Dorr

Gold Ore Roasting Techniques

In the case of many complex gold and silver ores roasting before cyanidation is essential if satisfactory extraction of the precious metals is to be obtained. In such cases no practical amount of grinding or prolonged contacting of the raw ore with cyanide solution will affect more than a certain low extraction; in other cases, while the extractions may be acceptable, the consumption of cyanide is prohibitive, and roasting or, occasionally, pyritic smelting of the material is the only alternative. Roasting, which is today almost always carried out following concentration of the values into a small bulk, must be used if it becomes necessary to decompose the minerals with which the gold is associated in order to expose it to solvent action. Many of the cyanicides in the raw ore are broken up and rendered harmless by roasting, but the calcine may contain new compounds that must be removed by water or acid washing before cyanide treatment can be successfully carried out.

There were instances, where at one time the whole of the ore was roasted before cyanidation, but this practice has been for the most part discontinued on account of the large plant and high overall costs involved. With ….Read more

Gold Metallurgy & Leaching in Cyanicides

Cyanidation as applied to ordinary gold and silver ores is a relatively simple process. When cyanicides {cyanide-consuming elements) are encountered in small amounts in the treatment of such ores, the various schemes already discussed, such as use of a lead salt or wasting barren solution, can usually be resorted to and successful operation maintained. When, however, the problem concerns the treatment of an ore that does not respond to these simple expedients, certain modifications of the cyanide process must often be considered.

In the present chapter the various cyanicides and methods of controlling them are discussed, as well as the causes of refractoriness in certain ores, together with recommended treatment procedures. Where these fail, it may be desirable to incorporate cyanide regeneration or roasting into the treatment scheme. Here is a list of methods to help you on how to manage gold leaching when cyanide consumers are present.

Recovering Gold from Iron Sulphides

While the oxidized iron minerals ordinarily have little effect in cyanidation, the sulphides—pyrite, marcasite and pyrrhotite—tend to decompose in cyanide solution.

Pyrite is the most stable and least troublesome of the sulphides. Flotation concentrates high in pyrite content are frequently cyanided without undue consumption of reagents. Marcasite decomposes ….Read more

Gold Shaker Tables

Overflow from the grinding mill runs over Gold Shaker Tables which are flat and smooth and made of a fibrous wood known as “yolombo.” They are arranged in series much the same as blanket or amalgamation plate tables. The grain of the wood in the deck runs with the flow of pulp, and a sharp-pointed tool is used to scratch its surface in a crosshatched pattern. The fibrous wood stands up along the scratches and forms an ideal trap for free gold. Twice a day the tables are cleaned and the high-grade concentrate is amalgamated by hand in a wooden batea or gold pan.

During the early days of operation referred to, nine wooden mills totaling 66 stamps, one ten-stamp California-type stamp mill, and three sand-leaching cyanide plants were in operation. Use of this equipment was discontinued.

diester shaker table

Bumping Shaker Tables

The gravity concentrators used in gold and silver milling plants are usually of the bumping-table or the endless-belt (Vanner) type. The former consists of a riffled deck carried on a supporting mechanism that permits adjustment of slide slope and connected to a “head” mechanism that imparts a rapid reciprocating motion ….Read more

Gold Flotation

Though the gold recovery methods previously discussed usually catch the coarser particles of sulphides in the ore and thus indirectly recover some of the gold associated with these and other heavy minerals, they are not primarily designed for sulphide recovery. Where a high sulphide recovery is demanded, flotation methods are now in general use, but in the days before flotation was known, a large part of the world’s gold was recovered by concentrating the gold-bearing sulphides on tables and smelting or regrinding and amalgamating the product. Though the modern trend is away from the use of tables, because flotation is so much more efficient.

GOLD FLOTATION

The flotation process, which is today so extensively used for the concentration of base-metal sulphide ores and is finding increased use in many other fields. In 1932 flotation plants began to be installed for the treatment of gold and silver ores as a substitute for or in conjunction with cyanidation.

Flotation Machine

The principles involved and the rather elaborate physicochemical theories advanced to account for the selective separations obtained are beyond the scope of this book. Suffice it to say that in general the sulphides are air-filmed and ufloated” to be ….Read more

Gold Recovery Methods

Regardless of subsequent treatment, it is considered best to recover free gold as early and as completely as possible in the flow of pulp. Gravity concentration in various forms is made use of for this purpose and amalgamation used for recovering the gold from the gravity concentrate.

Concentration is also extensively practised where a large part or all the gold or silver is intimately associated with base-metal sulphides, and the technical and economic considerations involved indicate that such a step in the flow sheet shows advantages over direct cyanidation. Flotation methods, with or without some gravity concentration, are usually employed for this purpose.

The concentration of gold and silver ores has two principal objects in view:

  1. The recovery of free gold.
  2. The collecting of gold or silver values which are associated with sulphide minerals into a relatively small product for subsequent treatment. The latter may involve (a) shipping to a smelter, (b) selective grinding for cyaniding in the mill circuit, (c) cyaniding in a separate circuit with or without roasting.

Recovery of Free Gold

Methods for the recovery of free and relatively coarse gold can be classified as:

A. Direct amalgamation, using

  1. Mercury added to mortar boxes or ball mills.
  2. Amalgamated copper plates.
  3. Tray-type ….Read more

Counter Current Decantation Design Formula

The principle of Counter Current Decantation is simply that, when water or solution is to act upon solids, both are made to pass, in contact, in opposite directions, so that at each end the strongest or most potent portion of either is acting upon the weakest or most exhausted portion of the other.

The recovery of dissolved gold and silver from slime pulp in the cyanide process, as first practiced, employed intermittent decantation.

Attempts to make this process continuous instead of intermittent were made as far back as 1902, but without success. The invention of the Dorr thickener furnished a means of continuous slime settling on a large scale and made the Counter Current Decantation process possible.

Although flow sheets and operation will vary with conditions, the following will give a general idea of the Counter Current Decantation process.
After the ore has been reduced to a uniform fineness by wet grinding and classification and the major portion of the solution removed, it is agitated continuously in a series of about three agitators with cyanide solution, where most of the gold and silver is dissolved. The mixture of solids and solution is fed continuously to the first thickener of the C.C.D. series, consisting of ….Read more

Rotary Disk Filter Flappers

The concentrate is filtered on two 8 by 10 ft. Oliver filters. One of these filters is used for each roaster. The filter cake discharges onto a conveyor belt which feeds the charge directly to the roaster.

The concentrate filter cake is at best a sticky, puttylike mass. If the moisture exceeds 20 per cent it becomes difficult to handle the cake on the belt and at the charging chute.

The best filtering technique which could be obtained on the 26 per cent sulphur concentrate produced a cake with from 21 to 22 per cent moisture. This was too sticky for continuous use and introduced an excessive amount of moisture into the furnace, which slowed the commencement of roasting.cross-section

To reduce the moisture content of the cake, a device was copied from the Consolidated Mining and Smelting Company in their Sullivan concentrator, which, according to them, was originally developed at Granby. This device consists of a flapper fixed on a revolving shaft parallel to the axis of the filter drum. The flappers are made of old conveyor belt about 12 in. wide and the length of the filter.

The single flapper reduces the moisture from 21 to 22 per cent ….Read more

Gold Leaching Agitation Tanks

The mixing of solids suspended in a fluid medium is still largely an art. The development of fundamental laws governing the operation is complicated by the large number of variables involved, some of which can hardly be evaluated in mathematical terms.

Practical studies of the circulation patterns of the more common types of agitators are to be found in chemical engineering literature. In one of the more recent articles on this subject the author summarises a few rules for agitator design as follows:

For ratio of tank diameter divided by impeller diameter, take 4:1 for simple blending of light material; 3:1 for the average job, and 2:1 for heavier density or high viscosity material. The approximate impeller speed should be 700 peripheral feet per minute for turbine-type impellers and 1000 to 1500 ft. per min. for the propeller type. The horsepower requirements can be estimated from data given in the various engineering handbooks.

Because in certain cases power input appears to be directly related to agitator performance, i.e., in gas absorption and emulsification, a rather intensive study has been made of the problem. Thomas Hooker investigated the power function M versus agitator Reynolds number Re. Secondary dimensions such as pitch, liquid depth, and ….Read more

Leaching Agglomerated Slimes

More recent work along these lines is reported in T.P. 790, A.I.M.E.’ by 0. C. Shephard and C. F. Skinner presented at the New York meeting in 1937, under the title of “Stabilizing Agglomerated Slimes for Gold Cyanide Leaching.” The paper describes the development of a method of stabilizing agglomerated slimes by the formation of a cementing substance in the glomerules. The conclusions based upon agglomeration tests were as follows:

  1. The porosity and permeability of finely ground ores can be greatly increased by moisture agglomeration.
  2. Variations in the amount of solution used in agglomeration causes a noticeable difference in the amount of permeability. The maximum permeability is reached between 10 and 18% moisture.
  3. A point of saturation occurs when too much solution is added, causing the glomerules to break down to a runny mud. Beyond 22 per cent moisture, none of the materials tested had a measurable permeability by the method used.
  4. The amount of solution necessary to give permeability by agglomeration increases with the fineness of the particles.
  5. The permeability of loose beds of agglomerated material decreases with packing, but the permeability decrease becomes less as packing progresses.

Vat Leaching of Finely Crushed Gold Ore

Vat leaching is carried out in vats ranging in capacity from 30 to 1200 tons. Sand for leaching is separated from slime in cones, V boxes, classifiers, and in collecting vats filled by distributors—the overflow in each case being slime or finer portion of the ore. As a rule, leaching is a simple process, involving a vat of well-mixed neutralized sand, ample contact with strong and weak cyanide solutions, water washes, aeration of the sand and solution, and rapid filling and discharging. It is a cheap and effective process for clean ores when fine grinding is not necessary for good extraction.

cyanide-vat-leaching http://www.groundtruthtrekking.org/photo/cyanide-vat-leaching/

The importance of classification prior to leaching cannot be over-emphasised. Sands that are essentially free from colloidal material behave quite differently from the same type of sand with a small percentage of colloid. The two following examples from plant practice are illustrative.

The Golden Cycle mill at Colorado Springs, Colo., grinds roasted siliceous ore in rod mills through 20 mesh before sand-slime separation at about 200 mesh. Prior to the development of the Dorr bowl classifier at this plant, this separation was made in Dorr classifiers which produced a sand containing about 15 ….Read more

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