH) 2 is commercially available in forms with relatively high surface area, preferably from about 13 m 2 /g to 32 m 2 /g for use in the process of the invention. The use of hydrated lime with high surface area facilitates the rapid suppression of the release of hydrogen sulfide gas.

The following examples are merely illustrative of various aspects of the invention, and are not exhaustive nor limiting in their disclosure of the scope of the invention.

EXAMPLES

In the absence of calcium hydroxide, the treatment of a solid-containing waste soil material with calcium sulfide under a variety of conditions may result in the formation of hydrogen sulfide, as is indicated by the results set out in Table 1.

TABLE 1 Treatment without Calcium Hydroxide Control Mix 1 Mix 2 Mix 3 Mix 4 CaS, 0 0.5 1.0 2.0 Phosphate,% 0 1.0 1.0 1.0 CaCO 3 0 0.7 1.4 2.8 5.6 Ca(OH) 2 0 0 0 0 0 Water, 0 8 8 9 pH, 2hr 7.7 6.3 7.0 8.1 8.6 TCLP Pb, mg/1 34.4 1.68 2.16 1.26 0.12

H

2 S Odor Strong Strong Strong Strong Hydrogen sulfide release remained significant in additional tests in which the amount of limestone added was doubled. However, as shown in Table 2, in a mixture in which all of the limestone was replaced with hydrated lime (calcium hydroxide), no hydrogen sulfide odour was detected. Further, in a mixture in which hydrated lime was used at half the previously tested limestone dosage, no hydrogen sulfide odour was detected.

Table 2 shows the results obtained in tests of four mixtures containing 200 grams each of contaminated soil, for which the percentages of reagents are given by wet weight of soil. The soil was contaminated with lead. Leaching values were determined in accordance with the US EPA TCLP test (USEPA SW-846, Method 1311, in which mg/l is considered acceptable). The reagents used in these tests were as follows: CaS dry calcium sulfide, 50% strength; phosphate triple super phosphate; limestone and hydrated lime commercially manufactured products. In these tests, the mixing cycle was as follows: Add phosphate and alkaline reagent mix 1 /2 minutes; add CaS mix 1 V/ minutes; add water mix 2 minutes.

-6-

I

TABLE 2 Comparative Tests Mix 2 Reference CaS, 1.0 Phosphate,% 1.0 CaCO 3 1.4 Ca(OH) 2 0 Water, 9 pH, 2hr 7.0 TCLP Pb, mg/1 2.16

H

2 S Odour Strong )NT did not test.

Mix 2A 1.0 1.0 2.8 0 9 7.4

DNT

Strong Mix 2B Mix 2C 1.0 1.0 0 1.4 9 12.3

DNT

None 0 0.7 9 11.2 1.72 None Table 3 shows results obtained in additional tests using lower CaS additions. In these mixtures, batches were mixed in the same sequence as the tests for which the results are shown in Table 2. In Table 3, data from test mixes IA and 1B are shown for comparison with data from the same Mix 1 as shown in Table 1. As shown in Table 3, Mix 1A included an amount of lime approximately equal to the stoichiometric equivalent of the amount of limestone used in Mix 1. For comparison, Mix IB used approximately /2 of the Mix 1 dosage of limestone.

-7- TABLE 3 Comparative Tests with Reduced Ca(OH) 2 Mix 1 Mix 1A Mix IB Reference CaS, 0.5 0.5 Phosphate,% 1.0 1.0 CaCO 3 0.7 0 0 Ca(OH) 2 0 0.5 0.35 Water, 8 8 8 pH, 2hr 6.3 9.7 8.1 pH, 24hr 6.5 10.1 8.7

H

2 S Odour Yes None None To test alternative mixing protocols, an additional trial mix (Mix 2C-R, for which data are not tabulated) was made using hydrated lime and other ingredients in the same ratios as used in Mix 2C of Table 2, with all of the phosphate, calcium hydroxide and calcium sulfide added together at the beginning of the mixing cycle. As in the case of Mix 2C, no H 2 S odour was detectable.

Table 4 shows data from treatment of a waste slag from a secondary lead smelter.

The ingredients shown in Table 4 were added (in a Hobart N-50 mixer) in the same manner as described for the tests shown in Table 2. A 250 gm sample of the slag was used for each mixture. Hydrated lime was used to partially replace the limestone in the mixtures, and minor changes in the dosages of the alkaline materials were made based on the pH of the resulting mixtures. TCLP results were obtained on selected mixes 1 day after treatment.

TABLE 4 Slag Treatment Control Mix 1 Mix IA Mix 2 Mix 3 Mix 4 Mix 4A CaS, 0 0.5 0.5 1.0 1.5 2.0 Phosphate,% 0 1.0 1.0 1.0 1.0 1.0 CaCO 3 0 0.4 0.5 0.9 1.4 1.9 Ca(OH) 2 0 0.3 0.5 0.5 0.7 0.8 0.6 Water, 0 6 5 6 6 6 pH, 2hr 8.4 11.5 11.6 12.3 12.6 12.0 TCLP Pb, mg/1 153 DNT 2.93 2.39 0.05 DNT 0.7

H

2 S Odour Slight None None None None None DNT Did not test.

Although the present invention has been described in some detail by way of example for purposes of clarity and understanding, it will be apparent that certain changes and modifications may be practised within the scope of the appended claims.

Claims (3)

1. A process for treating contaminated solid waste comprising one or more leachable, toxic, environmentally harmful metal, said process comprising: contacting the contaminated solid waste with a mixture comprising: (1) a sulfide selected from the group consisting of calcium sulfide, calcium polysulfide, sodium sulfide and sodium hydrosulfide, said sulfide being in an amount between about 0.5% to about 8% of the weight of the contaminated solid waste, (ii) an inorganic phosphate comprising calcium phosphate, said inorganic phosphate being in an amount between about 1% to about 8% by weight of the weight of the contaminated solid waste, and (iii) calcium hydroxide being in an amount of at least 0.25% by weight of the contaminated solid waste; establishing the pH of the resulting mixture to greater than about 8; stabilizing said metal to significantly reduce its leachable concentration; and ensuring the presence of sufficient water to suppress the emission of hydrogen sulfide during contacting.

2. The process of claim 1 wherein the metal is selected from the group consisting of arsenic antimony barium beryllium cadmium chromium copper lead mercury nickel selenium silver (Ag), thallium thorium vanadium and zinc (Zn).

3. A process of claim 1 or 2, substantially as described herein and with reference to any one of the Examples. Dated this 7 th day of December, 2000. SOLUCORP INDUSTRIES LTD By its Patent Attorneys MADDERNS om A OVI