Bennini, Aldo (Via Alla Selva, 6 Muzzano, CH-6933, CH)
| 1. | Method for the treatment of waste contaminated with heavy metals, characterised in that the following steps are carried out: determination of the acidity of the contaminated material, partial neutralisation of the material if the latter is acidic, the addition of a solution of orthophosphoric acid and an alkali metal salt thereof in water, forming a homogeneous paste from the mixture formed, the addition of calcium oxide (CaO) and/or calcium hydroxide (Ca (OH) 2) to the paste, and allowing the paste to dry in the air. |
| 2. | Method according to Claim 1, characterised in that the contaminated material is stirred. |
| 3. | Method according to Claim 1 or 2, characterised in that between 50 kg and 350 kg pulverulent calcium oxide and/or calcium hydroxide is added per tonne of waste to be rendered inert. |
| 4. | Method according to one of the preceding claims, characterised in that the pulverulent calcium oxide or the calcium hydroxide is of the type that is suitable for building applications. |
| 5. | Method according to one of the preceding claims, characterised in that the solution of orthophosphoric acid and an alkali metal salt thereof contains mono, diand trisodium phosphate and/or mono, diand tripotassium phosphate and/or mono, diand tricalcium phosphate, in the anhydrous form or in the form of the hydrate. |
| 6. | Method according to Claim 5, characterised in that the mono, diand trisodium phosphate and/or mono, diand tripotassium phosphate and/or mono, diand tricalcium phosphate are used in an amount of between 5 1 and 150 1 per tonne of waste to be rendered inert. |
Numerous technologies are disclosed in the prior art with which metals can be immobilised in stable matrices, as a result of which they acquire chemically inert properties and as a result of which the transfer thereof can be prevented, so that this type of waste can be dumped under less stringent conditions (the transfer tests with these materials fall within the limits imposed by national and international legislation).
Frequently these types of waste have a composition which requires large quantities of reagents, such as, for example, cement, to obtain an adequate degree of stabilisation, or for which expensive substances have to be used in combination with long periods of action. Other solidification techniques make use of additives such as slags, finely ground brick, grit and sand mixed with bentonite, as a result of which an excessive increase in the final weight and volume takes place, and moderate results are obtained in respect of restricting transfer of the metals.
To reduce the proportion of binder when rendering inert flue gas produced in the mining industry, the interfering ions (chlorides) are washed with water.
A number of the systems that have been used recently use aqueous solutions of chemical products which are particularly expensive, as a result of which inadequate data are known with regard to the physical and the physico-chemical conditions which are necessary for the various types of waste or with regard to the quantities which have to be used in order to achieve the desired result.
There is thus a need, in special cases, to be able to use a versatile method with which special pre-treatments are not necessary and with which inexpensive reagents available on a large scale can be used, which reagents can be added in limited quantities in order to minimise the increase in the weight finally to be dumped.
By means of the present invention it is possible to comply with the abovementioned
requirements as well as to obtain a number of other advantages which will become clear below.
To this end the present invention is characterised in that the following steps are carried out: -determination of the acidity of the contaminated material, -partial neutralisation of the material if the latter is acidic (pH between 0.5 and 3), -the addition of a solution of orthophosphoric acid and an alkali metal salt thereof in water, -forming a homogeneous paste from the mixture formed, -the addition of calcium oxide (CaO) and/or calcium hydroxide (Ca (OH) 2) to the paste, and -allowing the paste to dry in the air.
The calcium hydroxide or the calcium oxide can be added in powder form in an amount of between 50 and 350 kg per tonne of waste to be rendered inert. Good results have been obtained with pulverulent calcium oxide or calcium hydroxide of the type that is suitable for building applications.
The concentrated aqueous solution of orthophosphoric acid and of the alkali metal salt thereof can be the commercially available"Fissa Met"solution for rendering inert, which contains orthophosphoric acid and mono-, di-and trisodium phosphate (NaH2PO4, Na2HP04, Na3P04), mono-, di-and tripotassium phosphate (KH2P04, K2HP04, K3P04), or mono-, di-and tricalcium phosphate (CaHPO4, Ca (H2PO4) 2, Ca3 (PO4) 2), in the anhydrous form or in the form of the hydrate and which is used in an amount of between 5 and 150 litres per tonne of waste to be rendered inert.
The method according to the present invention is highly versatile and can be used on waste from various sources, such as metal slags, demolition debris or scrap, ash from incinerators or solid industrial and inorganic chemical waste material from towns, special installations, hospitals, industry and the like.
A description of the general characteristics according to the present invention has been given above. The characteristics, advantages and the way in which the method has to be used will be better understood with reference to the appended example, which gives a detailed description of a specific embodiment.
Example The method according to the invention for rendering waste inert was used on fly ash originating from the incineration of waste from a hospital. This material was placed in a mixer. Calcium hydroxide (slaked lime) in an amount of 90 kg per tonne waste was added by pneumatic means from suitable silos simultaneously with 50 litres of the"Fissa Met" solution for rendering inert, which is a concentrated aqueous solution of orthophosphoric acid and of the disodium hydrogen salt thereof (disodium monohydrogen phosphate (Na2HPO4)), which was added from another container, together with just enough water to guarantee intimate contact of the mixture. The mixture had to remain in a stirrable composition and was stirred for several minutes.
At the end of this step the wet mixture was placed on a suitable surface so that said mixture was able to dry completely, or said mixture was dried immediately in a stream of hot air (where heat recovery was possible).
When the material obtained by the abovementioned treatment was subjected to a leaching test, it was found that the release of lead, cadmium, copper and selenium remained within the acceptable limits to enable the material to be dumped immediately without further treatment in a protected dump.
The initial original values of the metal concentrations in the eluate with a solution of 0.5M acetic acid before and after treatment are given for comparative purposes in appended Tables 1 and 2."T. Q." in Table 2 gives the initial concentration of the heavy metals.
Table 1 Metal Eluate (mg/1) Eluate (mg/1) Acceptance beforethe after the limit treatment treatment Lead 10,500 0. 200 0.200 Cadmium 6,400 0. 020 0.020 Copper 0.1000.100 Selenium 0.030 0. 030 0.030 Table 2 Fly ash from RSU Metal slags Metallurgical fly ash Various materials incinerator Tests T.Q. Eluate Inert T.Q. Eluate Inert T.Q. Eluate Inert T.Q. Eluate Inert (mg/l) (mg/l) eluate (mg/l) (mg/l) eluate (mg/l) (mg/l) eluate (mg/l) (mg/l) eluate (mg/l) (mg/l) (mg/l) (mg/l) Lead 1400 8.00 <0.02 87,000 10.00 0.20 30,000 250 0.20 12,500 100 0.20 Cadmium 400 <0.02 <0.02 160 0.12 0.01 525 15 <0.02 25 0.40 0.02 Copper 75 <0.10 <0.10 - - - 1650 10 <0.10 0.10 15 0.10