|Mercury ore processing includes oxidizing roasting of cinnabar:|
HgS + O2 = Hg + SO2.
Roast gas pass through dust chamber and enter the tubular cooler made of stainless steel or Monel metal. Liquid mercury drips into iron receivers. It is refined as the liquid mercury thread flows through the 1-1.5 m tall tower with 10% HNO3, flush with water, dried up and sublimated in vacuum.
Hydrometallurgical extraction of mercury from ores and concentrates is also possible. The process includes HgS dissolving in sodium sulphite following with mercury substitution by aluminium. Methods of mercury production by sulphide solutions electrolysis are also worked out.
Preparation of Mercury
|Mercury is extracted from cinnabar by calcination at a high temperature, either alone in contact with air, when sulphur dioxide and mercury are formed, or along with iron or lime with the exclusion of air. With iron, ferrous sulphide and mercury are obtained, and with lime, calcium sulphide, calcium sulphate, and mercury. The first method is most frequently used, the two latter being only employed for rich ores. Since the temperature employed is higher than the boiling-point of mercury, the metal is collected by condensation. |
The old type of furnace, long used in Peru and Spain, was discontinuous. The ore in lumps was placed in a shaft on a perforated arch, and heated by the hot gases from the fire below. This has been superseded almost everywhere by a continuous furnace, of which the Exeli, Novak, and Czermak-Spirek shaft furnaces are examples. In these, the heating gases enter at the side, or the furnace is charged with a mixture of ore and charcoal.
In California, the Huttner and Scott shelf furnaces are extensively used, and are especially suitable for finely divided ores. The ore, in small pieces of about one inch diameter, is placed upon inclined shelves, sloping at an angle of 45° alternately in opposite directions, so that the ore slides down to fill up some of the space between the shelves. The furnace gases pass horizontally between the surface of the ore and the shelf above. The ore, after roasting, is discharged at the bottom of the furnace.
Multiple-hearth, mechanically stirred furnaces have also been used, and even, in very recent times, rotary furnaces, such as are used for cement-burning. For certain types of ore, reverberatory furnaces are more satisfactory than shelf furnaces.
When the ore is treated with lime or iron, retort furnaces are necessary. The mixture is heated in cast-iron or clay retorts of cylindrical or pear-shaped form, and may contain 1-5 cwt. of ore. The method is more costly than the air-roasting process, and produces vapours which are more injurious to the workmen. It has therefore practically fallen into disuse.
In all the processes employed, a mercurial soot known as " stupp " is obtained. It consists of finely divided mercury, together with mercury sulphate and chloride, and the sooty products and ash of the fuel. It accumulates in the condensers, and is collected and treated for the recovery of mercury. Some of the mercury may be removed mechanically by pressure or friction. The residue may be mixed with ore and returned to the furnace.
In the earlier types of condensing apparatus, part of the condensation was effected in a series of " aludels," or pear-shaped fireclay condensers, glazed on the outside and with a narrow neck fitting into the wide end of the succeeding one. Forty to forty-five of these were arranged in parallel rows, and communicated ultimately with condensing chambers. The aludel furnace used at Almaden in Spain was introduced into Idria about 1750, but was soon altered to the form known as the Idrian furnace, in which the aludels were replaced by large brick chambers. In the present-day furnaces the condensers consist of cast-iron pipes of elliptical section. Where acid vapours are likely to condense, the pipe is lined with cement to avoid corrosion of the iron, or glazed stoneware may be used. These condensers are water-cooled. From the condensers the vapours pass into flat wooden flues or dust chambers, in which the soot poor in mercury is collected.
Purification of Mercury
|Since mercury is used in the construction of many instruments of precision, and for other scientific purposes, much attention has been given to the question of the purification of mercury. The methods employed generally resolve themselves into one of three types. Purification from mechanical impurities is effected by filtration through leather, muslin, or filter paper pierced by small holes. Purification from dissolved impurities - that is, foreign metals - may be brought about by distillation in vacuo, or by some chemical method, such as the oxidation of the metallic impurities by heating in a current of air, and subsequent removal of the oxides by filtration, or the solution of the impurities by some modification of Lothar Meyer's method, in which mercury is allowed to fall in a finely divided stream through dilute nitric acid, acidified mercurous nitrate, or ferric chloride. The test for pure mercury is the ability to form spherical globules on a clean glass plate, without showing any tendency to adhere to the plate.|