Use of Chelating Agents for Enhancing the Settling of Slimes

This invention relates to a process for separating finely-divided solids that are suspended in an aqueous medium and, more particularly, to a process for treating a colloidal suspension of clay, alone or in combination with other solid materials, to enhance the settling of the solids.

Clays having a micaceous or platey structure such as bentonite (a montmorillonite clay) acquire the properties of self-suspension, swelling and gelatinizing when they are mixed with water. The clays are advantageously used for these unique properties in many industries. An example of such a beneficial use is in the well drilling industry where bentonite is a component of drilling fluids to suspend the cuttings and carry them out of the well bore.

These same properties of some clays present considerable problems in other industries when it is desired to remove the clays and other materials from suspension in an aqueous medium. For example, many foundries use a sand washing operation that produces as an undesirable by-product a wash water containing on the order of about 0.2 to about 1.2% by weight of suspended bentonite, carbonaceous material and fine sand. The disposal of this waste product has become a problem in view of recent stringent pollution regulations. The use of standard filtering methods and apparatus for separating the finely-divided material from the waste water before discharging or recycling the water is not entirely satisfactory since the fine suspended particles rapidly blind the filter media.

The colloidal behavior of certain clays upon exposure to water presents a very serious problem when hydraulic methods are employed to effect concentration of ore materials containing the clays. These operations, as exemplified by phosphate ore beneficiation processes used in Florida, produce tremendous volumes of aqueous suspensions of ultrafine material that are extremely difficult to de-water. These suspensions are commonly called "slimes." These slimes, which are encountered by metallurgists and mineralogists, are different from the slimes formed by certain microorganisms or fungi such as Myxomycetes slime molds so that treatments for destroying the latter slimes have no application in solving the "slime problem" encountered in the mineral industry.

Mining of phosphate ore in Central Florida is done by an open-pit method. The practice is to strip the overburden to expose the phosphate ore, called the "matrix," for mining. The thickness of the matrix ranges from one to fifty feet, and it is typically composed of about 1/3 phosphate, 1/3 silica sand and 1/3 clay. The matrix is mined with electric-powered walking draglines and the mined ore is hydraulically conveyed from the mining site to the beneficiation plant. In the typical phosphate beneficiation process, the matrix is initially sized and washed to produce a -3/4 inch + 16 mesh (Tyler Standard Series) pebble product. The -16 mesh material is then subjected to a desliming operation to remove substantially all of the -150 mesh waste material prior to the beneficiation of the -16 + 150 mesh material.

The suspension of the -150 mesh waste slime material is continuously transferred to a settling pond built in a mined-out area. The solids slowly settle to produce an upper layer of clarified water that is recycled to the plant for further use, until the settled slimes reach the top of the dam. The mineralogical composition of the slime solids will vary depending upon the location where the matrix was mined. The occurrence of specific minerals in phosphate slimes is somewhat consistent, but the relative amounts of the components vary considerably depending upon the manner in which the sedimentary phosphate rock deposits were formed. Montmorillonite and attapulgite are present in Florida phosphate slimes along with other materials such as fluorapatite, quartz, fluorspar and heavy minerals. The montmorillonite and attapulgite, which are well-known for for their colloid-like behavior in water, cause the characteristically slow settling property of the slimes.

It is necessary to continuously replace filled dams with newly constructed ones in view of the slow settling property of the slimes. Slimes that are transferred to a dam or impounding area from a beneficiation plant may contain as little as about 1.5 to 3% solids. The solids content is increased to only about 15% after three months of settling, and from that time on further densification is extremely slow. A state of equilibrium of only about 25% solids is reached after several years of storage. Therefore, the water content of settled slimes is so great that the storage volume required for the slimes is greater than the volume of the matrix that was removed.

The volume of a quantity of slimes does not appreciably decrease even after many years of storage. Evaporation over a number of years forms a thin crust of material containing about 20% water on the surface of the slimes. At this level of moisture, the crust is virtually impervious to the transfer of moisture so that further evaporation from under the crust does not take place. The slime solids cause the formation of an impervious layer that prevents seepage of water through the dams and the bottom of the impounding area. It has been estimated that there are over 1.5 billion tons of phosphate slime solids being stored in dams in Florida.

Considerable effort has been expended to find an economic and efficient method for bringing about the complete and rapid settling of the extremely fine solid materials. A solution to this problem would be of significant benefit since there are phosphate values in the slimes that could be recovered, substantial areas of land are used for storing the slimes, and the storage of the slimes poses an environmental problem. Also, the quantity of water associated with the stored slime solids, which is estimated to be about 4.5 billion tons, has increasing importance since the availability of water in the Central Florida area is becoming more restricted.

Although considerable research has been conducted by the industry, as well as governmental and other organizations, to find a suitable method for de-watering slimes, no completely satisfactory solution has yet been found. The various methods that were considered include conventional filtration, pressure filtration, hydrocycloning, centrifugation, electrical methods such as electrophoretic and electroosmotic methods, and the addition of various chemicals such as flocculants and dispersants to modify the properties of the clay.

Reagents which are known to be flocculants for other systems do not to any significant degree bring about the accelerated settling of slime solids. A report on the evaluation of a variety of flocculating reagents is given in the Bureau of Mines Report of Investigations 6163 entitled "CHEMICAL AND PHYSICAL BENEFICIATION OF FLORIDA PHOSPHATE SLIMES," published in 1963. The maximum settling over a 24-hour period was obtained by using 18 to 140 pounds of citric acid per ton of solids. The most rapid initial settling was obtained in 10 minutes with certain high molecular weight polymers, but after 24 hours the settled value was less than that obtained when citric acid was used.

As reported in our U.S. Pat. Application Ser. No. 643,447, filed Dec. 22, 1975 now Pat. No. 4,017,392, we have found that the settling of solids from an aqueous suspension of finely-divided solids including colloidal argillaceous material, such as phosphate slimes, is enhanced by the addition to the suspension of an organic peroxide having a molecular weight not in excess of about 250.

This invention is based on the discovery of a process for treating an aqueous suspension of finely-divided material including colloidal argillaceous minerals to accelerate the settling of the solids. In accordance with this invention, the rapid settling of the solids in suspension is brought about by the addition to the suspension of a small but effective amount of at least one chelating agent. The process of this invention has a variety of applications in industry, but it is especially useful for settling solids from slimes resulting from a hydraulic treatment of clay-containing ores.

It is, therefore, a primary object of this invention to provide a process for treating aqueous suspensions of finely divided solids including clays that exhibit colloidal behavior in water.

Another object of this invention is to provide a process for accelerating the settling of finely-divided material including colloidal argillaceous minerals from an aqueous suspension thereof.

Still another object of this invention is to provide a process for accelerating the rate of settling of the solid content of slimes resulting from the hydraulic treatment of ore materials.

A further object of this invention is to provide a process for treating phosphate slimes to accomplish the rapid settling of the solid content thereof.

A still further object of this invention is to provide a process using a chelating agent for treating phosphate slimes.

These and further objects of this invention will become apparent or be described as the description thereof herein proceeds.

In accordance with this invention, the settling of finely divided solids from an aqueous suspension of the type described below is enhanced by adding to the suspension a small but effective amount of at least one chelating agent. Chelating agents that are effective in the process of this invention may be categorized into two classes of compounds, namely, hydroxylated alkane dicarboxylic acids (malic acid and tartaric acid) and carboxymethylated amines (nitrilotriacetic acid and N,N,N.sup.1,N.sup.1 -ethylenediaminetetraacetic acid).

The place, time and method of adding a chelating agent in accordance with this invention are not critical and this flexibility is one of the advantages of this invention. For example, the chelating agents of this invention may be continuously added to slimes as they are introduced into the impounding areas, or the settling agents may be added to an inactive impounding area. The only requirement is that the treating agent be sufficiently distributed among the suspension being treated to accomplish the settling. They may be added neat, but are sometimes advantageously added in an aqueous solution containing, as for example, a chelating agent content of from about 0.1 to about 25%. The use of very dilute solutions has the disadvantage of using excessive quantities of water, especially when slimes are being treated.

The amount of the agent utilized in accordance with this invention is the quantity that is necessary to produce the desired degree and rate of settling of the slime solids. There will be imcomplete settling of the slime solids if less than an optimum quantity of a settling agent is used. On the other hand, there will be no advantage in using quantities in excess of the optimum quantity, and the use of an excess quantity might even have an adverse effect. The proper amount to achieve the desired result will be readily ascertainable by one skilled in the art. This will depend upon the compound or compounds being used and the composition of the volume of slimes being treated, including the total quantity of solids in suspension, the age of the suspension, the percentage of clay in the suspension, the relative amounts of the different suspended solids, and the like. The composition of the slimes will vary in any general area, depending upon the specific area being mined. In general, a chelating agent (or mixture of chelating agents) will be employed in amounts ranging from about 20 to about 200 pounds per ton of suspended mineral solids.

The aqueous suspensions that can be treated according to the process of this invention to accelerate the settling of the finely-divided content thereof generally include those suspensions that result from the presence of clay exhibiting colloidal behavior in water. Attapulgite and montmorillonites are among the types of clays that possess the properties of self-suspension, swelling and gelatinizing in water so as to become suspended in water along with other finely-divided material that might be present. Although the method of this invention has a variety of applications in industry such as for treating wash water from a foundry sand recovery process as hereinabove described, it perhaps has its greatest utility in treating aqueous suspensions, commonly called "slimes," resulting from hydraulically concentrating an ore material containing a clay of the type described above. This invention is particularly useful for treating phosphate slimes resulting from the hydraulic treatment of phosphate ore such as currently being used in Florida.

The percentage of solids in suspensions which respond to treatment by the present invention is not critical. Desirable results may be obtained over a wide range of solids content. The method of this invention will generally be used when the solids content is as low as about 0.01% or as high as about 35% by weight, most commonly from about 1 to about 10% by weight, but it is also useful for treating suspensions having lower and higher solids contents.

The following non-limiting examples are illustrative of the present invention.

This experiment illustrates the beneficial effect of using one of the chelating agents of this invention for flocculating and settling finely-divided material from an aqueous suspension. The suspension which was utilized in this experiment was a sample of phosphate slimes containing 6.35% by weight of suspended solids. Samples of 100 milliliters each of the slime were added to three 200 milliliter graduated cylinders. The compound dl-malic acid was added to two of the samples in the amounts of 0.1% and 0.3%. The third sample was used as a control. The graduated cylinders were each inverted several times to allow thorough mixing and the volume of clear water at the upper portion of each sample was observed and recorded over a seven-hour period. The results of this experiment, expressed as percentage of total volume as clear water above solids, are given below in Table 1.

The effectiveness of dl-malic acid in bringing about the rapid settling of solids from the phosphate slimes is readily apparent from a review of the data given in Table 1. It will be noted that the addition of dl-malic acid increased the settling rate of the solids from the suspension. For example, the use of 0.3% dl-malic acid accomplished more settling in one hour than took place in seven hours in the control sample. This increased settling rate is especially advantageous in the phosphate industry since it will produce large volumes of water for recycle to the ore treating operation in a relatively short time period.

The procedure of Example I was repeated utilizing four 100 milliliter samples of a slime containing 3.18% of suspended solids. Two of the samples were used as controls and 1-tartaric acid was added to the other two samples in amounts of 0.1% and 0.3%. The difference between the rates of settling of solids in the samples is shown in Table 2.

Still another experiment was conducted to test the effectiveness of N,N,N.sup.1,N.sup.1 -ethylenediaminetetraacetic acid in bringing about the rapid settling of suspended solids from phosphate slimes. The general procedure of Example I was utilized in this experiment with a phosphate slime containing about 5% solids. One sample was used as a control, and the compound under evaluation was added to a second sample in the amount of 0.1% and to the third sample in the amount of 0.3%. The results of this experiment are shown in Table 3.

The results shown in Table 4 were obtained when the procedure outlined in Example I was again repeated to test the effectiveness of 0.1% nitrilotriacetic acid when it was added to phosphate slimes containing about 6.35% solids.

It will be apparent from the foregoing examples that certain chelating agents are effective in enhancing the settling of finely-divided solids from aqueous suspensions. The scientific principle upon which the chelating agents are effective in bringing about the rapid settling is not understood, especially since the effectiveness of various chelating agents is very unpredictable. Some compounds which are known to be useful as chelating agents are not effective for improving the settling of slimes even though they are closely related in structure to chelating agents that are effective. As for example, nitrilotriacetic acid, N(Ch.sub.2 COOH).sub.3 is effective as demonstrated in Example IV, whereas iminodiacetic acid, HN(CH.sub.2 COOH).sub.2, and glycine, H.sub.2 NCH.sub.2 COOH, are ineffective. Another example is the effectiveness of N,N,N.sup.1,N.sup.1 -ethylenediaminetetraacetic acid (EDTA), as compared to the ineffectiveness of two commercial chelating agents, N-hydroxyethyl-N,N.sup.1,N.sup.1 -ethylenediaminetriacetic acid and diethylenetriaminepentaacetic acid. Another compound that is structurally related to EDTA but is ineffective in the process of this invention is 1,2-cyclohexanediamine tetraacetic acid. Two diamine chelating agents were also found to be ineffective in promoting faster settling of slimes. These are N,N.sup.1 -disalicylidene ethylenediamine and N,N.sup.1 -disalicylidine propylenediamine, both of which were tested at 0.1%.

Other chelating and complexing agents which were found to be ineffective at 0.1% and 0.3% are glycolic acid, oxalic acid, aconitic acid, itaconic acid, lactic acid, gluconic acid, succinic acid, and 1,2,3-propanetricarboxylic acid.

Although this invention has been described in relation to the specific embodiments, one skilled in the art may make obvious modifications without departing from the intended scope of the invention as defined by the appended claims.