Background of the Invention [0002] Disposal of contaminated waste materials is a major problem throughout the world. Waste materials contaminated with heavy metal ions and other inorganic ions requires complex and costly processes before these waste materials can be properly disposed of. One of the most difficult forms of waste to be disposed of are the mercury contaminated materials.

[0003] Treatment of mercury bearing waste by encapsulation is well-known in the prior art. Rechichi in U. S. Patent No. 6,399, 848 discloses a method''of encapsulating mercury by adding the mercury to a settable slurry composition comprising of calcium carbonate and a caustic magnesium oxide. Upon the setting of the slurry composition, the slurry composition encapsulates the mercury and provides low level of leaching.

[0004] The addition of sulfur or sulfur compounds to stabilize mercury waste is also not new. In U. S. Patent No. 4,844, 815, Ader et al describes a method for reducing the leachability of mercury containing waste to an acceptable level by using a mixture of elemental sulfur and cement kiln dust [00051 Similarly, Kaczur et al. in U. S. Patent No. 4,354, 942 discloses a process for stabilizing soluble mercury deposits by using inorganic sulfur compounds including sulfides, alkali metal thiosulfates, alkaline earth thiosulfates, iron thiosulfates, alkali metal dithionites, and alkaline earth metal dithionites. By using these inorganic sulfur compounds, the soluble mercury is converted to insoluble mercury compounds which is less prone to leaching and thereby reducing the leachability of the final products.

[0006] Heat treatment of some sulfur treated mercury wastes have further proved to be more effective in producing insoluble mercury compounds. Adams in U. S.

Patent No. 5,347, 072 and in U. S. Patent No. 5,562, 589 discloses a method to stabilize a mixture of inorganic substrates containing metals, metals ions, inorganic ions and up to 10 % organic chemicals with sulfur by heat treatment at an elevated temperature greater than the melting point of sulfur to produce an inert carbon sulfur polymer.

[0007] In U. S. Patent No. 6,399, 849, Kalb et al. mixed mercury-contaminated waste with a sulfur polymer cement under an inert atmosphere before heating the mixture to produce a molten product as a monolithic final waste form. In order to improve the leaching properties of the final product, additional sulfur polymer cement could be added as additive.

[0008] Autoclaving is a well-known process used for various applications such as sterilizing of equipments. However, since autoclaving involves the increasing of temperatures via steam, it can be also be used for enhancing the properties of materials. Suzuki et al in Patent publication No. 2002-255671 provide a method for producing inorganic hardened body from fiber and cement by using an autoclaving process.

100091 Ladle furnace slag is currently a waste material available at no cost and is a by product of steel manufacturing processes. Ladle furnace slag is considered one of the most difficult waste products to dispose of. Current practices simply disposes of ladle furnace slag by burying them in landfills. While this may appear a simple solution, this practice can create problems for regions where land is a highly priced commodity like in the island state of Singapore.

[0010] There have thus been several attempts at recycling ladle furnace slag by re- introducing ladle furnace slag as a raw material in construction industry. Caijun Shi and Shunfu Hu in the publication"Cementitious properties of ladle slag fines under autoclave curing conditions", Cement and Concrete Research 2430 (2003) (uncorrected Proof Copy) disclosed the use of ladle slag fines in production of construction products. Particularly, the publication disclosed that ladle slag fines when combined with siliceous materials, Portland cement and hydrated lime and after undergoing a hydrothermal process forms a high strength cementitious monolith compound suitable for the construction industry.

10011l However, such processes disadvantageously involves several steps and requires the addition of several-additives and raw materials (e. g. Portland cement and hydrated lime) before a high strength monolith compound is achieved.

Purchasing such additives and raw materials can disadvantageously add to the cost of recycling.

[0012] Alternative methods for stabilizing and disposing of heavy metals such as mercury and ladle furnace slags in ways which comply with environmental regulations are always in constant need. It can thus be seen that there exists a need for a simple and effective method for stabilizing waste heavy metal compounds that can overcome the disadvantages of the existing prior art.

Summary of the Invention 10013] The present invention seeks to provide a method for stabilizing and encapsulating waste mercury compounds using ladle furnace slag.

10014] Accordingly, in one aspect, a method for stabilizing and encapsulating mercury compounds using ladle furnace slag, comprising the steps: preparing a matrix slurry having a cementitious matrix, the matrix slurry comprising of ladle furnace slag particles and at least one alkaline activator ; precipitating mercury ions of the mercury compounds into the cementitious matrix of the matrix slurry by adding and mixing the mercury compounds into the matrix slurry ; and enhancing precipitation of the mercury ions via thermal treatment.

[0015] In another aspect, the present invention provides, a method for stabilizing and encapsulating waste mercury compounds comprising the steps: preparing a matrix slurry having a cementitious matrix, the matrix slurry comprising about 55% by weight ladle furnace slag particles, about 18% by weight of 2M sodium hydroxide solution and about 27% by weight of sodium silicate; adding and mixing waste mercury compounds amounting to about 1% by weight of ladle furnace slag particles into the matrix slurry ; and autoclaving the matrix slurry at 180°C for 12 hours.

[0016] In a further aspect, the present invention provides, a method for stabilizing and encapsulating waste heavy metal compounds comprising the steps: preparing a matrix slurry having a cementitious matrix, the matrix slurry comprising about 61% by weight ladle furnace slag particles, about 8% by weight of 2M sodium hydroxide solution and about 31% by weight of sodium silicate ; adding and mixing waste heavy metal compounds amounting to about 0.5% by weight of ladle furnace slag particles into the matrix slurry; and autoclaving the matrix slurry at 180°C for 12 hours. [0017] In yet another aspect, a cementitious matrix composition for stabilizing and encapsulating waste heavy metal compounds comprising : about 55% by weight ladle furnace slag particles, about 18% by weight of 2M sodium hydroxide solution and about 27% by weight of sodium silicate; wherein the ladle furnace slag particles are of 45 um and below.

[00181 In yet a further aspect, a cementitious matrix composition for stabilizing and encapsulating waste heavy metal compounds comprising: about 61% by weight ladle furnace slag particles, about 8% by weight of 2M sodium hydroxide solution and about 31% by weight of sodium silicate; wherein the ladle furnace slag particles are of 250 urn and below.

Brief Description of the Drawings [0019] A preferred embodiment of the present invention will now be more fully described, with reference to the drawings of which: [00201 FIG. l illustrates a flowchart for a method for stabilizing waste heavy metal compounds using ladle furnace slag in accordance with the present invention ; Detailed description of the Drawings [0021] A method for stabilizing waste heavy metal compounds using ladle furnace slag with a preferred embodiment is described. In the following description, details are provided to describe the preferred embodiment. It shall be apparent to one skilled in the art, however that the invention may be practiced without such details.

Some of the details may not be described at length so as not to obscure the invention.

[00221 The present invention advantageously utilizes one waste from one industry to stabilize waste heavy metal compounds and reduce the leachability of these waste heavy metal compounds to acceptable levels. Particularly, ladle furnace slag is used to form an activated matrix slurry for stabilizing waste heavy metal compounds such as mercury compounds.

[0023] Ladle furnace slag under normal conditions is a non-reactive material comprising mainly of gamma dicalcium silicate (y-Ca2SiO4), gehlenite (Ca2A12Si0) and bredigite (Cal4Mg2 (Si04) 8). However, when subjected to an alkali-activated process with thermal treatment, the non-reactive ladle furnace slag undergoes a chemical reaction and forms a durable cementitious matrix capable of advantageously stabilizing mercury ions. The mercury ions are precipitated into stable heavy metal compounds like mercury sulfides and are encapsulated by the matrix slurry as the matrix slurry sets into a monolith structure.

[0024] In addition to the above compositions, ladle furnace slag also differs in the amounts of impurities and additives found in the slag due to the various processes and practices in various countries. For example, ladle furnace slag used in the production of construction products in the publication by Caijun Shi and Shunfu Hu,"Cementitious properties of ladle slag fines under autoclave curing conditions", Cement and Concrete Research 2430 (2003) (uncorrected Proof Copy) uses ladle furnace slag that contains substantial amounts of fluorides which were added as fluxing agents.

100251 The ladle furnace slag used in the present invention, however uses calcium aluminate as a fluxing agent and therefore differs from the ladle furnace slag as used in the"Cementitious properties"publication. loo2s] Referring to FIG. 1, the method 100 in accordance with the present invention starts with the steps of preparing 110 a matrix slurry comprising the durable cementitious matrix. The matrix slurry comprises of ladle furnace slag particles mixed with at least one alkaline activator, examples of which are sodium silicate and sodium hydroxide solution. In the present description of the invention, the matrix slurry is mixed with both sodium silicate and sodium hydroxide solution. The ladle furnace slag particles have first been sieved before being added into the matrix slurry. The sieved ladle furnace slag particles are sieved according to at least two sizes. Non-limiting examples of possible sieved sizes are 250um and below, 75, um and below and 45gm and below.

[0027] Next, the step of precipitating 115 the heavy metal ions is performed by adding and mixing the mercury waste compounds into the durable cementitious matrix of the matrix slurry. The mercury ions begin to advantageously precipitate 115 into the matrix slurry forming mercury oxides, mercury hydroxides and mercury sulphides.

[0028] Sulfur is bonded in ladle furnace slags during steel desulfurization in the slag reaction process. Most of this sulfur is found in the ladle furnace slag as calcium sulfides and also free form sulfur. The presence of sulfur and sulfur compounds in the ladle furnace slag particles advantageously aid in the precipitation 115 of the mercury ions into the durable cementitious matrix of the matrix slurry. This advantageously omits the step of adding additional sulfur or sulfur additives into the matrix slurry.

[0029] Subsequently, the step of setting 120 the matrix slurry is performed to improve the manageability of the matrix slurry. The matrix slurry may be left to stand in a mold or in a container. Once the matrix slurry has set into a relatively manageable state or into a slurry block, i. e. the matrix slurry has set sufficiently to allow manipulation and ease of transport, the slurry block is demolded from its mold or removed from the container.

[0030] The slurry block then undergoes the step of enhancing 125 of precipitation of the mercury ions into the durable cementitious matrix via thermal treatment. A preferred means of the thermal treatment is by way of autoclaving. The slurry block is placed in an autoclaving unit and undergoes autoclaving treatment at about 180°C for about 12 hours. This autoclaving process helps to further release the sulfur from the ladle furnace slag particles into the durable cementitious matrix of the slurry block for further precipitating 115 of any remaining mercury ions in the slurry block.

10031] At this stage, recrystallization of the less soluble mercury compounds occur. which results in these less soluble mercury compounds being encapsulated within the durable cementitious matrix.

[0032] The final product after autoclaving is a solid monolith resembling a block of concrete. The solid monolith may then be used as substitute aggregate for replacing natural aggregate in land reclamation projects.

10033] The following non-limiting examples further serve to illustrate the preferred embodiments in accordance with the present invention and it will be appreciated that various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention.

Example 1 [0034] A matrix slurry is first prepared using 30 g of ladle furnace slag having particle size of 45, um and below, 15 g of undiluted laboratory grade sodium silicate solution (Na2SiO4) and 10 g of 2M sodium hydroxide solution (NaOH) as the alkaline activator.

10035] Mercury nitrate of up to 0. 3 g is used to simulate mercury waste and is added to the matrix slurry and well mixed The matrix slurry is allowed to harden and set for about 24 hours before being autoclaved for 12 hours at an autoclaving temperature of 180°C. The resulting solid monolith (cylindrical in shape for this present example) was found to have a compressive strength of around 12 N/mm2.

A Toxicity Characteristics Leachate Procedure (TCLP) test was performed on the resulting monolith and results showed that amount of leachable mercury present was less than 0.2 ppm.

100361 The composition of the materials used amounted to about 55% by weight of ladle furnace slag particles of 45 lim and below, about 18% by weight of 2M sodium hydroxide solution and about 27% by weight of sodium silicate. The amount of waste heavy metal compounds to be encapsulated by the matrix slurry amounts to about 1% by weight of the ladle furnace slag particles.

Example 2 10037] A matrix slurry is first prepared using 30 g of ladle furnace slag particles having particle size of 250 um and below, 15 g of undiluted laboratory grade sodium silicate solution (Na2SiO4), and 4 g of 2M sodium hydroxide solution (NaOH) as the alkaline activator.

[0038] Mercury nitrate of up to 0.15 g is used to simulate mercury waste and is added to the matrix slurry and well mixed. The matrix slurry is allowed to harden and set for about 24 hours before being autoclaved for 12 hours at an autoclaving temperature of 180°C. The resulting solid monolith (cylindrical in shape for this present example) was found to have a compressive strength of around 15 N/mm2.

A Toxicity Characteristics Leachate Procedure (TCLP) test was performed on the resulting monolith and results showed that amount of leachable mercury present was less than 0.2 ppm.

[0039] The composition of the materials used amounted to about 61% by weight of ladle furnace slag particles of 250, um and below, about 8% by weight of 2M sodium hydroxide solution and about 31% by weight of sodium silicate. The amount of waste mercury compounds to be encapsulated by the matrix slurry amounts to about 0. 5% by weight of the ladle furnace slag particles.

[0040] The above description and examples serve to illustrate the preferred embodiments in accordance with the present invention and it will be appreciated that various modifications and improvements can be made by a person skilled in the art without departing from the scope of the present invention.