Froth flotation is a common process applied to the art of separating or concentrating minerals from ore or the like. Briefly, the flotation process usually comprises grinding crushed ore, classifying the ground ore in water, treating the classified ore by flotation to concentrate one or more minerals while the remainder of the minerals of the ore remain behind in the water pulp, thickening and filtering the separated concentrate and thereafter treating the same for ultimate use of the separated minerals. In carrying out the flotation step, a chemical reagent, called the "collector" is added to the water-dispersed ore and air is introduced into the pulp to form a froth. This froth, containing those minerals that are wetted by the collector and consequently have an affinity for air bubbles, is then scooped away from the pulp.
A host of selective collecting agents have been developed that are used for forming water-repellent, air-avid surfaces on one mineral or a class of minerals. These collectors are anionic or cationic, and while many of them have been used satisfactorily, they often are limited by their solubility and handling characteristics, selectivity, effectiveness, stability, cost, etc.
In recent years, the enrichment of non-magnetic taconite iron ore deposits by a selective flocculation/desliming process, followed by froth flotation, has become an important commercial process. The application of this process to a large ore body located on the Marquette Range in Michigan (the Tilden Mine) is described in a paper:
Villar, J. W. and Dawe, G. A., "The Tilden Mine--a New Processing Technique for Iron Ore", Mining Congress Journal, October, 1975, Vol. 61, No. 10, pg. 40-48.
The process described in this paper utilizes a cationic flotation system following the selective flocculation/desliming step. The purpose of the cationic flotation system is to remove silica from the deslimed ore to produce an iron ore concentrate of commercial grade.
The cationic flotation system employs an amine collector. The principal amine collector utilized in the Tilden process has been an ether amine of the following general structure: ##STR1## where R--O-- is derived from a mixture of normal alcohols consisting predominantly of C.sub.8 and C.sub.10 carbon number alcohols. In use, the amine is typically partially neutralized (.about.30 percent) with acetic acid to improve water dispersability.
Other mono ether amines offered commercially for iron ore flotation are products where R--O-- is derived from normal C.sub.10 alcohols, methyl branched C.sub.10 alcohols, normal C.sub.12 -C.sub.14 alcohols, and normal C.sub.16 -C.sub.18 -C.sub.22 -C.sub.26 alcohols. Other products which have been mentioned in the patent literature include products derived from normal C.sub.6, C.sub.7, C.sub.8, C.sub.9, C.sub.10, C.sub.11, C.sub.12, C.sub.13, C.sub.14, C.sub.15, etc. alcohols and various iso C.sub.8 alcohols (see U.S. Pat. No. 3,363,758 for other starting alcohols).
Other products known for cationic flotation of iron ores include fatty amines, fatty beta-amines, various ether diamines (see U.S. Pat. Nos. 3,363,758 and 3,404,165) and, more recently, blends of alkyl amines/mono ether amines and alkyl amines/amino ethers (see U.S. Pat. No. 4,168,227).
The collector used in a cationic flotation process for iron ore is desired to achieve many, sometimes conflicting objectives. These requirements are outlined as follows:
1. Produce an Iron Ore Concentrate of Acceptable Quality
The final product must contain a sufficiently high iron content (generally 60+ weight percent Fe) and not exceed a given silica content to meet commercial standards. It is desirable that silica contents not exceed 5-6 weight percent SiO.sub.2. In some cases, high purity (2-3 weight percent SiO.sub.2) iron concentrates are required.
2. Recover the Maximum Quantity of Iron Consistent with Acceptable Quality
Iron recovery is of major economic importance to the plant operation. For example, improving iron recovery by 1 weight percent from a crude ore assying 35% Fe increases the return per ton by about 25 cents.
3. Achieve Acceptable Results with a Variety of Iron Ore Types
Variations occur in the specific type of iron ore encountered in day-to-day mining operations. A given deposit of ore may vary significantly in the amount of desired contained iron ore minerals (e.g., martite, hematite, magnetite, geothite, etc.) and in undesired gangue (quartz, clays, etc.). Commercial iron ore technology does not permit controlling the precise composition of the crude ore being fed to the concentrator (although attempts are made to minimize gross changes through control of mining and ore blending operations). Thus, a successful collector must give acceptable results with the normal commercial variations in ore types fed to the concentrator.
4. Be Sufficiently "Persistent" to Yield Acceptable Results Through Several Stages of Froth Flotation
Sharp separations between the undesired silica mineral particles and the desired iron-containing mineral particles are not obtained in a single stage of froth flotation. Thus, in commercial practice, to remove enough silica in the Rougher Flotation cells to achieve commercial purity iron ore concentrate in the underflow, considerable amounts of iron ore are also removed in the froth. Loss of this iron would make the process uneconomic. Thus, the froth product from the Rougher Flotation cells is subjected to several subsequent cleaner froth flotation stages to further separate the desired iron ore from the undesired silica.
In theory, collector could be added at each stage of Rougher and Cleaner froth flotation. However, in commercial practice, collector is often added only to the Rougher cells. Even if additional collector is added at some stage of the cleaning process, this causes complications in process control.
As a practical consequence, a commercial collector must "persist" (i.e., continue to cause the silica mineral particles to float) through several stages of cleaner flotation.
It should be noted that this requirement for a successful collector has heretofore not been recognized in iron ore flotation as a specific property of a collector which should be determined.
5. Require Minimum Quantities of Collector to Achieve Acceptable Operations
While costs of collector are relatively small versus, for example, the value of improved iron recovery and/or the cost of unsatisfactory operations, these collector costs are still an important operating cost. It is general commercial practice to minimize the amount of collector used. Thus, collectors which achieve satisfactory operations at minimum treating rates are desired. Stated another way, a collector which gives a relatively-flat dose-response curve is preferred. Specifically, when the collector dosage (in lbs. collector per long ton of ore) is plotted against % Fe recovery (the response) at a given grade or quality of iron concentrate, the slope of the resulting curve should be as small as possible, optimally zero.
6. Continue to Achieve Equal or Improved Response at High Dosages
When plant operations become more difficult (for example, from changes in ore quality, lower water temperatures, and other factors), it is necessary to increase collector dosage to attempt to achieve target quality from the froth flotation operations. With some collectors, an increase of dosage resulted in a loss in selectivity between silica and iron ore, resulting in a drop in Fe recovery, i.e., the slope of the response--dosage curve referenced in (5) is negative at high dosages.
7. Continue to Achieve Good Performance Under Cold Weather Conditions
On the North American Continent, major iron ore deposits are located in Michigan, Minnesota, and Canada. It has been found that performance of the cationic flotation process becomes poorer when water temperatures drop, even though the specific mechanisms which cause this effect are not well understood. It is obviously desirable that a collector suffer the minimum drop in performance under cold water flotation.
It is also desirable, though less important, that a collector have good physical handling properties under cold weather conditions in its concentrated form. Obviously, lower viscosities and lower freezing points offer advantages in product unloading, pumping, and storage. Energy is saves through minimizing the need for heating the product.
It is an object of the invention to provide a cationic collector reagent which performs better than known collectors in meeting the foregoing requirements for the concentration of mineral ores, particularly for iron ores.