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Sorption of Detrimental Impurities from Sewage

#K-1875


Purification of Industrial and Domestic Sewage from Arsenic, Chrome and other Detrimental Impurities

Tech Area / Field

  • ENV-EHS/Environmental Health and Safety/Environment

Status
3 Approved without Funding

Registration date
23.08.2010

Leading Institute
Chemical and Metallurgical Institute, Kazakstan, Karaganda

Supporting institutes

  • Scientific and Analytical Center Biomedpreparat LLC, Kazakstan, Stepnogorsk\nJSC «Kaustic», Kazakstan, Pavlodar reg., Pavlodar

Collaborators

  • Colorado School of Mines, USA, CO, Golden\nUniversita degli Studi di L`Aquila / Dipartimento di Chimica, L`Aquila

Project summary

Objective of the Project – application of carbon sorbent from candle coal for the extraction of detrimental impurities – arsenic and chrome from industrial solutions and sewage and use of tar obtained as a by-product of coal pyrolysis for deep concentration of nonferrous metal ores.

Current situation in the field

Adsorption is a most popular method of arsenic and chrome extraction from sewage. Activated carbon, synthetic sorbents, industrial wastes (ash, slag, sawdust, etc.) are used as the sorbents. Mineral sorbents – clay, silica gel, alumogel and metal hydroxides are used less often because of high energy of interaction with water molecules which sometimes exceeds the adsorption energy. Activated carbon sorption has the following advantages: 1) Purification to maximum allowable concentration (MAC); 2) Combined extraction of impurities of different nature; 3) No secondary contamination of sewage; 4) Reusability of water after pH normalization. Porous structure of the carbon sorbent has strong influence on a sorptive capacity. Sorbents with pore size around 2 nm (nanoporous) are most suitable for Cr (VI) ions sorption. Numerical values of the sorption may be as high as 0.8-0.9 g/g. According to the literature, low pH (2-3) promotes redox reactions on the surface of a carbonaceous material – recovery of Cr (VI) to Cr (III) with simultaneous oxidation of the sorbent surface which leads to increasing sorption capacity. The stronger the surface charge, the more oxygen-containing groups the surface has and the higher the degree of interaction of Cr (III) ions with the sorbent surface. Cleaning of sewage from the arsenic is usually done with modified sorbents made of coal with several additives, such as kerogen shales, naphtha residue which allow to significantly improve the sorption properties and the selectivity of the sorbent. Arsenic sorption capacity of the semi-coke adsorbent made of pyrobitumen is 1.5-1.8 times higher than that of the active carbon of Norit (Netherlands) and Futamura and Dioxon (Japan) although use of semi-coke and polycondensate increases the technology cost. Main properties required for the carbonaceous sorbents are: high sorption capacity, mechanical strength and selectivity. Traditional methods of microporous carbon sorbents manufacture include two stages of the thermal treatment (carbonization and activation) that provide formation of adsorbents pores. The nature and properties of carbonaceous material’s surface contain the possibility of directional chemical modification allowing to obtain sorbents with controlled physical-chemical characteristics. In this case the sorption capacity depends on the activation method that also determines the surface state, microporous structure development, regularity and imperfection of the structure. Sufficient chemical and thermal stability of the sorbent with wide range of pH creates the possibility of thermochemical regeneration and modification of the sorbent for specific purposes. The porosity nature (size and distribution of pores) of the feed stock is usually not considered in the process of adsorbent manufacture from candle coal. However, porosity characteristics strongly influence the properties of the adsorbent. In the table 1 we represent comparative data on physical-chemical properties of Kazakhstan coal and well-studied candle coal (grain size 2-5 mm).

Table 1 Physical-chemical properties of coals.



Parameter

Mesoporous coal

Kazakhstan coal

Technical composition, %
Ad

-

5,3

Wd

-

5,7

Vdaf

37

39,0

Bulk density, g/cm3

0,675

0,670

Specific surface area, m2/g

50 - 110

92

Iodine activity, %

1,3

2,5

Clarifying capacity for methylene blue, %

12

18

The characteristics represented in the table 1 show that the properties of the Kazakhstan coal are very close to that of the mesoporous coals recommended as a valuable feed stock for manufacture of adsorbents with high sorption capacity, i.e. developed mesoporous structure.

The necessity of processing refractory nonferrous ore with complex composition often causes degradation of the process parameters – lower quality of concentrates and growth of metal loss with final tailings. Thus, the production and commercialization of the new reagents and flotation techniques for various types of ore have become an urgent problem. Based on the study of interaction nature of various flotation agents with the sulfide minerals surface we propose new flotation agent that would have both collective and foaming properties. Our technology is unique in that it allows to avoid the necessity of regeneration of used carbon sorbent. The sorbent, saturated with chrome may be used in metallurgy while the stripped arsenic together with coal pyrolysis tar is suitable for flotation agent synthesis. Such technique allows multipurpose utilization of the products obtained from the candle coal.

The influence of the Project on the progress in the field

Sewage purification with a sorbent made of domestic raw materials has certain economic advantages as it allows to reduce the treatment costs. The new flotation agent allowing to intensify the ore concentration process is of great interest for the nonferrous metallurgy as it also represent significant economic benefits.

Qualification of participants

Qualification of our specialists and availability of the groundwork for the Project is seen from the list of published works available at our website http://www.hmi.kz

Other organizations expected in the Project have considerable experience in the monitoring of hazardous substances (e.g. mercury) in the sewage.

Expected results and application

Discontinuance of natural reservoirs pollution with arsenic, mercury, chrome and other impurities would help to solve the global environmental problem, particularly urgent for industrial regions of Kazakhstan. Arsenic, chrome, mercury and ions of other heavy metals that ingress the environment may have cancerogenic, mutagenic and teratogenic action. Modern technologies do not ensure efficient purification of sewage from dangerous substances. The content of the latter in the liquid wastes is 10-20 times higher than MAC, which makes sorption technology an indispensable stage in the treatment of sewage and drinking water and in the development of closed water supply systems for industrial enterprises.

Correspondence of the current Project to ISTC goals

Expected results of this Project meet the goals of ISTC – solution of environmental problems and environment enhancement in the industrial regions.

Scope of work

The following tasks are to be accomplished within the Project:


Task 1. Development of technology of multipurpose processing of Kazakhstan candle coal.
Task 2. Development of technology of sorption purification of industrial solutions and sewage from organic and non-organic detrimental impurities.
Task 3. Development of techniques of tar stock application for improvement of flotation concentration process.

Foreign collaborators

The following scientists have given their consent to participate in the Project: Mr. Corby Anderson (USA) and Mr. Francis Veglio (Italy)

Research methods and approaches

Methodology includes production of high-quality carbon adsorbent by a new method based on combined carbonization and activation in single device. The second stage is analysis of sorption characteristics of the new sorbent by standard methods and development of flowchart of sewage purification. The third stage includes research on production of flotation agent from arsenic and tar obtained as a by-product of coal pyrolysis.


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