DE4314775A1 - Process for on-site remediation and chemical long-term stabilisation of deposits having sulphidic contents - Google Patents

Abstract

In a process for on-site remediation and chemical stabilisation of deposits having sulphidic contents, in particular from ore mining and ore treatment, in which the heavy metal cations and radio element cations are immobilised by an acid-base- and redox-buffer capacity, comprising hydroxides, carbonates and silicates of alkaline earth metals, aluminium and iron(II, III), being generated or stabilised in the deposit, which capacity converts the heavy metal cations and radioelement cations into poorly soluble or insoluble compounds which are included in precipitates of the major constituents such as iron hydroxide and gypsum, the bicarbonate required for the neutralisation and the reducing agent are generated in concentrations of 1-10 milligram-equivalents/l of leachate water by in-situ development of carbonic acid and reducing substances by bacterial oxidation in humus and calcium- or magnesium-carbonate-containing additives and together with the precipitation water or groundwater are fed to the sulphide-containing deposit.

Description

The invention relates to a method for on-site renovation and chemical Stabilization of deposits with sulfidic components, especially the Ore mining and ore processing, through which a reduction of the Removal of heavy metals and radio elements with the Leachate is reached.

Such deposits consist of the tailings or the residues ore processing that remains after the materials have been applied. they contain in addition to the gait ore shares, their extraction either not the target of the mining activity was or was not economical. These shares subject to weathering. The result is water-soluble chemical forms of heavy metals and radio elements associated with the Leachate into the ground and surface waters of the area can and thus lead to pollution of the biosphere.

Especially in the residues of hydrometallurgical ore processing, such as it is typical for the extraction of uranium, the heavy metals lie as a result chemical leaching partially in comparison to the original chemical and mineralogical conditions changed connections. Dependent on the heavy metals and the pH value and the redox potential Radio elements in both water-soluble ionic forms and in poorly soluble compounds, e.g. B. carbonates and hydroxides.

In the acidic environment, the heavy metal compounds dissolve, so that with the Progress of the oxidation process an increasing heavy metal pollution of the Leachate.

At a sufficiently low pH value (pH <4) occurs in addition to the purely chemical Sulfide oxidation is a bacterial process in the foreground in which various Types of sulfur bacteria that consume sulfides to form sulfates. This Process runs significantly faster than atmospheric oxidation, so it with the entry of the deposit into the phase of bacterial sulfide oxidation too comes to the dissolution of the more stable heavy metal compounds. That leads to increased discharge of heavy metals.

A group of known methods for remediation assume that the deposit (possibly after neutralization by adding lime) Covering with a material as dense as possible (floors that are practical are impermeable to water, plastic films, bitumen layers, etc.), insofar as technically possible to lock against atmospheric influences and to prevent the seepage of precipitation (DE 38 12 705).  

Other known methods (DE 35 26 508) are based on solidification or the inclusion of the masses containing heavy metals in a matrix siliceous materials, in concrete bodies (also using setting ash) and various organic polymers (e.g. phenol resins). By adding certain chemicals, such as. B. carbonates, Pyrophosphates and tartrates that are injected into the deposits are said to Heavy metals are converted into poorly soluble compounds.

The procedures based on this are costly and grant under the there is no humid climate in Central Europe with cyclical frost periods Long-term protection. According to experimental results found in the scientific literature published, the seal is after a hundred such cycles at the latest, H. after 30-50 years no longer exist due to the driving effect of frost. The targeted fixation of certain heavy metals by forming a connection with certain counterions such as B. alkaline tartrate or sodium carbonate (DE 35 26 508) can simultaneously mobilize other heavy metals such as B. Uranium and thorium, so that the application of this principle to certain Deposits must remain limited.

As deposits, the deposits usually lie freely on the ground or are enclosed as mud ponds by dams or natural valley slopes.

The shipment of such heavy metal deposits (tailings), especially those that use radioactive heavy metals in large quantities included (deposits from uranium ore mining and processing, Halden of nonferrous metal extraction z. B. the historic silver and Copper mining and mining of uranium-bearing coal) is for the sake of Occupational and environmental protection and for economic reasons only for limited quantities acceptable. Extensive deposits in the area of more 100,000 tonnes must therefore be renovated on site.

The present invention has for its object an inexpensive Process for on-site remediation and chemical stabilization of Deposits with sulfidic components, especially those of ore mining and Specify ore processing that provides long-term protection against atmospheric oxygen and thus against the mobilization of heavy metals as metal cations or light soluble complex ions and their discharge through leachate guaranteed.

The object is achieved in that in the deposit controlled leachate is added so that it is in contact with the Built-in or deposited aggregates Solution and balance processes reducing and neutralizing Connects with the acid-soluble compounds of the Deposit respond. This ensures that the soluble chemical inventory  deposition in the geochemical state range (pH / pE range) is returned, or is stabilized there, in which the heavy metals and natural radio elements, except radon, in poorly soluble compounds occur. This state is characterized by pH and pE values that correspond to the correspond to natural groundwater (pH range between 6 and 8, pE between -1 and 4). The type of exposure to neutralizing and reducing acting substances is determined by the initial chemical state (type and quantity the dissolved calcium-magnesium-iron compounds) and the desired Acid bases and buffer capacity determined in the state of stabilization. The Acid-base buffer capacity consists mainly of calcium, magnesium and iron (II) carbonates, iron (II, III) and aluminum hydroxides and iron (II) - or Iron (II) magnesium silicates. The redox buffer capacity is made easy by the decomposable iron (II) compounds (iron (II) carbonate, iron (II) silicate, Iron (II) hydroxide) built up.

The necessary size of the buffer capacity is determined by the inevitable Fluctuations in the oxygen content of the rainwater and unavoidable infiltration irregularities determined. With typical Sulfide levels of 1-3% are values for the acid-base buffer capacity in the range from 50-100 milligram equivalent / kg of deposit mass, for the redox capacity from 10-50 milligram equivalent / kg. The necessary quantities of neutralizing aggregates are in the range of 5-10 kg / t deposit mass, those for the reducing supplements in the range of 0.5-1 kg / t.

This stabilization state is achieved in that the Remediation (neutralization) phase necessary high base entry of 1-10 Milligram equivalent / l through the dissolution of calcium carbonate carbonated leachate is realized. The necessary According to the invention, carbonic acid is produced by natural bacterial degradation processes compostable or humus-containing components in the aggregates.

After the buffer capacity has been built up, the deposit is chemically stabilized. To maintain this state (stabilization phase) is essential lower base entry sufficient, that for oxygen-saturated leachate is 0.5 milligram equivalents of neutralizing substances per liter. This Entry corresponds to the acid equivalent of that from oxygen from sulfides newly formed acid. In the stabilization phase or after consumption of the compostable or humus-containing components of the aggregate is through the remaining stock of undissolved alkaline earth carbonates and by the at Neutralization in the deposit released carbonic acid, which by conversion in soluble bicarbonates are first bound in the leachate and with them the Deposit is fed back, secured. The bicarbonate is in one  Cyclic process available as long as undissolved alkaline earth carbonates available.

When the oxygen content of the leachate is reduced by half the saturation concentration is sufficient to maintain the Stabilization state due to the solubility of calcium carbonate achievable base entry, so that under this condition the deposit also remains in the chemically stable state as long as undissolved Calcium carbonate is available. The reduction in oxygen content the leachate is covered according to the invention by covering the deposit an oxygen-consuming layer, e.g. B. a cultural layer.

To ensure short times for the restoration of the deposit on the one hand and to limit the amount of necessary additives for a long time Stabilization phase on the other hand is a with the superimposed top layer extensively uniform and controlled leakage of the Rainwater set by the surface, the same or little is greater (1.1-2 times) than the rate of leakage through the Deposit layer.

Due to a low seepage rate during the stabilization phase is also the discharge of pollutants in the basic and Surface water around the deposit further decreased.

The seepage speed is set using the cover layer known methods of surface design, the selection of Installation materials and the degree of their mechanical strengthening.

Based on the inventory of deposition of sulfidic materials, the Size of the acid and redox Buffer capacities, the set leakage and an expected value for the period of maintaining steady state will be under Consideration of the type of aggregates, their minimum quantities for remediation and Long-term stabilization dimensioned.

Moderate to poorly soluble additives come as neutralizing additives (Solubility <0.01-1 milligram equivalent / l) Materials considered Contain alkaline earth hydroxides and poorly soluble carbonates. Join in preferably waste products such as mortar-containing demolition compounds, lignite ash, Fly ash, calcareous soils (marl) are used, depending on the hydrogeological conditions, either by incorporation into the deposit or applied as a cover. As reducing supplements are Sewage sludge, humus-containing or compostable materials e.g. B.  Municipal waste suitable, preferably as a cover layer or in the upper Deposition layer can be used incorporated.

The neutralizing and reducing additives are preferably in Combinations introduced. The introduction of aggregates can also in In combination with mineral soils with low permeability, a To achieve coverage of controlled leakage.

The inventive method has the further advantage that the before Remediation of soluble heavy metal ions not only through the formation of their own hardly soluble minerals are immobilized, but that additionally by Inclusion or adsorption in the poorly soluble compounds of the Main components (gypsum, calcium carbonate, iron hydroxide, calcium aluminum Silicates) their thermodynamic activity and thus their solubility towards the Eigenminerals is reduced by orders of magnitude. This effect can targeted by adding certain mixed crystal formers (e.g. barium compounds in Form of reducing spar residues or wall paint residues on demolition materials) for specific restraint, d. H. fixation of radium.

Embodiment

A concrete residue stockpile of uranium ore processing is considered. A former sludge pond of a processing plant contains the residues a multi-year uranium production from ores of different origins.

The deposit is about 800 m long in a natural brook valley (after piping and superstructure of the stream and later relocation) and upstream and downstream through dams up to 40 m high Tailings material delimited.

During the course of uranium production, there were approximately 3.36 million t of processing residues (Tailings) washed in up to a maximum layer thickness. The residue material comes from the sulfuric acid leaching of the ores and was after Pre-neutralization with a pH of about 3 and one Production process corresponding Eh value of approx. 600-900 mV (pE = 10-15) spent. After a 20-year rest in which the mud pond settled was left alone, as additives in the sense of the invention and commercial waste up to 9 m thick. By the onset Composting with landfill gas formation was in addition to methane about 5 billion grams Equivalent carbon dioxide is generated by the calcium bicarbonate reaction with the Calcium sources in the garbage (mainly demolition waste) are 6-7% as Bicarbonate with which leachate was transferred to the tailing layer. Next this neutralizing surcharge was made with the leachate from the  Aggregates that reduce the waste layer, mainly sulfide in the tailing layer brought in.

As a result of infiltration with those with neutralizing and reducing Impacted leachate was used in the formerly acidic and oxidized tailing layer built up a buffering capacity with respect to the Base capacity with an average of about 250 milligram equivalents of 1 kg Residual material and its reduction capacity (poorly soluble Fe (II) - Compounds) is approximately 150 milligram equivalents / kg.

The state parameters in the pore water of the residue layer after the preliminary On-site renovation is at pH 6.6-7.5, the redox potentials at -60 to 180 mV (pE = -1 to 3). Taking into account the composition of the water body the heavy metal compounds lie in the residue layer Condition in the form of their poorly soluble sulfates (lead, calcium), - carbonates (Magnesium, partially calcium, iron), - hydroxides (iron, zinc, copper, uranium, Thorium u. a.), - and as sparingly soluble metalates (arsenic, vanadium, phosphorus u. a.) - before. Most of these compounds are in the gypsum, which is approx. 16% by weight. is included, so that the buffer capacities mentioned only one Record a fraction of the total buffer capacity. That to about 1.3 kg in the Total radium is present as barium sulfate in gypsum locked in.

As a result of this on-site renovation, the dissolved heavy metals are already in the free water of the residue layer only in very low concentrations (see table).

In the surface water that runs right next to the deposit, the has proven hydraulic contact with the water body of the deposit no significant increase in pollutant values after passing the Evidence of deposition area.

A stabilization of the deposit was finally made several meters high Layer of brown coal ash deposited. The renovation is done by applying a culture layer, the seepage to 10-20% of the middle Precipitation is set, completed.  

table

Concentration of heavy metals and dissolved parts after the on-site renovation

Claims (7)

1. Process for on-site remediation and chemical long-term stabilization of deposits with sulfidic components, in particular from ore mining and ore processing, in which the heavy metal and natural radio element cations are immobilized, characterized in that an acid base in the deposit - and redox buffer capacity, consisting of alkaline earth aluminum iron (II, III) hydroxides, - carbonates and - silicates is generated and stabilized, which converts the heavy metal and radio element cations into poorly or insoluble compounds, which in precipitation of Main components such as B. iron hydroxide and gypsum are included, and that carbonic acid-developing additives in combination with alkaline earth-containing additives are incorporated or stored, through which the leachate is passed and enriched with carbonic acid so far that an increased dissolution of alkaline earth carbonates from the additives to neutralize the acid present in the uncleaned deposit (neutralization phase) a bicarbonate input of 1-10 milligram equivalent / l is guaranteed, which leads to the precipitation of the heavy metals dissolved in the form of poorly soluble hydroxides and carbonates and that after neutralization of the pore water in the deposit to maintain the neutral state (stabilization phase) a carbonate entry is guaranteed according to the solubility of calcium carbonate. 2. The method according to claim 1, characterized in that in the The carbonic acid remediation phase mainly through natural bacterial Degradation processes in compostable or humus-containing parts of the Aggregates is generated. 3. The method according to claim 1 and 2, characterized in that after Completion of the renovation phase or after consumption of the compostable or humus-containing proportions of the additives, d. H. in the Stabilization phase, through the remaining stock of undissolved Alkaline earth carbonates and due to the neutralization in the deposit released carbonic acid by converting it into soluble bicarbonates first bound in the leachate and with it the deposit again is supplied, bicarbonate entry mentioned in claim 1, the at least corresponds to the equivalent of the acid newly formed by the sulfide oxidation, is secured.   4. The method according to claim 1 to 3, characterized in that the The permeability of the cover layer can be greater than the permeability the deposit. 5. The method according to claim 1, 2, 3 and 4, characterized in that as Aggregates industrial waste products with basic and reducing Ingredients such as ash, alkaline earth metal hydroxides and carbonate and waste products containing hydroxide, demolition waste, municipal waste, sewage sludge, Reduction residues in combination for incorporation or as Underlay in connection with mineral soils with low permeability be used. 6. The method according to claim 1 to 5, characterized in that a Cover the deposit with an oxygen-depleting layer, e.g. B. a cultural layer. 7. The method according to claim 1 to 6, characterized in that the Additives also for the specific retention of the radium Contain barium compounds with the metal ions to be retained poorly soluble mixed crystals, e.g. B. form barium sulfate.