Analyze the cyanide presents in the pulp for controlling the process

During the process its necessary take samples and analyze the cyanide presents in the pulp for controlling the process, and for environmental reasons. The main terms involved are free cyanide, weak acid dissociable cyanide, and total cyanide.

Free Cyanide (CNF). Only hydrogen cyanide and the cyanide ion in solution can be classed as "free" cyanide. The proportions of HCN and CN- in solution are according to their equilibrium equation; this is influenced by the solution pH. Methods used to detect free cyanide should not alter the stability of weaker cyanide complexes, as they may otherwise be included in the free cyanide result. Methods used to detect free cyanide should be clear of interferences due to the presence of high concentrations of more stable cyanide complexes or other cyanide forms. If not, the interference must be quantified and allowed for in the result.

Weak Acid Dissociable Cyanide (CNWAD). Unlike the definition of "free cyanide" which identifies the specific cyanide species being measured, WAD cyanide refers to those cyanide species measured by specific analytical techniques. WAD cyanide includes those cyanide species liberated at moderate pH of 4.5 such as HCN(aq) and CN-, the majority of Cu, Cd, Ni, Zn, Ag complexes and others with similar low dissociation constants. Methods used to measure WAD should be free from interferences due to the presence of high concentrations of more stable cyanide complexes or other cyanide forms. If not, the interference must be quantified and allowed for in the result.

Total Cyanide (CNT).This measurement of cyanide includes all free cyanide, all dissociable cyanide complexes and all strong metal cyanide including ferro-cyanide Fe(CN)6-4, ferri-cyanide Fe(CN)6-3, and portions of hexacyano cobaltate Co(CN)6-3 and those of gold and platinum. Only the related or derived compounds cyanate (CNO-) and thiocyanate (SCN-) are excluded from the definition of total cyanide. Methods used to determine total cyanide must be shown to be capable of quantitatively determining all stable complexes of cyanide, including the cobalt cyanide complex. If methods determine other analytes as well (e.g. include SCN-), those analytes need to be determined separately and allowed for in the total result.

To accurately determine if there is enough cyanide or caustic in a solution, a titration is performed. This indicates the ppm (parts per million) of cyanide that have not been complexed with other elements. The term that is used to describe this amount is free cyanide. To maintain the levels of reagent in solution at their optimum level there should always be some free molecules of either reagent. There are chemicals that will react to each of these two reagents in a predetermined manner. By measuring the amount of chemical that is used to obtain that reaction it is possible to determine the amount of reagent that is free in the slurry. The reagents and methodology used in performing the titration will change depending upon lo cal circumstances and the preferences of the plant metallurgists.

While the taking of either aqueous or solid samples may appear easy, the collection of correct samples, both in terms of location and with respect to the analytes to be monitored, is fraught with difficulties. Any sampling must have as its aim the collection of a representative portion of the substance to be analyzed. When this portion is presented for analysis, the parameters to be determined must be present in the same concentration and chemical or biological form as found in the original environment from which the portion was removed. Samples representative of a site, or of a portion of a site, provide information that is often extrapolated to include the whole area under investigation. This is true whether the entity being sampled is a contaminated section of land, surface water, an industrial outfall, or a drum containing waste material. Therefore, samples must be representative of the specific entity being sampled, but not necessarily representative of the entire area of which it is part.

The overall objectives of a sampling program must be considered in the development of the sampling plan. Sampling may be performed for one of several purposes: Maximum, minimum and average values for a near steady state stream with the aim of monitoring compliance versus set specifications (process control, environmental criteria). Such data can illustrate the likelihood and magnitude of occurring non-compliance provided enough data points have been analyzed from samples.

Process, residue, and effluent stream analysis could have this type of objective. Even aquifer sampling (bore-holes) would fit this description. Often the relative mass-flows have to be known for proper data integration. Maximum, minimum and average values derived from the analysis of "batch streams" such as treated backfill portions or detoxified waste batches usually require a minimum of one data point per batch to insure a representative sample. The major objective remains one of compliance and/or verification of effective management procedures for the batch streams involved.