Process for using fatty esters of alkanolamine hydroxyalkylates as oxidized coal conditioners

Abstract

 Alkanolamine hydroxyalkylates and fatty esters are used as conditioners in froth floation processes, especially for cleaning coal. The preferred alkanolamine hydroxyalkylates are the ethoxylates and propoxylates of mono-, di-, and tri-ethanolamines. Suitably, said hydroxyalklates are esterified with tall oil fatty acids.

Description

[0001] This invention concerns froth flotation of oxidized coal and coal-containing solids. In particular, this invention involves a process for the use of a novel conditioner class: fatty esters of alkanolamine hydroxyalkylates.

[0002] Coal occurs naturally in several different forms. Depending upon local geology, one should expect to find varying amounts of undesirable materials present with the carbonaceous solids. For example, sulfur content may be objectionably high in raw coal. Likewise, ash and generally noncombustible materials (such as sand) may be found along with the coal. Solid ores in general and coal in particular have long been the subject of study relating to the separation of the desired product from the undesired balance.

[0003] Froth flotation is well known as an effective method of carrying out such separation. This process exploits a physical difference between the wanted and unwanted solids, wherein one type of solid particle is preferentially wetted. Agitation of slurry of the solids creates a bubbly surface region in which the less wetted type of the pulverized solids accumulates.

[0004] Consider a solid mass of materials X and Y, wherein X is characterized as more wettable than Y under certain conditions. Froth flotation separation of X from Y first requires that the solid mass be pulverized into small particles. If the ore is non-homogeneous the resulting particles will have different compositions - some particles with more X and some with less X. Next, the particles are agitated in a carefully chosen liquid flotation agent to form a froth. The agent is generally a mixture that includes, as well known in the art, various collectors, frothing agents and conditioners, all directed toward enhancing the separation. The wetted particles, for example predominantly X, remain in the bulk liquid while the unwetted particles, say predominantly Y, are located in the air bubbles of the froth. A simple embodiment of the general idea appears in U.S. Patent 2,389,763. That reference involves removal of silica and siliceous materials from manganese and magnesium ores. The flotation agent in said reference is aliphatic carboxylic acid esters of nontertiary alkylamines. Experience has taught that the precise structure of the flotation agent is critical to the effectiveness of a given separation. This result comes as no surprise, since liquid-liquid extraction and chromatography also exhibit critical sensitivity to small changes in chemical structure of the mixture components.

[0005] The specific context of the present invention involves oxidized coal. Such coal has generally been known to be somewhat separable from solid ash, using an air-water system for froth flotation where the water contains a carefully chosen flotation agent. The agent, typically a liquid, may contain a number of components, such as chemicals intended to adjust pH or induce bubble formation. Successful separation of coal from ash has depended on flotation agent components making the coal particle surfaces sufficiently hydrophobic. These components of the flotation agent are often called "conditioners". Their function, generally, is to enhance recovery selectivity and efficiency of the froth flotation separation. Well known conditioners include alkanolamines and their derivatives and these function reasonably well in industrial separation processes. However, since froth flotation of coal depends critically on the exact makeup of the liquid flotation agent, there remains the question of exactly what flotation agent is appropriate.

[0006] U.S. Patent 4,474,619 teaches the use, as a component of a flotation agent, of a conditioner that is a condensate of an alkanolamine with at least 0.8 equivalent of a fatty acid. Condensation reactions such as esterification are generally reversible, so the composition of the products will depend strongly on the composition and stoichiometry of the reactants. This reference notes that the preferred ratio of fatty acid or fatty ester to alkanolamine is about 2:1 or 3:1 to yield the most effective conditioner. The flotation agent when added to a slurry to be separated also includes a refined kerosene (Soltrol 100) serving as the collector and a frother (Dowfroth 1012).
Furthermore, this reference recognizes the efficacy of diethanolamine condensed with a commercially available mixture of rosin acids, oleic acid, linoleic acid, steric acid, etc. This mixture of tall oil fatty acids is generally denominated TOFA, while diethanolamine is denoted by DEA. Thus, one may speak of a 1:2 condensation of DEA/TOFA, for example. A comparison of the present invention with U.S. Patent 4,474,619 appears later in this discussion, in Example 5 below.

[0007] The present invention is an improved process for separating ores, especially oxidized coal, by the froth flotation process, characterized in that a suitable ore is (1) classified to a size range selected for froth flotation and then (2) floated in a froth flotation agent comprising a collector and a new alkanolamine derivative conditioner. A key element of the invention is a conditioner that is an alkoxylated alkanolamine. The new conditioner may be esterified alkanolamine hydroxyalkylates.

[0008] The present invention improves upon the prior art by alkoxylating the alkanolamine. This alkoxylation incorporates ether linkages into the conditioner which distinguish it structurally from coal conditioners of the prior art while providing unexpected improvements in clean coal recovery.

[0009] In one aspect, the general flotation process of the invention comprises crushing an oxidized coal to a size suitable for froth flotation and floating the sized coal in a frothing, aqueous medium comprising a fuel oil collector and an effective amount of a conditioner corresponding to the formula:

wherein:
R is any hydrocarbyl group;
R₁ is H or an alkyl;
R₂ is H or an alkyl;
x is an integer of 1 or more; and
A is H or

-

-R₃, wherein R₃ is an alkyl of 2 to 36 carbon atoms.

[0010] The above conditioner may be an ethoxylated alkanolamine or a propoxylated alkanolamine or a mixed ethoxylated and propoxylated alkanolamine or ester thereof. For example, the process of the invention might employ a 1:3 reaction of diethanolamine (DEA) with propylene oxide (PO), designated DEA-3PO. That product will then be esterified with TOFA, thus designated as DEA-3PO-3TOFA.

[0011] Preferred alkoxylated alkanolamines of the invention are the mono-, di- or triethanolamine propoxylates, wherein the number of repeating units of x are 1 to 100. Preferably, the propoxylates are esterified such that the conditioner is the esterified product of a stoichiometric amount of tall oil fatty acids and mono-, di-, triethanolamine propoxylate. The TOFA:EA ratio is preferably 2:1 to 5:1. The ester is typically characterized by x = 1 to 100 and A is
-

-R₃

[0012] The process of the invention is particularly suitable for beneficiating or cleaning coal having oxidized surfaces. This process is useful for cleaning oxidized bituminous coal. As exemplified below, the conditioners of the invention are suitable for use with conventional frothing agent components such as frothers acids to assist in dispersion of the components, and the like.

[0013] In order to facilitate discussion of the present invention in greater detail, specific examples of the conditioners and the processes of using them follow. MEA represents monoethanolamine and TEA represents triethanolamine.

Flotation Procedure

[0014] The coal used was a highly oxidized type containing 14.7 percent ash (available as Republic Steel Banning #4). Oxidized bituminous coals were satisfactorily beneficiated by the process of the invention.

[0015] The flotation apparatus was a Galigher Agitair equipped with a 3000 ml cell, operating at 10 rpm for the froth collecting paddle. About 200 g coal was charged to the apparatus with about 2800 ml deionized water and conditioning of the slurry began at 900 rpm. After 2 minutes the pH was adjusted to 7.0 by addition of 11.0 ml 1N NaOH, followed by 5 minutes of additional conditioning. Then 0.5 ml of the experimental conditioner of interest was added, where the formulation of the flotation agent component including the conditioner was (by weight percent):

% 5.0 Conditioner
% 47.5 Soltrol 100
% 47.5 Deionized water.

Next, another, 0.25 g Soltrol 100 was added. Whenever acetic acid constituted an additional component of the experimental formula, it was present at 5 percent and Soltrol was again made up to fix the Soltrol/coal feed ratio at 2.5 kg/ton (2.75 kg/tonne).

[0016] After another minute of conditioning, 0.04 ml Dowfroth 1012 (DF-1012) was added, followed by one more minute of conditioning. Experimental protocol required the running of a standard, wherein experimental conditioners were omitted and DR-1012 and Soltrol were at concentrations of 0.04 ml DF-1012/2.5 kg Soltrol/ton (0.9 tonne) coal feed. The actual frothing took place upon a flow of 9 liters/minute of air into the cell, with the froth being collected for 4 minutes. Coal concentrate (froth product) was dried at 110°C. Ash content was determined by loss of weight on ignition of a gram of the coal at 750°C for 1.5 hours.

Synthesis of Hydroxyalkylated Alkanolamine

[0017] Propoxylates of MEA,DEA, and TEA were made at atmospheric pressure using a glass RB reactor-condenser apparatus cooled by Dowtherm. A side-arm vented and calibrated addition funnel was equipped with nitrogen inlet, and the set up included magnetic stirring and thermostatically controlled heating lamps. Alkylene oxide was stoichiometrically added to stirred alkanolamine under nitrogen at between 50 and 190°C. After the reaction commenced, makeup alkylene oxide was added to achieve theoretical final product weight. The reaction took place under base catalyzed conditions (0.25 percent KOH by weight).

Esterification

[0018] Esterified alkanolamine propoxylates were prepared, for example, by reacting TOFA with a hydroxy­alkylated alkanolamine in a stoichiometric ratio of TOFA to alkoxylated alkanolamine in the range of 2:1 to 5:1. The reaction was carried out in a glass RB reactor equipped with a steam condenser-Dean Stark trap-cold water condenser assembly, nitrogen sparge tube, magnetic stirring, and thermostatically controlled infrared heating lamps. Reaction occurred under nitrogen at 200°C for 2 hours, followed by an hour at 225°C.

[0019] Infrared analysis verified product structure for both syntheses.

Experimental Results and Examples

[0020] The following tables of results illustrate the effectiveness of the present process. For the purposes of these tables the term "coal concentrate" denotes the solid material recovered from the froth, and "coal head" means the raw feed charged to the cell, such that:
coal concentrate % = 100 x [g coal concentrate/g coal head];
and
clean coal recovery % = coal concentrate % x [1-ash fraction].

Example 1

[0021] These runs were done using monoethanolamine (MEA) as the alkanolamine precursor to the flotation conditioner.

Flotation Reagent Type	Coal Recovery %	Clean Coal Recovery %
MEA-10PO	40.9	35.9
MEA-10PO-2TOFA	68.2	61.0
MEA-36.15PO	51.0	45.4
MEA-36.15PO-2TOFA	68.2	61.5
MEA-36.15PO-8TOFA¹	70.1	63.5
Standard	29.7	25.8

¹Minor amounts of unreacted TOFA present as evidenced by infrared spectrophotomertic analysis.

[0022] The esterified monoethanolamine propoxylates are particularly useful in coal recovery. The nonesterified MEA-36.15PO reagent is also satisfactory. Generally, at least a 40 percent clean coal recovery is desirable.

Example 2

[0023] These runs were done using DEA as the alkanolamine precursor to the flotation condition.

Flotation Reagent Type	Wt. Coal Head (g)	Wt. Coal Conc. (g)	Ash (%)	Coal Recovery (%)	Clean Coal Recovery %
DEA-2PO	200.8	80.5	12.6	40.1	35.0
DEA-2PO-2TOFA	200.6	140.6	10.9	70.1	62.5
DEA-2PO-3TOFA	201.2	144.1	10.2	71.6	64.3
Standard	201.3	59.7	13.0	29.7	25.8
DEA-10PO	201.4	87.8	11.4	43.6	38.6
DEA-10PO-2TOFA	200.3	136.5	9.4	68.2	61.7
DEA-10PO-3TOFA	201.9	139.2	9.3	69.0	62.5
DEA-10PO 5TOFA¹	200.6	141.4	9.4	70.3	63.9
STANDARD	201.3	59.7	13.0	29.7	25.8

¹Minor amounts of unreacted TOFA present as evidenced by infrared spectrophotomertic analysis.

Example 3

[0024] These runs were done using TEA as the alkanolamine.

Flotation Reagent Type	Wt. Coal Head (g)	Wt. Coal Conc. (g)	Ash (%)	Coal Recovery (%)	Clean Coal Recovery %
TEA-2PO	200.9	77.1	12.6	38.4	33.5
TEA-2PO-2TOFA	200.4	141.6	11.1	70.7	62.8
TEA-2PO-5TOFA¹	201.2	145.0	11.2	72.1	64.0
Standard	201.3	59.7	13.0	29.7	25.8
TEA-10PO	201.5	79.6	12.5	39.5	34.6
TEA-10PO-2TOFA	201.4	136.1	11.2	67.6	60.0
TEA-10PO-3TOFA¹	201.0	141.8	10.7	70.6	63.0
STANDARD	201.3	59.7	13.0	29.7	25.8

¹Minor amounts of unreacted TOFA present as evidenced by infrared spectrophotomertic analysis.

Example 4

[0025] These runs done with acetic acid treated reagents.

Flotation Reagent Type	Wt. Coal Head (g)	Wt. Coal Conc. (g)	Ash (%)	Coal Recovery (%)	Clean Coal Recovery %
DEA-2PO-3TOFA	200.7	142.3	11.2	70.9	63.0
DEA-10PO-3TOFA	201.2	133.9	10.9	66.6	59.3

[0026] Comparison of these results with those of Example 2 shows that acetic acid treatment slightly elevated the ash fraction, diminishing clean coal recovery. Nevertheless, industrial conditions may call for such acid treatment in order to further the dispersion of the conditioner. In such an event, the present process still operates. The acid employed may be an inorganic acid as well as the organic acid used above.

Example 5

[0027] These runs illustrate the merit of the present process, as compared with another process which uses the nonalkoxylated conditioner. The first entry below is not an embodiment of this invention.

Flotation Reagent Type	Wt. Coal Head (g)	Wt. Coal Conc. (g)	Ash (%)	Coal Recovery (%)	Clean Coal Recovery %
DEA-2 TOFA²	200.8	135.6	10.1	67.5	60.7
DEA-2PO-2TOFA	200.6	140.6	10.9	70.1	62.5
DEA-10PO-2TOFA	200.3	136.5	9.4	68.2	61.7
Standard	201.2	59.7	13.0	29.7	25.8

²Not an embodiment of the present invention and included to exemplify prior art patent U.S. 4,474,619.

Claims

1. An improved froth flotation process for cleaning coal using alkanolamine hydroxyalkylates or fatty esters thereof comprising the steps of,

(a) crushing the coal to a size suitable for classification by froth flotation,

(b) floating the sized coal in a frothing aqueous medium comprising a fuel oil collector and an effective amount of a conditioner corresponding to the formula,

wherein,
R is any hydrocarbyl group;
R₁ is H or an alkyl;
R₂ is H or an alkyl;
x is an integar of l or more, and
A is H or -C-

₃

and
R₃ is an alkyl of 2 to 36 carbon atoms.

2. A process as claimed in Claim l, wherein the alkanolamine hydroxyalkylates are selected from ethoxylates, propoxylates and mixtures thereof.

3. A process as claimed in any one of the preceding claims, wherein the alkanolamine hydroxyalkylates are hydroxyalkylates of mono-, di-, or tri-ethanolamine.

4. A process as claimed in any one of the preceding claims wherein x is an integer from l to l00.

5. A process as claimed in Claim 4, wherein the conditioner is selected from monoethanolamine propoxylate, diethanolamine propoxylate, and triethanolamine propoxylate.

6. A process as claimed in Claim 5, wherein at least one A is -

- R₃.

7. A process as claimed in Claim 6, wherein R₃ includes tall oil fatty acids alkyl radicals.

8. A process as claimed in Claim 7, wherein tall oil fatty acid stoichiometry is in a ratio to the ethanolamine hydroxyalkylate of from 2/l to 5/l.

9. A process as claimed in any one of the preceding claims, wherein the coal to be cleaned has an oxidized surface.

l0. A process as claimed in Claim 9, wherein the coal to be cleaned is bituminous coal having an oxidized surface.

11. A process as claimed in any one of the preceding claims, wherein the frothing aqueous medium further includes an effective amount of a conventional frothing agent.

12. The use as a conditioner in a froth floatation process of an alkanolamine hydroxyalkylate as defined in any one of Claims l to 8.