GB2156243A - Froth flotation - Google Patents

Abstract

Polyglycol ethers of general formula (I> R<1>O-(-CxH2xO-)n-H (I> in which R<1> is an organic radical of one or more carbon atoms, x is an integer from 2 to 5 and n has an average value of from 0.1 to 5, provided that when x is 3 and R<1> is an alkyl radical of less than 5 carbon atoms, then n has an average value of from 0.1 to 1.5, are efficient frothers for mineral treatment and demonstrate advantages over known frothers, especially for coal for power generation.

Description

SPECIFICATION Froth Flotation This invention concerns froth flotation, more especially it concerns the froth flotation of minerals, particularly coals using novel frothers.

The froth flotation of minerals, using a mixture of a "collector" and a "frother" is well known, although there is considerable debate in the literature as to the mechanism by which this works. In the case of coal, polyglycol ethers are established frothers. The general formula for a polyglycol ether is R1 O(CxH2xO)nH (I) in which R' is an organic radical, xis an integer, and n is an average number.

Such compounds may be derived from the reactions of an alcohol of formula R1OH with alkalene oxide molecules having the general empirical formula CXH2xO. These alkalene oxide molecules are three membered or larger heterocyclic ring compounds containing oxygen within the ring; carbon atoms in the ring may be substituted or unsubstituted. The nature of the reaction forming the polyglycol ether is such as to give a mixture of products having a spread of values of n, and hence an average value of n is conveniently used to identify the main product. According to the alkalene oxide used, the alkalene radical in the polyglycol ether may be straight chain or branched.

One particular polyglycol ether is used almost exclusively in commercial froth flotation products in Great Britain; "Teefroth G" conforms to the general formula (II) R'O(CH2CHR"O)nH (II) in which R' is an isobutyl radical, R" is a methyl radical, and n has an average value of 3.

We have discovered that certain other polyglycol ethers demonstrate particuiar advantages in the froth flotation of minerals and especially coals. The present invention provides the use of polyglycol ethers of general formula (I), in which n has an average value of from 0.1 to 5, xis an integer from 2 to 5 and R' is an organic radical of one or more carbon atoms and x is 3 provided that when R' is an alkyl radical of less than 5 carbon atoms and x is 3 then n has an average value of from 0.1 to 1.5, as frothers in the froth flotation of minerals, especially coal.

The invention also provides a froth flotation mixture comprising the specified polyglycol ether and a hydrocarbon collector. The invention further provides a method of treating minerals in a froth flotation cell comprising the operation of the cell using a frother which is the specified polyglycol ether.

Although the invention has particular relevance to the froth flotation of coals and will be described specifically hereinafter with respect to coals, it is not to be understood as being restricted thereto.

In the production of coals for power stations, for steam raising in general and coals for industrial combustion, it is desirable to have a high froth yield, together with a high ash-in-tailings. The froth yield is the weight percentage of solids in the dried froth against the solids content of the initial charge of coal slurry to the flotation cell, and the ash in tailings is the weight percentage of ash (mineral matter remaining after combustion) in the solids residue from the froth flotation cell liquor, on a dry basis. Since the recovered fine coal from the flotation cells is going to combustion, clearly a high yield is important together with minimising the ash content by maximising the ash rejected from the cell (ash-in-tailings).

Preferred frothers for use with coals for power stations and the like, taking into account the criteria set out above, are those of general formula (II) in which R' is an organic radical containing five or more carbon atoms. The organic radical is suitably a substituted or unsubstituted alkyl radical and is preferably a branched alkyl radical, especially of 6 to 10 carbon atoms. A particularly advantageous R' radical is methyl hexyl and a particularly advantageous value of n is in the range 0.5 to 1.5, especially about 1. Preferably, R" is a methyl radical.

Tests of the frothers of the present invention demonstrates improvements in the particlar criteria appropriate to "power station coal" over the industrial standard of "Teefroth G". Certain other practical advantages are also unexpectedly observed. The frothers of the present invention do not in general exhibit a strong odour, unlike some of the alcohols which are used as both as starting materials and as frothers, and some commercial froth flotation mixtures can be judged as offensive. Also, the frothers of the present invention demonstrate a very short conditioning time. That is, the frothers of the present invention are more effective in the first minute or two of operation than currently available commercial frothers.Frequently, coal preparation plants incorporating froth flotation cells operate, for one reason or another but often because of operation at greater than design throughputs, at rather low residence times for coal slurry in the cell, of the order of 1 to 2 minutes. The frothers of the present invention also demonstrate less variation in froth yield with dose rate of froth flotation mixture, and in froth yield with ash-in-froth than commercial frothers. Again, this is particularly important in practice, since froth flotation cells do not operate in steady state conditions, at equilibrium, but are exposed to considerable variations in feed slurry density. The frothers of the invention, therefore, exhibit the desirable attribute of maintaining a good froth yield despite relatively wide variations in effective dose rate caused by rapid changes in the feed slurry density.

The frothers of the invention may be used with any suitable collector, and these are suitably hydrocarbon oils which may be single cuts or blends.

The invention will now be described by way of example only.

EXAMPLE 1 A froth flotation solution was prepared containing 20% by wt. of a compound of general formula (II), in which R'=2-ethylhexyl, R" =methyl and n has an average value of 1, with a standard hydrocarbon oil blend used in commercial froth flotation solutions. This was identified as Solution A.

A comparison Solution B was prepared containing 20% of 2-ethyl hexanol (a starting material forthe frother used in Solution A), in the same hydrocarbon collector as Solution A.

A coal feed from the National Coal Board's Dodworth Colliery coal preparation plant, which coal is mainly sold for electrical power generation, was treated in a laboratory froth flotation cell, using Solution A and Solution B. The results of froth yield against dose rate are plotted in Fig. 1, and froth yield against ash in froth in Fig. 2. The Solution B had a strong and unpleasant odour. It can be seen that Solution A gives a larger froth yield at lower dose rates, and that this varies less with variations in dose rate, from Fig. 1.

EXAMPLE 2 A further froth flotation solution (Solution Al) according to the invention was prepared, containing the same hydrocarbon collector as above, and 20% by weight of a different compound of formula (II) in which R'=iso-butyl, R11 =methyl and n had an average value of 1. This was tested in the laboratory froth flotation cell and compared a commercial froth flotation solution (X) containing 20% of "Teefroth G" in a hydrocarbon collector. The % froth yield was plotted against dose rate and the resulting graph is shown in Fig. 3. It can be seen that Solution Al is more effective at lower dose rates and exhibits less variation in froth yield with dose rate.A plot of froth yield against ash in froth for both Solution Al and Solution X (Fig. 4) shows virtually identical overall performance, and it is believed that the advantages demonstrated in Fig. 3 are significant in real terms.

EXAMPLE 3 A froth flotation solution (Solution C) containing 20% by wt of "Teefroth G" in the same hydrocarbon collector as Solution A, was prepared and tested in the same cell and with the same coal feed as in Example 1. The froth was removed after 1 minute, 2 minutes, etc. and analysed to give data on % froth yield and % cumulative ash in froth for each time interval. This data is given in Table 1 below and compared with the test results for Solution A.

TABLE 1

Solution A Solution C Time interval (mins) % Yield % Cumulative ash % Yield % Cumulative ash 87.5 12.8 80.4 13.0 1-2 10.1 16.4 11.6 15.2 2-3 1.3 16.9 3.5 16.9 3--4 0.8 2.7 18.3 17.6 0.3 0.3 1.8 19.4 It is seen that Solution A, according to the invention, is significantly more effective in the first minute, and in the first two minutes (cumulative) in obtaining a high froth yield.It is believed that this will be reflected in appreciably better froth yields in practice in operational froth flotation cells and the solution could be particularly useful in plants with no conditioning facilities.

EXAMPLE 4 Using Solution A according to the invention, and Solution C as described in Example 3 above, tests were carried out in the laboratory froth flotation cell, using the same coal as in Example 1. The results of % froth yield against dose rate were plotted in Fig. 5, from which it can be seen that Solution A demonstrates improved froth yield at lower dose rates. The shallower slope of the Solution A curve leads to the advantage of stabilising plant performance over widely varying feed rates. Figure 6 shows a plot of froth yield against ash in froth for each test solution, and Solution A demonstrates improved performance especially over the froth yields equivalent to lower dose rates.

Claims (14)

1. The use of polyglycol ethers of general formula (I) R'O(CxH2xO)nH (I) in which R' is an organic radical of one or more carbon atoms, xis an integer from 2 to 5, and n has an average value of from 0.1 to 5, provided that when R1 is an alkyl radical of less than 5 carbon atoms and xis 3, then n has an average value of from 0.1 to 1.5, as frothers in the froth flotation of minerals.

2. The use as claimed in claim 1, wherein in the ether of formula (I), R' is an organic radical of five or more carbon atoms.

3. The use as claimed in claim 1 or 2, wherein in the ether of formula (I), R1 is a substituted or unsubstituted alkyl radical.

4. The use as claimed in claim 3, wherein in the ether of formula (I), R1 is a branched alkyl radical of 6 to 10 carbon atoms.

5. The use as claimed in any one of the preceding claims, wherein in the ether of formula (I), xis 3.

6. The use as claimed in claim 5, wherein in the ether of formula (I), R' is a 2-ethyl hexyl radical and n has an average value in the range 0.5 to 1.5.

7. The use as claimed in any one of the preceding claims, wherein the mineral is coal.

8. The use as claimed in claim 7, wherein the coal is a coal used in power stations, steam raising or industrial combustion.

9. The use as claimed in claim 1, substantially as hereinbefore described.

10. A froth flotation mixture comprising a polyglycol ether of general formula (I), as defined in any one of claims 1 to 6, and a hydrocarbon collector.

11. Afroth flotation mixture as claimed in claim 10, substantially as hereinbefore described.

12. A method of treating minerals in a froth flotation cell, comprising the operation of the cell using a frother which is the polyglycol ether of general formula (I), as defined in any one of claims 1 to 6.

13. A method as claimed in claim 12, wherein the mineral is coal.

14. A method as claimed in claim 13, wherein a hydrocarbon collector is also present in the cell liquor.