GB2112808A - Beneficiation of hydrophobic or hydrophilic materials - Google Patents

Abstract

A process to remove hydrophilic substances from hydrophobic solid materials includes forming an aqueous slurry and incorporating a low-boiling nonpolar, water insoluble, bridging hydrocarbon, for example pentane or hexane, to selectively agglomerate the hydrophobic solid which can then be removed. The process useful for coal fines, and for producing a metallurgical grade coal and a low ash steam coal from coal containing vitrinite, ash and pyritic sulfur.

Description

SPECIFICATION Beneficiation of hydrophobic or hydrophilic materials The present invention relates to a method of separating substances from a solid wherein the substances have a different affinity for water than the pure solid, such as removing impurities from coal, or removing gangue minerals from phosphates.

There are a variety of known techniques for removing impurities from solids, based on differences in characteristics between the pure solid and its impurities. For instance, materials can be separated based on their size, their density, their ability to hold an electrical charge, or their magnetic characteristics. These methods are useful for most solid separation applications, but there are some solids that cannot be economically separated by these methods because the pure solid and its impurities are too similar in these characteristics.

A solution to this problem is to use a different characteristic, such as affinity for water, to separate the solid from its impurities. In one known method, ash (a hydrophilic impurity) is separated from coal (a hydrophobic solid) by forming a coal slurry, mixing oil into the slurry to produce agglomerates, and recovering the agglomerates as product. Most of the ash remains in the aqueous phase of the slurry.

A major disadvantage of this method is that the oil used to agglomerate the coal becomes part of the product. This means that one is selling oil at the price of coal. This also means that this process could not be used to separate other hydrophobic materials from their hydrophilic impurities whenever oil would not be a desirable part of the final product. It is possible to try to recover the oil from the agglomerates, but this would require extremely high temperatures (in excess of 260"C) and, even at these high temperatures, the oil recovery would not be complete.

Pyritic sulfur is not normaly removed by this process. The fuel oil has components in it which activate the surfaces of both the coal and the pyritic sulfur to make both more hydrophobic, thus the pyritic sulfur is agglomerated with the coal.

Another method of separating two solids is by froth flotation; Froth flotation is a process for separating finely ground valuable minerals from their associated gangue. The process is based on the affinity of properly prepared surfaces for air bubbles. A froth is formed by introducing air into a pulp of finely divided ore in water containing a frothing or foaming agent. Surface modifying reagents (collectors) may be also added to increase the affinity of the mineral surface for air bubbles. Minerals with a specific affinity for air bubbles rise to the surface in the froth and are thus separated from those wetted by water. As a first step, the ore must first be ground to liberate the intergrown valuable mineral constituent from its worthless gangue matrix.The size reduction, usually to about 208 microns (65 mesh), reduces the minerals to such a particle size that they may be easily levitated by the bubbles.

Froth flotation can be used to produce a metallurgical grade coal. In froth flotation of bituminous coal, the fraction most easily and rapidly floated is rich in vitrinite, a constituent of coal, with a low ash content and good coking properties. Vitrinite is the material needed to make a good metallurgical grade coal. The remaining fraction has a high content of ash and pyritic sulfur. It would be advantageous if this ash and pyritic sulfur could be removed from the remaining fraction.

It would also be advantageous if a separation method could achieve a better separation of two differing solids than has been achieved by the prior art processes. It would also be advantageous if a separation method was more energy efficient than the prior art processes. It would also be advantageous if a separation method could separate two solids without agglomerant being in the final product.

The present invention overcomes the deficiencies of the prior art by selective agglomeration of the hydrophobic material. In the present invention, an aqueous slurry is formed of the hydrophobic material and hydrophilic substances; a nonpolar water insoluble bridging hydrocarbon is used to selectively form agglomerates of the hydrophobic material; and the agglomerates are separated from the slurry containing the hydrophilic substances. Preferably, the bridging hydrocarbon is recovered and recycled. An essential element of this invention is the bridging hydrocarbon used. It is essential that the bridging hydrocarbon have a low boiling point (700C or less), such as butane, pentane, hexane and mixtures thereof.

In one embodiment of the present invention, the hydrophobic material and hydrophilic substances are ground in a slurry so that the particle size distribution of the hydrophobic material and hydrophilic substances has at least 90% of the particles less than 10 microns in size and the agglomerates are formed by subjecting the hydrophobic material, the hydrophilic substances, and the bridging hydrocarbon to high shear agglomeration and low shear agglomeration.

Preferably the initial slurry of hydrophobic material should contain 10 to 20% by weight solids and the separation step should be carried out using a screening means or a centrifuge.

In its broadest application the present invention involves separating hydrophilic substances from a hydrophobic material by forming an aqueous slurry of the hydrophobic material and hydrophilic substances, then selectively agglomerating the hydrophobic material in such a way as to agglomerate the hydrophobic material but not the hydrophilic substances. This selective agglomeration is carried out by the use of a nonpolar, water insoluble bridging hyrocarbon. After the selective agglomeration takes place, the agglomerates can be separated by a screening device ora centrifuge, and the bridging hydrocarbon can be recovered and recycled.

This process can be used to remove hydrophilic substances from hydrophobic materials, or it can be used to remove hydrophobic impurities from hydrophilic substances. When the impurities are hydrophilic, the final products are agglomerates of hydrophobic material, and the slurry containing the hydrophilic impurities is a waste stream. When the impurities are hydrophobic, the final product is a slurry of hydrophilic substances which may be dried, and the agglomerates of hyrophobic material is waste.

In one particularly advantageous embodiment, oxidized or low rank coal is mixed with water to form an aqueous slurry wherein coal and the hydrophilic substances (ash and pyritic sulfur) associated with the coal are dispersed in water and the resulting slurry has from 30 to 40% by weight solids, the coal is ground in the slurry so that the particle size distribution of the coal has at least 90% of the particles less than 10 microns in size.Water is then added to the slurry to give a 10 to 20% by weight solids slurry and pentane and fuel oil are mixed into the slurry so that the pentane is 30 to 40% by weight on a pentane and dry coal basis, and the fuel oil is less than 5% by weight on a dry coal and oil basis; then the slurry is subjected to both high shear agglomeration and low shear agglomeration to form agglomerates of the coal (the ash and pyritic sulfur remain dispersed in the siurry); then the coal agglomerates are separated from the slurry by passing the slurry through a screen and the coal agglomerates are heated in the absence of air to remove pentane; then the pentane is recovered and is recycled.

In another particularly advantageous embodiment, phosphate rock is mixed with water to form an aqueous slurry wherein the phosphate rock and gangue minerals are dispersed in water and the resulting slurry has from 10% to 20% by weight solids; the pH of the slurry is adjusted to between 10 and 11; pentane and oleic acid are added to the slurry; then agglomerates of phosphates are formed; the phosphate agglomerates are separated from the slurry by passing the slurry through a screen and the phosphate agglomerates are heated in an inert atmosphere to remove the pentane; then the pentane is recovered from the inert atmosphere and this pentane is recycled.

The present invention can be used to separate any hydrophilic substances from a hydrophobic material.

This invention is especially useful in separating gangue minerals from phosphates, and in separating ash and pyriticsulfurfrom coal.

The first step in this invention is forming an aqueous slurry of the hydrophobic material and hydrophilic substances. Preferably this slurry has a solids content of from 30 to 40% by weight prior to grinding. When there is no grinding step, the slurry should have a solids content of from 10 to 20% by weight.

As a preferred additional step, the hydrophobic material can be ground in the slurry so that the particle size distribution of the hydrobphobic material and the hydrophilic substances has at least 90% of the particles less than 75 microns in size, more preferably, less than 10 microns. Such a grinding step would be used whenever the hydrophilic substances are fine grained. The grinding step helps to liberate the hydrophilic substances from the hydrophobic material. The grinding step occurs prior to the addition of the bridging hydrocarbon, otherwise agglomerates would form during grinding and reduce the grinding efficiency.

An agglomerant is added to the slurry in order to selectively agglomerate the hydrophobic material. This agglomerant is a low-boiling nonpolar, water insoluble hydrocarbon having a boiling point of 70"C or less.

This agglomerant may be butane, pentane, hexane or a mixture thereof. The slurry should contain from 10 to 40% of the agglomerant on an agglomerant and dry hydrophobic material weight basis.

The agglomerant should be low boiling so that it can be readily recovered at low temperatures and can be recycled to reduce the agglomerant requirement. High-boiling hydrocarbons, such as fuel oil, are hard to recover, even at temperatures of 260"C and higher. If fuel oil is used as an agglomerant, extremely high temperatures are required to recover the agglomerant and these high temperatures represent a severe penalty in energy requirements. Even at these high temperatures, fuel oil recovery is incomplete. For these reasons, low-boiling agglomerants are preferred over fusel oil. As a general rule, increases in agglomerant boiling point cause recovery of the agglomerantto be more difficult since the agglomerant is more strongly adsorbed on the hydrophobic material suface.

The aggiomerant should be nonpolar for a better distribution of the organic between the aqueous phase and the hydrophobic solid. As polarity increases, more agglomerant is lost in the aqueous phase.

The agglomerants should be a hydrocarbon, instead of other nonpolar insoluble agglomerants such as freon, because these hydrocarbons are cheaper than other nonpolar agglomerants and because halogens in the product could cause problems downstream, such as corrosion.

One advantage of using as agglomerant either butane, pentane, hexane or mixtures thereof, is that these agglomerants result in a greater degree of removal of impurities than when fuel oils are used.

Another advantage of these low-boiling agglomerants is that they have lower densities than other agglomerants. In agglomeration, there is an optimum volume of agglomerant that is needed to give good, easily separable agglomerates. The energy required to remove the agglomerant depends upon the weight present. Thus, if two liquids of equal heat of vaporization are used, the energy required to remove equal volumes will be less for the liquid of lower density.

When the hydrophobic material is coal, the agglomerant needs to have a low viscosity to achieve low ash in the final product. high viscosity increases the time needed to form agglomerates and with fuel oils, increases the ash and sulfur content of the product.

If an agglomerant-free product is desired, then the agglomerant must be volatile, it must be recoverable at a reasonable temperataure (30 -70 C) and it should not be strongly absorbed into the hydrophobic material.

The agglomerants of the present invention satisfy these criteria.

Preferably the agglomerant is added to the slurry to a premixer to give a homogeneous feed. In the premixer, a surface conditioner can be added to make the hydrophobic material more hydrophobic (5% or less by weight on a hydrophobic material and surface conditioner basis). Fuel oil is a preferred surface conditioner for oxidized or low rank coal. A high molecular weight organic acid is a preferred surface conditioner for phosphates.

If the slurry had been ground, the slurry is diluted to a solids content of from 10 to 20% by weight prior to agglomeration.

The hydrophobic material is selectively agglomerated and the hydrophilic substances remain dispersed in the slurry. The hydrophobic material can be subjected to either low shear agglomeration alone or in combination with high shear agglomeration. Low shear agglomeration is sufficient to selectively agglomerate phosphates but both high shear agglomeration and low shear agglomeration are preferred when agglomerating coal.

Whenever high shear agglomeration is used, it must be followed by a period of relatively low turbulence so that the agglomerates formed in the high shear zone can form a more compact, more easily separable product. The agglomerates coming out of the high shear zone are quite small and would cause separation problems if the subsequent period of relatively low turbulence is missing.

After the agglomerates of hydrophobic material are formed they can be separated from the slurry by any known separation technique. Preferably the agglomerates are removed from the slurry by using either a screen or a centrifuge. A sieve bend is a particularly advantageous screening means because of its low cost.

After the agglomerates are separated from the slurry they are heated or flashed to remove the agglomerant. To maximize recovery of the agglomerant, the product leaving the heated zone should be discharged at a temperature in excess of the boiling point of the agglomerant. An inert atmosphere or vacuum should be used in the heating step to reduce the chance of either the hydrophobic material or the agglomerant from thermally decomposing.

An advantage of the present invention is that the low-boiling agglomerants of the present invention do not require high temperatures in order to be removed, thus saving energy.

The agglomerant can be recovered from the inert atmosphere and can be recycled. In one agglomerant recovery process the agglomerant and the inert gas are passed through a bag filter for dust removal, then the agglomerant and inert gas are passed through a compressor and an agglomerant recovery condenser, which recovers the agglomerant from the gas. The gas leaving the condenser is passed through a carbon adsorption system which further removes agglomerant. The agglomerant is then recycled as a source of make-up agglomerant for the premixer and the insert gas is recycled to the heating zone.

The invention will be illustrated by the following Examples which set forth particularly advantageous method embodiments. While the examples are provided to illustrate the present invention, they are not intended to limit it.

Example! A series of runs were made using a Sunnyside (Utah) coal. In each run a Sunnyside (Utah) coal having 5.58 weight percent ash and ground to a median particle size of 5.4 microns was mixed with water to form an aqueous slurry of 10 weight percent solids; an agglomerant was added to the slurry so that it constituted 36 weight percent on a coal and agglomerant basis; the agglomerant was used to selectively form agglomerates of coal; and the agglomerates were separated from the slurry and heated in an inert atmosphere to remove the agglomerant. When the agglomerant was an oil, the product coal was extracted with pentane to remove the oils so that product ash is on an oil-free basis. The moisture-free weight percent ash for each product is shown in the following table.

TABLE I Effect of Agglomerant on the Product Ash in Coal Agglomerant Weight % Ash in Product Pentane 1.01 Kerosene 1.14 White Oil (Heavy) 1.35 No. 2 Fuel Oil 1.42 No. 4 Fuel Oil 2.24 Thus, in operation, a low-boiling nonpolar, water insoluble bridging hydrocarbon, such as pentane, gives excellent ash removal.

Example Il A series of run were made using a Pittsburgh Seam coal. In each run, a Pittsburgh Seam coal was mixed with water to form an aqueous slurry; the slurry was ground to a specified median particle size; an agglomerant was used to selectively form agglomerates of coal; the agglomerates were separated from the slurry; and the agglomerates were heated in an inert atmosphere to remove the agglomerant. The moisture-free weight percent ash for each run is shown in Figures 1 of the accompanying drawings. The moisture free weight percent sulfur for each run is shown in Figure 2 of the drawings.

These Figures show that the use of a low-boiling nonpolar, water insolublbe bridging hydrocarbon, such as pentane, give superior ash removal and sulfur removal than No. 4 Fuel Oil. These Figures also show that optimum removal of ash and sulfur is achieved when the slurry has been ground prior to agglomeration such that the particle size distribution has a median of less than 5 microns.

Example 111 An aqueous slurry was formed containing an Illinois No. 6 coal having an ash content of 32.92 weight % on a dry coal basis. A No. 4fuel oil was added to the slurry as a surface conditioner (weight ratio oil/coal=0.033) pentane was then added to the slurry such that the slurry contained 40 weight % pentane on a pentane and dry coal basis. The pentane was used to selectively form agglomerates of coal and the agglomerates of coal were separated from the slurry. These agglomerates had a product ash of only 4.10 weight %, which was a 88% ash reduction.

Example IV A series of runs were made to show the effect of fuel oil concentration on product ash. In each run, an Illinois No. 6 coal was mixed with water to form an aqueous slurry. No. 6 fuel oil was used at various levels to selectively form agglomerates of coal and the agglomerates were separated from the slurry. The results of these runs are shown in the following table.

Effect of Oil Concentration on Product Ash in the Agglomeration of Illinois No. 6 Coal Weight % Ash in Product Weight Ratio of Oil to Coal Moisture-free 0.167 3.17 0.033 2.64 0.017 2.47 0.007 2.01 Thus, in operation, the presence of increasing amounts of fuel oil in a slurry has an adverse effect on weight % ash in the final product.

Example V A series of runs were made using an unweathered western phosphate rock. In each run, an unweathered western phosphate rock having a particle size distribution such that at least 50% of the particles are less than 400 mesh, and containing 20.73 weight percent P2O5, was mixed with water to form an aqueous slurry; the pH of the slurry was adjusted to a particular level; oleic acid and hexane were used to selectively form agglomerates of phosphate; the phosphate agglomerates were separated from the slurry; and the agglomerates were heated in an inert atmosphere to remove the hexane. The results of these runs are shown in the following table.

TABLE II Effect of pH on P205 Recovery From Unweathered Western Phosphate Rock Product Grade P205 Recovery (Wt. % P2 05) (Wt. /O) pH 30.26 21.2 7.2 28.32 43.0 7.5 29.58 42.2 7.9 30.04 46.2 9.1 31.04 76.4 11.0 30.09 70.7 11.9 Thus, in operation, selective agglomeration of phosphate using hexane as an agglomerant is an effective means of beneficiation of phosphate rock, but such beneficiation must occur at a pH of at least 10.

Example Vl Another series of runs were made to determine the effect of recycle solvent on ash content. In each run an Illinois No. 6 coal having 10.87 wt. % ash and ground to a median particle size of 3.9 microns was mixed with water to form an aqueous slurry of 10 wt. % solids; the agglomerations were carried out with different weight ratios of recycle solvent to coal; in each case the wt. % recycle solvent plus pentane was 40% on a coal, recycle solvent and pentane basis.

Wt. Ratio Recycle Solvent to Coal WL % Ash in Product 0.3333 3.82 0.1667 3.43 0.0333 2.08 0.0233 1.80 Thus in operation, decreasing the ratio of recycle solvent to coal decreases the wt. % ash in the product.

In one embodiment of the present invention, small quantities of fuel oil are added to low grade coal to make the coal more hydrophobic. Some coals are difficult to agglomerate by use of hydrocarbons alone (kerosene, light fuel oils, e.g. No.2, hexane, pentane, etc.) because they have more hydrophilic surfaces.

Examples are low rank coals (subbituminous), coals which have been oxidized, or coals such as Illinois No. 6.

In such cases, No. 4, 5,6 fuel oils or heavy crude oils such as Kern River can be mixed in with the hydrocarbon agglomerant to make the surface of the coal more hydrophobic. The fuel oil stays on the product and is not recovered, but the economic penalty is not severe because of the small amount of fuel oil used (less than 5% by weight on a dry coal and oil basis). The agglomerant constitutes from 30 to 40% by weight.

In another embodiment of the present invention, phosphates having a particle size of less than 500 microns are surface conditioned prior to agglomeration with high molecular weight organic acids at a pH of greater than 10 to make the phosphates more hydrophobic so they could be separated from gangue minerals such as clays, calcite, dolomite, silica, etc. The phosphates are conditioned with oleic acid or other fatty acids or high molecular weight organic acid having surfactant properties.

The present invention could also be used to recover coal from coal preparation plant tailings ponds and to recover coal from coal preparation plant fine coal circuits. Because these fines are difficult to remove from the process water, these coal fines usually are stored in tailing ponds as waste. Since these fines have significant BTU content in the form of coal and, since it is costly to maintanin these tailing ponds, it would be advantageous to recover these fines as a useful product. This can be accomplished by the process of the present invention.

Also the present invention could be used in conjunction with short residence time froth flotation to separate metallurigcal grade coal from lower grade coals, and produce a byproduct suitable as a fuel for power plants. A metallurgical grade coal and a low ash steam coal are produced by forming an aqueous slurry of coal containing vitrinite, ash and pyritic sulfur; adding a froth flotation reagent to the slurry; subjecting the slurry to froth flotation to produce an underflow and an overlow; filtering and drying the overflow to produce a metallurgical grade coal; then selectively agglomerating the underflow in such a way as to agglomerate the coal, but not the ash and pyritic sulfur. This selective agglomeration is carried out by the use of a nonpolar, water insoluble, bridging hydrocarbon.After the selective agglomeration takes place, the agglomerates can be separated by a screening device or a centrifuge, then the bridging hydrocarbon can be recovered and recycled.

Also the present invention could be used to recover carbonaceous components from coal liquefaction residue.

The low ash coal agglomerates of the present invention can be used as a feed for a coal liquefaction process. In this embodiment, the coal agglomerates are subjected to solution in a hydrogen donor solvent to produce a liquid product. When a low ash feed is desired, the agglomerant should be a low boiling, nonpolar, water insoluble, hydrocarbon fraction derived from the coal liquefaction process. When a feed having a higher ash content can be tolerated, then recycle solvent is a very economial source of agglomerant. U.S. Patent 3,594,304 describes an advantageous method of subjecting coal to solution in a hydrogen donor solvent.

Claims (16)

1. A method of separating a hydrophobic material from a hydrophilic substance comprising: (a) forming an aqueous slurry of the hydrophobic material and the hydrophilic substance; (b) incorporating in the slurry a nonpolar, water insoluble, bridging hydrocarbon having a boiling point of less than 70"C to selectively form agglomerates of the hydrophobic material; and (c) separating the agglomerates from the slurry containing the hydrophilic substance.

2. A method according to Claim 1, wherein said bridging hydrocarbon is recovered and recycled for use in step (b).

3. A method according to Claim 1 or 2, wherein said bridging hydrocarbon is butane, pentane, or hexane our a mixture of two or more thereof.

4. A method according to Claim 1,2 or 3, wherein said slurry in step (a) has a solids content of from 10 to 20% by weight.

5. A method according to Claim 1,2,3 or 4, wherein said separation step (c) is carried out using either a screening means or a centrifuge.

6. A method according to any preceding claim, wherein said hydrophobic material is coal and wherein said hydrophilic substance comprises ash and pyritic sulfur.

7. A method according to Claim 6, wherein the coal is a low rank or oxidized coal; a fuel oil is added to said slurry of step (a) such that the slurry contains less than 5% by weight fuel oil on a dry coal and oil basis, and said slurry in step (b) contains from 30% to 40% bridging hydrocarbon based on the weight of the bridging hydrocarbon and dry coal.

8. A method according to Claim 6, wherein the agglomerates of coal are subjected to solution in a hydrogen donor solvent to produce a liquid product.

9. A method according to Claim 8, wherein said liquid product is recycled to step (b) as a source of bridging hydrocarbon.

10. A method according to any one of Claims 1 to 5, wherein said hydrophobic material is phosphate rock; said hydrophilic substance is gangue mineral; the pH of the slurry in step (a) is adjusted to be in the range from 10 to 11; and there is added to said slurry a surface conditioner selected from oleic acid, fatty acids and high molecular weight organic acids.

11. A method according to Claim 1, 2 or 3, wherein the slurry of step (a) is ground so that the particle size distribution of the hydrophibic material and the hydrophilic substance has at least 90% of the particles less than 75 microns in size.

12. A method according to Claim 11, wherein the particle size distribution of the ground hydrophobic material and hydrophilic substance has at least 90% of the particles less than 10 microns in size.

13. A method according to Claim 11 or 12, wherein said slurry in step (a) has a solids content of from 30% to 40% by weight prior to grinding.

14. A method of processing coal fines from a coal preparation plant comprising: (a) forming an aqueous slurry of coal fines; (b) using in said slurry a nonpolar, water insoluble, bridging hydrocarbon having a boiling point of less than 70"C to form agglomerates of coal; (c) separating the agglomerates of coal from the resultirig slurry; (d) recovering the bridging hydrocarbon; and (e) recycling the bridging hydrocarbon to step (b).

15. A method of producing a metallurgical grade coal and a low ash coal comprising: (a) forming an aqueous slurry of coal containing vitrinite, ash and pyritic sulfur; (b) adding to said slurry to froth flotation reagent; (c) subjecting the resulting slurry to froth flotation to produce an overflow and an underflow, wherein said overflow is rich in vitrinite and has a low content of ash and pyritic sulfur; and wherein said underflow has a low content of vitrinite and is rich in ash and pyrite sulfur; (d) filtering and drying the froth flotation overflow to produce a metallurgical grade coal; (e) using a nonpolar, water insoluble, bridging hydrocarbon having a boiling point of less than 70"C to selectively form agglomerates of coal from the underflow; (f) separating the agglomerates of coal from the underflow containing the ash and pyritic sulfur; (g) recovering the bridging hydrocarbon; and (h) recycling the bridging hydrocarbon to step (e).

16. A method of separating hydrophobic material from hydrophilic substances, substantially as described in any one of the foregoing Examples.