Dewatering Method and Agent

1. Field of the Invention

The invention relates to dewatering methods and agents used in these methods, especially those methods and agents used for dewatering sludges.

2. Background Art

Dewatering of various types of sludges, such as mineral sludges generated by phosphatic ore mining, has been a subject of intense research due to the economic and environmental impact of disposing of the sludges or, alternatively, of recovering the solids from the sludges by dewatering processes. For instance, the mining and processing of phosphatic ores (apatites) in Florida generates an estimated 100,000 tons of phosphatic waste clay solid suspensions each day. Such suspensions, which are generated as sludge-like slurries, contain 3 to 10% solids, and have historically been pumped to large, above-ground holding ponds where water is decanted through spillways as the solids slowly consolidate under the impact of gravity to a 15-18% solids level. At this solids level the ponds slowly dehydrate and form a crust on their surface which retards further surface evaporation. Without additional physical efforts to dewater the mass, it then may take several decades to consolidate the crusted pond to a solids content of 25-35%. Because as much as 60% of the land surface in a mining area is covered with these ponds, they represent a considerable economic penalty to the mining industry and preclude the profitable use of thousands of acres of central Florida land. This conventional practice also ties up millions of gallons of water for years, losing much of the retained water to evaporation. The severe economic costs of these traditional practices of handling waste clays have thus prompted the mining industry to experiment with methods that dewater the clays more rapidly.

Several dewatering methods are used to obtain a more rapid consolidation. Such procedures have been extensively described in the literature and include (a) sand spraying, (b) dredge-mix, (c) sand-clay mix, and (d) sand-clay flocculation and settling. The most economical methods include the sand-clay mix and sand-clay flocculation methods. In the sand-clay mix method, clays are settled for about a year in a large pond until they reach 15 to 18% solids. The clays are then dredged and mixed with (flotation) tailings in a ratio which, on further consolidation, yield a stable, arable soil. However, the dredging and mixing process may require another three to four years.

The sand-clay flocculation process, known as the Estach process, involves the flocculation of the sand and clay. The mixture is processed through a rake type thickener, mixed with additional sand and then pumped into a disposal area. The 12% slurry consolidates rapidly to about 30% solids in the first year and reaches the 40% level in the next three years.

As indicated above, both of these methods still require years before a mined out area can be returned to productive use. Even further, actual practice indicates that there is no universal method due to site-specifics in regard to types of clays, location of settling areas, economics, etc.

More effective dewatering methods are described in the U.S. Bureau of Mines Reports of Investigations, e.g., Investigation Nos. 8611 (Large-Scale Dewatering of Phosphatic Clay Waste from Central Florida), 8349 (Flocculation Dewatering of Florida Phosphatic Clay Wastes), and others. The investigators in those reports used several polymer flocculating agents, including polyethylene oxide (PEO) polymers and polyacrylamides. The investigators found that PEO polymer of about eight million molecular weight alone was effective as a dewatering agent. They further found that PEO polymer alone was as effective as PEO polymer and polyacrylamides combined, and was more effective than the polyacrylamides alone. See for example, U.S. Pat. No. 4,931,190 to Laros. However, while rapid dewatering is achieved using these methods, the PEO polymer is quite expensive and dewatering after 90 days still does not reach a 30% solids level.

The method described in the above-mentioned '190 patent to Laros also employs PEO polymer, but employs it in combination with a cheaper polymer such as anionic polyacrylamide flocculating agents. By employing this reportedly synergistic combination of flocculating agents in conjunction with a sand slurry, Laros reportedly is able to consolidate a sludge containing 3 to 5% solids by weight into a filter cake having an excess of 40% solids. However, Laros still requires the use of expensive PEO polymer.

Further, PEO polymer is specific in regards to the type of clays for which it will induce flocculation. Because the type of clay varies from mine site to mine site, certain sites may not be able to employ PEO polymer in their dewatering processes.

Polymer flocculating agents have also been employed in other fields of dewatering. For example, PEO polymer and acrylamide polymers have been employed in a method of dewatering organic sludges. See U.S. Pat. No. 4,710,298 to Noda et al. Noda et al.'s method employs one of these polymers with a fibrous material which prior to being added to the sludge is pressed or watered to a specific gravity of at least 0.3 g/cm.sup.3. However, the method described therein cannot be economically and efficiently used to dewater sludges containing mineral solids such as phosphatic wastes.

The present invention generally relates to dewatering methods and agents and more specifically relates to methods and agents for treating, recovering, rehabilitating and recycling mineral residue of mining operations, in particular to the removal of water and the recovery of solid particulate from aqueous mineral sludges which result from such mining operations. The method and agents of this invention also have other applications as explained herein.

One embodiment of this invention is a method of dewatering a mineral-containing aqueous sludge having a dispersed solids content, the method comprising

(a) preparing a pulp of microfibers by disintegration of a fibrous material;

(b) providing an aqueous solution or emulsion of flocculating agent;

(c) activating flocculation of the solids in said sludge by introducing to the sludge said pulp of microfibers and said aqueous solution or emulsion of flocculating agent;

(d) allowing an accumulation of flocs to form; and

(e) recovering and separating the floc accumulation from the water of the aqueous sludge by a dewatering apparatus.

The microfiber additive is prepared by disintegration of a fibrous material, such as waste newspaper or the like, into an accumulation of finely divided individual fiber elements, preferably by means of mechanical agitation, such as by suspension in a liquid and agitation by an impeller. The mineral-containing sludge can be easily consolidated into a cake comprising 25-35% solids by using this method.

As indicated above, the method of this invention can be employed in treating, recovering, rehabilitating and recycling mineral-bearing earth residue which has been extracted during a mining operation. Specifically, that embodiment of the invention comprises

(a) preparing a pulp of microfibers by disintegration of a fibrous material;

(b) providing an aqueous solution or emulsion of flocculating agent;

(c) activating flocculation of the solids in said sludge by introducing to the sludge said pulp of microfibers and said aqueous solution or emulsion of flocculating agent;

(d) allowing an accumulation of flocs to form;

(e) recovering and separating the floc accumulation from the water of the aqueous sludge by a dewatering apparatus; and

(f) returning said recovered floc accumulation to the earth.

With modification by further adding clay to the flocculating agent and microfiber additive in a manner hereinafter set forth, the method according to another embodiment of this invention can be used to dewater organic sludges as well.

FIG. 1 is a flow sheet in schematic representation of a dewatering process employing the method according to this invention.

FIG. 2 is a micrograph of a flocculate which results from the dewatering method according to this invention.

A unique combination of components useful in dewatering mineral sludges is provided. For instance, flocculation within a dewatering process for sludges occurs in the simultaneous presence of a pulp of finely divided individual fibrous microstructures (hereinafter "microfibers") together with a dispersed field of flocculating agent, preferably in emulsion form. The combination, when properly prepared and introduced in the manner set forth herein, causes coagulation of the solids within the sludge to form flocculates (flocs) which possess enhanced mechanical strength and size characteristics over those produced by prior art methods. Because of these important physical differences, the flocs can be readily separated and recovered by straightforward and inexpensive dewatering techniques such as by gravitational flow of flocculated sludge through a static screen or under pressure through a press. Flocs produced by prior art methods are either too small to be separated by a static screen as just described or lack sufficient mechanical integrity and thus tend to disintegrate and pass through the sieves of strainers or press apparatus.

While not being bound by any theory as to how the mechanism of the present invention provides the advantages realized thereby over the prior art, it is thought that the flocculating agent and the finely divided fibrous microstructure, when properly dispersed within the sludge in accordance with the method of the present invention, act together in a synergistic manner to cause formation of the large size, mechanically reinforced flocs. It is believed that the dispersed flocculating agents, e.g., polymers, form bridging elements between the fine particles of the suspension within the sludge to establish the basic mechanism of floc formation. It is also believed that the large surface area to volume ratio presented by the fibrous microstructure forms an extended surface on which an accumulating network of flocculating agent and particles can form and be mechanically supported. As the flocs grow to a larger size, the fibrous microstructure acts as the support matrix for the floc structure and provides mechanical reinforcement and mechanical strength.

Even further, the method according to this invention provides a way to utilize cheaper flocculating agents such as polyacrylamides and avoids the necessity of more expensive flocculating agents such as PEO polymer. The present invention also provides a dewatering method which employs relatively low concentrations, e.g., 0.1% or less, of flocculating agent. According to this method, the flocculating agent need only consist essentially of a polyacrylamide polymer and the microfiber additive.

The flocculating agent used according to this invention preferably comprises an aqueous solution or emulsion of a polyacrylamide polymer flocculating agent well known in the art. In a more preferred embodiment, the method employs a moderately anionic polyacrylamide. Such flocculating agents are commercially available from Nalco in an aqueous emulsion form known as 7877 high molecular weight polyacrylamide flocculating agent. Such emulsions also typically comprise surfactants. Polyacrylamide flocculating agents are preferred because they are much less expensive than agents such as PEO polymer, and can be used at lower concentrations. Polyacrylamide flocculating agent can be initially prepared to about a 1% by weight solution or emulsion and then can be further diluted to a 0.1% or less solution or emulsion prior to addition to the mineral-containing sludge.

Other flocculating agents which may be used are those well known in the art. See, for example, U.S. Pat. No. 4,710,298 to Noda et al. Such agents include, but are not limited to, PEO polymers, starch, polymers or copolymers of cationic acrylic monomers, vinyl imidazolines, chitosan, polyethyleneimides and epihalohydrinamine condensates.

Another component of the dewatering agent comprises microfibers. A "microfiber" is defined herein as a finely divided individual fibrous microstructure resulting from the disintegration of a fibrous material, preferably processed through high sheer agitation. Dimensions of the fibrous microstructure depend on the material used to prepare the fibrous material. The average length of microfibers typically obtainable from high shear agitation of paper is about 7 mm. The average diameter of the fibrous microstructure is less than 100 microns and typically about 10-20 microns. However, depending on the degree of agitation and disintegration fibrous bundles having diameters of about 250 to 500 microns are also possible. See FIG. 2. Preferable sources of microfiber include, but are not necessarily limited to, organic fibrous materials such as newspapers, magazines and the like. Glass wool and cloth may be used as well.

Agitation means suitable for obtaining a pulp of microfibers include those used to disintegrate and pulp waste paper for recycling. Such agitation means comprise an impeller-equipped mixing tank. The impeller has dimensions suitable for obtaining the desired microfibers. A suitable impeller diameter dimension is about 10 inches. Ultrasonic induced vibration is another suitable agitation means.

In instances when the method employs newspaper as the source of the microfiber, it is preferable that the paper is agitated until the highest pulp density obtainable is achieved. The diameters of the original fibers used to prepare newspaper are typically less than 100 microns, and more typically about 10-20 microns.

For illustrative purposes, a method of using the above-described components is described with reference to dewatering a sludge resulting from phosphatic mining. Generally, these sludges contain waste phosphate, bauxite, talc, clay or clay-like solids, or other solid materials such as barite, all of which, unless specified otherwise, are hereinafter referred to as "clay" or "clay solids." The particle size of such solids are generally less than about 150 mesh and frequently less than 200 mesh. These measurements correlate to a solid particle size of generally less than 50 microns and frequently less than 25 microns. The sludges generated by phosphatic mining generally contain 3-10% by weight of these solid particles and are generally removed from the mining sites in a slurry stream. When using this method, the slurry is routed toward a clay holding tank. Further general processing of the clay sludge is described below.

Before introducing microfibers to the clay sludge, a sand slurry having 20% to 30% by weight solids is optionally added to the sludge. The sand to sludge ratio is about 1:1 on a dry solids basis.

The microfiber slurry is pumped for mixture with the sludge at a precalculated rate to a clay mixing tank where the microfibers are intimately mixed with the 3-10% clay-containing sludge, and sand if present. This mixing can be accomplished with either an agitator or static in-line mixer.

Prior to addition, the preferred microfiber source, i.e., newsprint, magazine or other mixed paper, is disintegrated into microfibers by high shear agitation in water in an impeller-equipped mix tank, thereby providing a pulp or microfiber slurry. The microfiber slurry is prepared at the highest pulp density that can be practicably handled by the agitator.

The clay-microfiber mixture stream is then transferred for mixture with a stream of a 0.1% diluted polyacrylamide flocculating agent. Flocculation takes place immediately as the two streams converge and are mixed by the static mixer. Sufficient time should be allowed after the addition of the polymer flocculating agent and the microfiber in order to allow the flocs to accumulate and form as large a diameter as possible. Diameters of 2 mm or larger, e.g., 12 mm, are particularly suitable and are greatly facilitated by the presence of the microfiber additive for reasons discussed above.

In alternative embodiments, the flocculating agent can be added to the clay slurry at the same time the microfiber is added. Thus, the flocculating agent can be added simultaneously to the addition of microfiber via a separate line, or it may be mixed with the microfiber and then added as a mixture of flocculating agent and microfiber to the clay slurry.

The flocculated slurry is then pumped to a dewatering apparatus, preferably a static dewatering screen or sieve bend, preferably a 10 mesh screen. The flocs are recovered on the screen and separated from the sludge as water passes through the screen openings. The flocs, by virtue of the strength imparted by the microfibers, are strong enough to withstand shear forces and maintain their physical integrity on the screen. The fibers also help the consolidated clays slide down the inclined screen, leaving a clean surface for the flocs that follow. The consolidated clay product resulting from screening typically contains 25 to 35% solids by weight. If a higher solids content is desired, a filter press or "squeeze" belt can be employed to increase the solids content to about 60% or more. Such squeeze devices are well known in the art.

As mentioned above, the method is preferably used with a screen. Screens are preferred for economic reasons and perform satisfactorily relative to other methods. See the examples below. However, in certain instances, other methods, e.g., filtration or centrifugation, may also be used as a dewatering apparatus.

The dewatered solid clays can then be recycled to mine cuts or reclamation areas. The method according to this invention thus provides a suitable method of recycling mineral-bearing earth extracted during mining operations. The dewatered clays produced according to this invention may also be used for other purposes, e.g., dried to form clay bricks, dried and ground to be used for kitty litter, matted as soil amendments, chemically processed to recover residual phosphate values, etc. The method also provides a suitable method of disposing of paper waste.

Water removed by the dewatering screen and the other devices can then be pumped to holding tanks, or thickeners, to allow any suspended solids to settle. The clarified water (overflow) is then pumped back to a washer to be reused. Some water can be used on site for polymer dilution or pulping the fiber.

The amount of each dewatering component in the above-described method will vary, depending on the clay being processed. The amount of flocculating agent depends on the type employed. An acrylamide polymer flocculating agent will generally be used in an amount in the range of 0.2 to 2 pounds per ton of clay solids being processed. The microfiber additive should be added in an amount of at least about 5%, and preferably in the range of 5 to 15% by weight of the dry clay weight.

Referring now to FIG. 1, there is shown a sludge dewatering system utilizing the method of the present invention in a preferred embodiment thereof. In the apparatus shown in FIG. 1, clay sludge, such as the effluent from a mining operation, is introduced into a holding tank 10 where it is accumulated for dewatering. The optional addition of sand is made from a sand storage tank 12 and the mixture is introduced to a mixer 14, which may be of a static or agitation type.

At this point, the microfiber is added from a microfiber slurry holding tank 16 through a metering pump 18 and the mixture of clay sludge and microfiber slurry is discharged at outlet 15. The microfiber slurry is prepared in a pulper 20 in which a fiber source, such as newsprint or other source, is mixed with water and emulsified by means of a high speed rotary cutter 22 driven by an electric motor 24.

Good results in the method of the present invention were obtained by using a proportion of about 5 pounds of paper in the form of scrap newspapers combined with about 95 pounds of water. In the embodiment shown in FIG. 1, the pulper 20 was approximately 23 inches in diameter and about 36 inches high and held about 40 gallons of microfiber slurry. The impeller was about 10 inches in diameter and was rotated at about 1725 RPM for about 0.5 hours while being driven by a 1/4 HP electric motor. It was found that a suitable microfiber slurry was produced under these conditions, although it will be realized, of course, that other conditions can be selected to provide a similar result and that the present invention is not limited to the particular mechanism shown in FIG. 1.

Preparation of polymer flocculating agent is initiated in a tank 26 where it introduced in the form of an emulsion, which can be in the form in which purchased from the supplier as described above. The polymer emulsion is homogenized in the tank 26 by a rotary impeller 28 and is then removed through a metering pump 30 where dilution water is added at 32. The diluted polymer solution is then fed into a mixer 34 where it is mixed to a homogenous form and discharged as a low concentration, i.e., 0.1%, solution at 35.

The diluted polymer solution from the mixer 34 is then mixed with the clay and microfiber slurry from mixer 14 in a static mixer 36 where flocculation occurs in a manner as previously described. The floc sized ranged from 2 to 12 mm in diameter. See FIG. 2. The flocculated sludge is then removed at 30 and is sent through a dewatering screen 40 where the flocculated clay is recovered and sent to reclamation at 42.

Water is recovered in a tank 44 along with any unrecovered portions of the clay sediment, which is allowed to settle for recirculation through the process at 46 as shown. Water is allowed to overflow from the tank 44 where it is removed at 48 and recirculated as process water.

In a typical process embodying the present invention, the recovered sludge at 42 may contain about 25% to 30% solid, which is adequate for reclamation purposes. An even higher density sludge can be recovered using a belt press mechanism 50 to remove additional water and achieve 60% or more solid content in the recovered sludge at 52. As explained above, the method of the present invention is particularly adapted to dewatering with the inexpensive screen method such as by use of the dewatering screen 40, and it is also able to withstand the more highly stressed forced removal under such methods as applied by the belt press mechanism 50.

As indicated above, an unexpected synergistic combination of components has been found for dewatering mineral sludges. For instance, a dewatering agent consisting essentially of an acrylamide polymer flocculating agent and the microfiber additive has been found to dewater an inorganic sludge more effectively than either alone. Further, a combination consisting essentially of those components is relatively inexpensive and efficient for dewatering most clay sludges. Cost savings will thus be realized over those methods which employ PEO alone or which employ PEO in combination with other flocculating agents.

In addition to dewatering mineral-containing sludges, the methods described herein can be modified to dewater organic sludges as well. Such organic sludges include lake sediments, sewage sludge, oil and fat emulsions, etc. The method of this invention is modified by adding a clay slurry having about 3-5% dry solids to the sludge in an agitated tank or in-line mixer prior to conducting the above-described steps. The volume ratio of sludge to clay slurry preferably ranges from 1:1 to 1:5.

Alternatively, dry clays can be added to the sludge before the above-described process is repeated. The amount of dry clay can be in the range of 20 to 100% of the dry weight of the sludge. The dewatered sludge product can then either be disposed of in a landfill or used in other applications (fertilizer, for example) depending on the nature of the sludge.

Tests conducted to illustrate how various dewatering apparatus can be used with the above-described dewatering components are described in the examples below. The tests also illustrate the effects which the invention has on dewatering methods employing the various dewatering apparatus.

In the following examples phosphatic clay slurries from different phosphate beneficiation plants in Florida, fiber from newsprint, magazines, baby diapers, filling pulp, cotton and glass fiber, and Nalco 7877 polyacrylamide flocculant were used.

The equipment included a variable speed mixer with 2.0 inch propeller, blender, Buchner funnel for filtration, 10 mesh plastic screen cloth, solid bowl centrifuge, perforate bowl centrifuge, and Microtrack particle size analyzer.

Abbreviations have been used below.

Their definitions follow:

atm--atmosphere

cc--cubic centimeter

cm.sup.2 --square centimeter

G--gravitational force

lb--pound

min--minute

sec--second

rpm--revolutions per minute

A calculated amount of fiber is pulped in a clay slurry using a blender. The required amount of 0.1% polymer is added to this slurry in a beaker and the mixture is stirred at 300 rpm using a variable speed mixer until flocculation is observed. Slower mixing (100 rpm) is continued for one more minute. The flocculated slurry is then filtered on a Buchner funnel using water aspirator to create vacuum of -0.5 atm.

Several tests were conducted using clays from different sources (at least six different locations) and fibers from newsprint, magazines, cotton and fiber glass. The flocculant, which is found to work well under different conditions, is 7787 high molecular weight polyacrylamide from Nalco. The following component ranges were used:

Amount of fiber/dry clay weight=5-15%

% solids (clays) by weight in the slurry=1-6%

Flocculant consumption=0.2-0.8 lb/ton clay

Testing indicates that 5% is the minimum amount of fiber to clay ratio by weight that is effective in producing significant improvement in the dewatering characteristics of the flocculated clay. For instance, below 5% fiber addition, the filtration rate is as slow as the flocculated clay in the absence of fiber. Also, the product is sticky and cannot be separated easily from the filtering media. Also, the less the fiber content, the less the solids recovery on the dewatering screen. There does not appear to be any significant increase in dewatering by using more than 15% fiber.

It is believed that the polymer consumption depends on the solids concentration in the feed slurry and on the type of clays. For instance, concentrated slurries consume more polymer than the diluted clays.

The data indicated in Table I below illustrate the beneficial effects of the invention. For instance, clay slurries were dewatered (1) as received from the mines, (2) as flocculated with flocculating agent alone, and (3) as flocculated with agent and fiber. The specific conditions for these tests follow:

Volume of clay slurry=200 cc

% solids in slurry=5.0% by weight

Amount of fiber=5% of dry clay's weight

Amount of polymer=0.6 lb/ton of dry clay

Concentration of polymer=0.1%

Filtration area=5.0 cm.sup.2

The data indicates that fibers enhance dewatering of phosphatic clays in terms of filtration rate and % solids in the filtered cake. The results also indicate that dewatered clays can be handled better in presence of fiber. For instance, the filter cake can be peeled easily, thus leaving clean filter media. Also, the dewatering screen does not get blinded since the material flows smoothly on the surface. Even further, the dewatered clays can be squeezed (pressed) for further dewatering without significantly breaking up the flocs. Thus, a belt press may be used if needed.

However, due to the cake thickness build-up on the filter, filtration may not be preferable for dewatering in situations where large outputs are desired.

A five-gallon clay sample was flocculated in presence of fiber and then evaluated using the following tests:

a--Solid bowl decanter test. Solid bowl decanter performance is evaluated by metering a sample into a basket centrifuge at different rates and gravitational levels. The supernate which reports as an overflow stream is analyzed for percent suspended solids. The cake is compacted on the bowl wall and is analyzed for percent total solids. From the data the expected percent solids, capture (recovery) and cake consistency for the centrifuge over the operating range can be determined. This data also displays the optimum retention time and gravitational force for an application.

b--Perforate bowl solids dewatering test. The perforate bowl is used to measure the maximum cake dryness possible with a given product. A thickened sample of feed is prepared to simulate the settled solids in a decanter centrifuge. This thickened material is spun in a perforated basket with an open cloth liner to measure the consistency of the cake with increasing dewatering times and at various gravitational levels.

These tests were conducted on a fiber-flocculated phosphatic clay sample. The conditions of flocculation were the same as in the beaker tests described above in Example 1 with the exception that the percentage of solids in the treated slurry was 5.93%. The size distribution of the clays, as determined by a L&N Microtrace Particle Size Analyzer, is given in Table II below.

This data indicates that the solids are very fine with an average weight diameter of 8.0 micron. The literature reports that filtration and/or centrifugation of unflocculated samples are difficult. See Bureau of Mines "The Florida Phosphate Slimes Problem: A Review and a Bibliography," Information Circular 8668, United States Department of Interior, 1975. However, flocculation of such clays in the presence of fibers is found to enhance the dewatering characteristics of the slurry as shown in Tables III and IV below. Solid bowl decanter test results are indicated in Table III. These tests were conducted at different gravitation forces and different times.

The results indicate that cakes of better than 19% solids can be obtained. Also, solids content and recovery can be increased as the force and retention times are increased. In order to measure the maximum cake dryness, a perforate bowl centrifuge was used as discussed below.

Perforate bowl solids dewatering test results are indicated in Table IV. In these tests, a thickened sample of feed was prepared to simulate the settled solids in a decanter centrifuge. This thickened material was spun in a perforated basket with an open cloth liner to measure the consistency of the cake with increasing dewatering times.

The data indicate that more than 35% solids in the product can be obtained with a one minute retention time. Normally such a high percentage can only be obtained after years of natural settling in clay disposal areas.

The flocculated slurry described above for the beaker flocculation method was also dewatered using a 10 mesh plastic screen cloth. The conditions used were the same as those used in the beaker method in Example 1, except that filtration area is not relevant for screens. The results obtained are indicated below.

As those results in Example 1, the results above illustrate the beneficial effect of the invention. In this Example, the tests specifically illustrate screen dewatering of clay slurries (1) as received from the mines, (2) as flocculated with flocculating agent alone, and (3) as flocculated with agent and fiber.

Since there are many modifications, variations and changes in details or methodology, it is intended that all matter in the foregoing description and Examples and shown in the corresponding drawings be interpreted as illustrative and not limitative.