WO1999047467A2 - Method for producing sand-lime bricks from lime residues-containing cyanide - Google Patents

Abstract

Lime residues containing cyanogen are treated with silica sand or silica sand residues in autoclaves at a temperature of at least 110 °C and at a pressure of 2 bars, whereby CaCN2 is broken down into CaCO3 and ammonia.

Description

Patent application

Process for the production of sand-lime bricks from limestone residues containing cyanide

Summary

Lime residues containing cyan are treated with silica sand or silica sand residues in an autoclave at at least 110 ° C and 2 bar pressure with water vapor, whereby CaCN ₂ is broken down into CaCO ₃ and ammonia.

State of the art

The production of sand-lime bricks from silica-rich sand and lime (CaO or Ca (OH) ₂ ) has been a very well-known technical process for decades.

Ost-Rassow, Textbook of Chemical Technology, pp. 401 - 402

The use of silicate minerals from mineral processing is also common practice.

Pigott, P.G., Valdez, E.G., Dean, K.C, Dry pressed building bricks from copper mill tailings, US Bureau of Mines Report of Investigations No. 7537, 1971

Cyan-containing residues from the chemical industry and from and mineral extraction pose a problem for the environment. They either have to be deposited in suitable landfills or have to be detoxified using complex processes.

Edmonds, C.J., Cyanide destruction. Gold mill effluents, Proc. Annu. Meet. Can. Miner. Process., 5 ^ (1973), pp. 275-286. Editor: Joe, E..G. Publisher: Can. Miner. Process., Ottawa, Ont.

Kroeger, Edwin Bane, Analysis and comparison of cyanide detoxification methods from spent heaps, dissertation, Univ. of Alaska, Fairbanks, AK, USA

It is known to use calcium cyanamide with steam under pressure according to the formula:

Decompose CaCN ₂ + 3 H ₂ O = CaCO ₃ + 2 NH ₃ in calcium carbonate and ammonia.

So far, many processes have been described to detoxify cyanide residues using specially developed processes.

The idea of combining a production process with the simultaneous detoxification of the cyan is new.

Lime residues containing cyan are to be used for the production of sand-lime bricks. Experiments carried out have shown, surprisingly, that with the same proportion of CaOH ₂ in the sand-lime mixture and with an SiO ₂ content of over 95% in the sand, the strength is reduced by the formation of CaCO ₃ during the decomposition of the CN ₂ increased by 8%.

The novel method according to the invention consists in mixing lime residues with a high content of CaO and / or CaOH ₂ , which are contaminated with cyan compounds, with silica sand of high purity and small grain size and pressing them into stones. The compacts are usually treated in autoclaves at at least 160 ° C and at least 6 bar for several hours.

The following implementations take place:

Si0 ₂ + Ca (OH) ₂ + H ₂ 0 = CaHSi0 ₂ + H ₂ 0 CaCN ₂ + 3 H ₂ O = CaCO ₃ + 2 NH ₃

After the autoclave process has ended, the ammonia / water vapor mixture is withdrawn from the autoclave and e.g. converted into ammonium sulfate with sulfuric acid and thus used for further use.

The process of stone production corresponds to the known manufacturing process of sand-lime bricks.

In this way, lime sandstones can be produced that meet the DIN specifications. Due to the formation of CaCO ₃ from CaCN ₂ , the stones are particularly strong.

A new variant of the process described above is to replace the usual silica sand with sand exits from mineral processing, especially those that are contaminated with cyan. The sands are also detoxified. The use of these silica sands reduces the risk to the groundwater caused by soluble heavy metals even when cyan is detoxified.

Kyle, JH, Stability of metal-cyanide and hydroxide complexes, Publ. Australas. Inst. Min. Metal. (1997), 2/97 (World Gold ^'97), pp. 163-169

The following example serves to explain the method according to the invention:

1,000 kg of silica sand or sand residues, e.g. B. from ore processing, with at least 95% SiO ₂ and a grain size less than 0.3 mm with 100 kg of a lime residue that contains at least 95% CaO or Ca (OH) ₂ and with 1%, corresponding to 1 kg, cyanides is contaminated, pressed to stones. If the lime residue consists mainly of CaO, CaO must be converted to Ca (OH) ₂ by adding water before stone pressing.

The stones are then treated with steam in an autoclave at 180 ° C and 10 bar pressure for 8 hours. Calcium hydrosilicates are formed and the cyan compounds are decomposed in 1.25 kg CaCO ₃ and 425 g ammonia. The ammonia-containing water vapor is removed from the autoclave and 425 g of ammonia are converted to 1.650 kg of ammonium sulfate with 1.225 kg of sulfuric acid.

Claims

claims 1. A process for the production of sand-lime bricks, characterized in that cyan-containing lime residues are used alone or together with CaO and / or Ca (OH) 2 as binders for quartz in the conventional sand-lime brick production. 2. A process for the production of sand-lime bricks, characterized in that silicatic exits of the mineral ore processing are used instead of or together with quartz sand. 3. A process for the production of sand-lime bricks, characterized in that the vapors formed during the hardening of the stones in the autoclave are drawn off from the steam and ammonia from the autoclave and the ammonia is chemically bound. 4. The method according to claim 1, characterized in that the cyan-containing residues are freed of any alkalis before they are used. 5. The method according to claim 2, characterized in that the silicate treatment exits are subjected to a process for the enrichment of the silica before they are used. 6. The method according to claim 1 and 2, characterized in that the cyan-containing lime residues and the silicate treatment exits are detoxified in an autoclave with steam at a temperature above 110 ° C and pressures above 2 bar by converting the cyan into ammonia.