High Temper " ¹ "** Process and Plant for Treatment and Resources Recovery of Hazardous Waste

Technical Field

The invention relates to an industrial process for the treatment of hazardous waste having various ori¬ gins, of both organic or inorganic nature, also mixed in certain limits, while it is provided by means of new se¬ ries of elementary processes to destroy their molecular structure -starting from a one which evolves at high tem¬ perature in a melted iron bath- and providing to the re¬ covery of the toxic metals, if they are present, or of other metals when it is convenient.

In all cases solid residuals or gases are genera¬ ted which result totally acceptable to the environment.

Background Art

Based on the state of technology and, in particu¬ lar, in regards to the recovery of heavy metals cadmium, mercury and zinc, including their oxides, it is possible the utilization of a reactor containing melted iron, in which carbon is present as solute and with a concentration close to saturation, with scope to separate the vapors of said metals by means of reducing gas current. The vapors end up in a reducing gas stream. It is also known that their condensation is possible in order to render the gases free from the presence of metals. Other gas treatments are known in which all residuals of said metals are eliminated prior to their release in the atmosphere, in order to meet air protec¬ tion requirements dictated by law. The above treatment is described in the Italian

Patent Application 12454 A/90 dated 27 April 1990 and

deals with the treatment of materials as they are, or under oxides form. The metals recoverable are mercury, cadmium and zinc.

The described treatment has proved to be suitable only for specific waste, that is for exhaust primary cells, and in particular when no other elements that may impede the performance of the process are present in the waste.

The elements that invalidate the efficiency of the process are the halogens; amongst them chlorine is the one found most frequently in the toxic waste when such waste are not formed by exhaust cells collected in a differentiated manner.

For exemple, in absence of a differentiated col- lection, the above mentioned metals - together with other like Cu, Pb, Ni, Cr, and As - are found in flying ashes produced from solid urban waste incinerators, when these incinerators are provided with equipment suitable for the capture of said ashes. The disposal of said ashes representes a great danger of soil pollution, in that, the metals are diffused by leaching. And this generates water pollution.

The flying ashes containing chlorine in considera¬ ble quantity can not be treated in a type of plant as described in the above mentioned patent application.

About the treatment of waste different in nature

(organic or inorganic) and of different origin, processes are known to specifically treat single hazardous waste or some kind of them; no process is known valid for both waste containing toxic metals and organic waste.

Very stable organic compounds create unsafe condi¬ tions during waste treatment by means of technical equipment available at this time.

Depending upon their heating value, waste are divided in combustible ones, with a low heating value and

inert ones .

Industrial furnaces and incinerators are conside¬ red today general purpose plants for waste treatment.

Proper utilization of known processes is generally limited to organic waste; genaral purpose plant are not yet designed to eliminate toxic metal.

In the latest treatment units, the incinerators are coupled with plants which perform physical and/or chemical treatment on metals containing waste (inor- ganic). Such units are capable to treat all the hazar¬ dous waste produced in geografically defined areas.

In regards to the energy recovery many attempts are made today aiming at energy recovery from waste.

However, no system has completely overcome the difficulty in dealing with energy generation unit to ensure the absence of toxic metals in the gas effluent and sometimes the presence of undecomposed organic micro- pollutants. Additional problems are posed by solid residuals containing heavy metals. Finally, the type (or level) of recovered energy is important; rarely the energy of the organic waste is produced in form of combustible gas, having a medium heating value, useful for utilization in many energy pro¬ duction systems.

Disclosure of invention

A first topic of the present invention is repre¬ sented by a process suitable for recovery of heavy and toxic metals even from waste containing halogenides, that is even when metals are present together with chlorine or other halogen elements.

A second restriction of the process described in the previous patent consists in the limited number of metals that can be recovered after the evaporation, according to the physical conditions determined by the process. Of great interest is the possibility to recover

lead which is an element frequently present in the hazar¬ dous waste.

Therefore, a second improvement which is also a second topic of the present invention is a special fea- ture obtained at reduced pressure by the same process, being the pressure approssimately one tenth of an atmo¬ sphere.

Such operational practice - which is a transpo¬ sition of the vacuum metallurgy - allows, when it is ap- plied to the process according to the instructions de¬ scribed later on, lead recovery in the metallic form.

It is, generally speaking, the lead recovery from exhaust secondary cells; being lead incontaminated by other metals, the process proves to be very simple. In fact it is possible to eliminate many preli¬ minary operations regarding the disposal and the refining steps which, following the procedures known in etal- lurgic technology, are utilized to reach the same result. The third and most important characteristic feature of the present invention allows to considerably increase the possibility of safe and efficient treatment of hazardous waste having different origin, to include among these organic waste; it is based on the feature of the new process obtained by a particular feeding techni- que: materials to be treated are fed from the bottom of a Reactor which contains melted iron, when the physical conditions stated earlier exist and the previously established improvements are effective.

The feeding through anular shaped conduits, con- centric with circular shaped ones, through which oxygen or air is fed, is adopted in the iron metallurgy process called OBM (or Q-bop) . Such technique is based on utilization of cooling effect corresponding to the heat required for evolution of the endothermic hydrocarbon de- composition reactions (pyrolysis or cracking) .

The scope is to cool down the refractories which are in direct contact with the reaction (decarbu- rization and other refining reactions) zones in those points in which the ducts carrying oxygen merge into the Reactor. On the contrary, the oxidation reactions of iron and other elements dissolved in the latter are charac¬ terized by a great production of heat.

Using this feeding technology, when properly transferred, according to the present invention, to a Reactor suitable for the treatment of hazardous waste in iron melted bath, the most favorable conditions are obtained for the required tranformations for both inor¬ ganic or organic waste. In the flame zone thermal flows are noticed to assume very high values and are useful for pyrolysis reactions of the complex molecules constituting organic waste.

Due to dissolved carbon and to the presence of reducing gases in said zones, conditions are found suitable for oxides reduction of those metals having high enough formation free energy.

In a treatment plant operating according to the present invention, the solutions mentioned above can be separately adopted, but, since each solution is carried out by a specific ^{^} equipment, the simultaneous utilization of all equipment provides a plant capable to treat a great variety of waste.

The single equipment can be used at different ti¬ mes. In the present invention many alternative ways for waste immission (gases, liquids or solids in reduced size) are available. Solid waste of bigger size are fed from the top, after being properly dried. A part from this last case, it is always possible to convey, using where necessary a transport gas, waste in selected reaction zones. In regards to the treatment of waste containing

chlorine, the presence of oxide compounds of evaporating metals must be excluded in the physical conditions exis¬ ting in melted iron bath. The same holds true in gas phase. It is known that, according to partitioning of the compounds in the phases present in the system, the oxides of some metals are transferred in the slag, those of heavy and volatile metals are reduced and transported by the gas phase, while the oxides of other metals are, in a partial or total amount, reduced and transferred in the liquid phase together with the iron (Cu, Ni, Co, Cr,- V, etc.).

When the chlorine deriving from waste is present in the system - or other elements called halogens - volatile metals are not found in the gas phase, rather their chlorides (halogenides) are present. The separation and the recovery provided for such metals (Hg, Cd, Zn, Pb) becomes therefore impossible.

In order to overcome such difficulty a new solution has been identified as illustrated below. The solution is based on the properties of a compound, that is calcium chloride (CaC12) that, according to the present invention, is found suitable to eliminate the chloride from the ^~ gas current exiting in the melted iron T bath. The CaC12 exists at liquid state in a wide temperature range, from 772°C up to 1.936 ^β C, and is very stable; the free energy of the reaction 1) is known in the range from 8390C to 1.484 C. The values are shown in relation to the temperature by 1.1): 1) Ca + C12 = CaC12

1.1) ΔG° * - 190.000 + 34 ,89 T In Fig. l the relation 1.1) is shown, together with the the chloride potential for other chlorides involved in the treatment process . Due to the greater stability of CaC12 , the

chloride can be fixed as CaC12 at liquid state; obtaining such compound by means of an addition of dry lime (CaO) made by proper procedures in the gas stream exiting the metallic bath. The new elementary process evolves according to a path involving reaction between a solid (liquid) and a component of the gas phase ( Cl, F, Br, I). Similar reactions are found in the modern desulphurization pro¬ cesses used for power plant waste gases: in these cases the element to be fixed is sulphur.

The reaction allowing the chlorine elimination is: 2) CaO + C12 = CaC12 + 1/2 02 2.1) j_) G ⁰ = - 37.860 + 8.94 T The value of the equilibrium constant is, at l.δOO^C: K(1.500°C) = 515

While at 1.000°C : K(1.000°C) = 37.1

The thermodynamics of chlorine elimination is then favorable in the temperature range of the process.

When combined chlorine is considered, for example, with zinc, the following simultaneous reactions must be considered which lead to the global reaction 7):

3) ZnC12 = Zn + C12 G-= 93.300 - 25,IT

4) CaO(s) = Ca +1/202 ΔG^= 153.000 - 25,93T

5) CO + 1/202 = C02 __G°= -67.000 +20,3T 6) Ca(l) + C12 = CaC12 ΔG ^o =-190.000 +34,89T

7) CaO(s)+ZnC12+CO=CaC12+Zn+C02

ΔG ^ύ = -10.250+4,21T About the thermodynamic constant one must observe that at 1.500°C K(7) = 2,2 and at 1.000 ^» C K(7) = 7,82

IZnI*IC02I

Where K(7) =

|ZnCl2|*|CO| If one further considers that the values of the ratio

|C0|/C02| are as high as 10.000 for the gas exiting from the iron bath, it must be concluded that the reaction 7) evolves in the right direction giving metallic zinc, plus CaC12. Similar considerations are valid for other chlori¬ de (CdC12, HgC12). The addition of CaO is effective also when floride or other halogen elements are present, due to the above mentioned conditions.

In order to operate within the required tempera- ture range, lime additions must be made, according to the present invention, in a point located at the exiting of gases from metallic bath ( in the upper part of the pyrolysis Reactor) to the initial part of gas collection ducts. In regards to the treatment of waste containing lead, it is known that the boiling temperature of the lead at atmospheric pressure is 1.747°C.

Some metallurgical processes are known to operate at 950° C, using a reduced pressure of 0,001 atm and producing pure lead ( apart from the presence of more volatile metals ) through vapor condensation. From frac¬ tional condensation technology different metals are obtainable separately.

According to the present invention, the utiliza- tion of the conditions existing in the iron bath, among which the temperature, a lower vacuum degree is necessary (about 0,1 atm) at a temperature of 1.250°C-1.300 ^β C.

The existing reducing conditions are able to pro¬ duce lead vapors free from their oxides. The same for Hg, Cd and Zn.

Similar conclusions made for the above mentioned metals are valid when lead halogenides are present.

In regards to the waste treatment of different na¬ ture (organic or inorganic) and having different origin, the known processes have their main limitation in the two

fundamental thermodestruction factors: the temperature and the residence time (time allowed to the compound to travel in the Reactor in a designated region where tem perature and other physical parameters exist, to be de- composed in simpler molecules). For particurarly stable organic waste it may happen that, given the residence ti¬ me in the Reactor, the temperature values are insuffi¬ cient compared to decomposition reaction requirements.

Inversely, it may happen that the compound does not have a sufficient residence time necessary for its destruction although the temperature distribution is appropriate.

As a result of the minimum and sometimes ina- deguate physical conditions of the actual processes, the destruction degree is about 99,999. This value is insuf¬ ficient when toxic waste are to be treated.

Residence time studies advise that a temperature increase from 1,500°C to 2,000° C corresponds to a one tenth reduction in the residence time. In absolute terms, the residence time is about 0,1 seconds at 2,000°C.

It is convenient to consider in depth the con¬ ditions existing inside the Reactor in which the pyroly¬ sis reactions evolve according to the present invention taking into account the improvements above illustrated. In the gas/metal flame volume a consistent tempe¬ rature increase is observed in respect to the average bat one. The latter is already sufficiently high (> 1.600°C) for the purposes of waste treatment. Said temperature in¬ crease is related to the combustion of the carbon dis- solved in the iron as well as to the iron oxidation reac¬ tion; being likely the latter the first step of the over all carbon combustion. In the interior part of the flame, temperatures exceding 2.400° C are noticed. Around the primary zone a secondary one exits where the endothermic reactions evolve; thus, said reactions utilize imme-

diately the heat developed. The heat flow, due to the physical parameters of the bath, assumes a very high value compared to that found in the existing system. The thermodestruction efficiency is satisfactory in a system operating according to the above conditions. For example, when hydrocarbons are injected through an annular duct, while oxygen is fed through a central one, they are decomposed giving CO and H2, regardless to the complexity of their undecomposed molecule. Same holds true when coal is injected according to the same proce¬ dure. No trace of byproduct has been found in practice.

The C02 formation must also be excluded in such a system, characterized by bottom injection of the oxidi- zing reac n . In order to exlpain this, it must be recalled the work of J. Wei (MIT):"A stoichiometric analysis of coal gasification", Ind. Eng. Chem. Proc. Dev., Vol.18., No.3,1979,555-558., for a system constituted by carbon, H20, 02; particularly for a system in which the carbon is present in excess. The melted iron Reactor fed from the bottom meets the conditions determined by Wei.

The kind of reactions taking place in the zone indicated as "metallic flame" are: i)- The cracking and the dealchilization of hydro- carbons having complex structure and with relatively low molecular weight. ii) The pyrolytic dehydrogenation resulting in the production of carbon which dissolves in melted iron. iii) Decomposition of organometallic compounds (e.g. zinc cyanide, lead acetate, etc.). iv) Hydrogenation of the compounds containing ete- roatoms (hydroxigenation, hydronitrogenation, hydrode- sulphurization) . v) Metal oxides reduction pertaining to Zn, Cd, Hg, Pb, Cu, Ni, based on their thermodynamic instability.

vi) Reduction of non metallic oxides as NOx. Particurarly important is the last kind of reaction, in that, it assures the absence of nytrogen oxides in the developed gas; the reaction is: 2NO + 2£= N + 2CO. When Hydrocarbons are fed, the following reactions must be considered: Cracking:

8) C.n/m H = n/2m H2 + C Gasification: 9) 02 + 2C = 2CO

10) C02 + C = 2CO

11) H20 + Q = H2 + CO

The reactions 10) and 11) are consequent to the argument exposed above (pag.^ j in dealing with theory of gasifi- cation processes.

The final product of treatment of the hydro carbons, resulting from gasification reactions are H2 and CO.

The compounds resulting from a modificaton of initial structure of hydrocarbon molecule by means of the substitution of a radical with an atom of halogen ele¬ ments (Cl, F, Br, I) or with groups like -NH2, -OH, -OOH undergo to dehydrogenation, cracking and gasification reactions, producing other gases like Cl, F, Br, I, N, apart from H2 and CO. Sulphur and, eventually phosphorus are captured by the slag phase.

For example, when ethylene chloride is considered, the overall reaction occurring in the new process is: 12) CH2C1.CH2C1 = 2C + 2H2 + C12 followed by oxidation reaction 9). While the carbon gives a contribution to the gasification reaction, chlorine develops and undergoes to the dehalogenation reaction 6), which is a characteristic feature of the present inven tion. Although the reaction mechanism may be more com-

plex and sometimes not easy to define, similar consi¬ derations apply to other toxic substances as threechlo- robenzene, threechloroethane, exachlorophenol, and poly- chlorured phenol and dioxin. Particularly important is the application of such process - using the first and the third feature (dehalo- gena ion and bottom injection) of this invention - to the toxic and letal gas (as well as liquid and solid) known as chemical weapons. As examples we mention the Phosgene (COC12) and the Ypryte (S(CH2-CH2-C1)2) . The advantages are evident if one considers the possibility to destroy a large number of such compounds, to include nerve gases.

One must consider the possibility to avoid the transportation of such materials and also the fact that the new plants have reduced capacity and can be easily transported.

In regards to the treatment of inorganic waste, one must take into consideration the effect of the immission of different metals (Cu, Ni, Mn, Cr, etc.) re¬ sulting from the reduction of their respective oxides. Furthermore, the increase of the slag weight is also to be considered. In order to maintain both the slag and metallic bath weights constant, a special equipment has been designed according to the present invention. Such equipment is illustrated in Fig.6.

Brief Description of Drawings

Fig.l: represents the values of free energy formation of chlorides starting from metals and chlorine in the temperature range of the metallic liquid phase.

Fig.2: shows the Plant layout in accordance with the first topic of the present invention. The process for solid waste treatment evolves by feeding, for example, said waste containing heavy metals and

halogen elements in the Reactor (2), after a previous drying stage in (1) . The Reactor is equipped with an Induction Heating and Melting Unit (3). The Reactor is crossed by a gas stream (4) introduced by tuyeres posi- tioned on the bottom of said Reactor; a gas flow produces the melted iron bath agitation. The gas produced (5) contains the chloride (halogenide) of the metals (Hg, Cd, Zn, Pb) and excess chlorine. A recycle gas is collected, according to the present invention, upstream the Combustion Unit (9) . Such gas is a reducing type and this avoids unsafe operations. The gas flow reaches a distributor (8), preferebly of toroidal shape, which is positioned around the duct collecting the Reactor gas.

The distribution duct (not shown in Fig.2) is provided with nozzles equally spaced along an internal circumference; through the nozzles the pulverized lime (13), transferred pneumatically by the recycle gas, is injected in the main stream of the gas to be dehalo- gened. A proper component of the gas velocity, given by the nozzles, provides for the required mixing. The gas cooled, approximately to 1.000 ^σ C reaches the Cyclone (10) depositing the calcium chloride (halogenide) together with dusts.

Fiσ.3: shows the Plant layout in accordance with the second topic of the present invention.

The layout applies, as no limiting example, to the treatment of exhaust lead secondary cells. Said figure shows the melted iron Reactor (2) equipped with Meltin and Heating Induction Unit (3) and with a Charging System (1) . The waste are previously frantumated. Said chargi system is provided with double camera sealing system as well as with devices for waste drying. Hot gas (21) is used for this scope. The recycle stream (4) provides mixing o the phases; the lead vapors are transported downstream by said gas current. A Vacuum Unit (7) is

positioned, according to the this invention, downstream to the lead Condensation Unit (6) . The Combustion Unit is indicated in (8) . The gas stream (11) reaches the electrofliters (12) and then a wet Dedusting Unit (20) before reaching the stack (13). The dusts collected in (14) are charged in the Reactor (2) transported by the current (15) . It must be observed that the vaporization heat is 207 kcal/kg, the enthalpy variation at 1760®C is only 60,3 kcal/kg. The total energy requirement is 0,311 kWh/kg.

Fig. : depicts the metallic flame structure. The physical stucture of the flame described before and the temperatures showed in the figure have been obtained from experimental works performed with the aid of hot model. Special techniques were used: transparent wall and cinefilms. Same physical parameters are present in the Thermodestruction and Recovery Reactor described in the present invention.

When waste are injected, through annular ducts directly in the flame zones, even very stable substances, reach a temperature of more than 2.000°C. The reactions (pyrolysis, cracking, dehydrogenetion, oxides reduction) occur at very high rate. The residence time required is, according to theoretical estimate, much lower than 0,1 second; the destruction efficiency can be estimated to be 99,9999.

Fiσ.5: shows the most complex Plant layout to in¬ clude the Energy Recovery Unit. In (1) is shown an indirect steam Drying Unit, being the steam produced by utilizing the heat content of waste gas in the Combustion Unit (9) . The recovery system is indicated by (33). The excess steam is sent to a Turbine (31). The electric energy generator is the Unit (32). The Plant layout includes other units already mentioned. Fig.6: Metal and Slag Weight Contol Unit.

The unit consists of internal refractory lined tubes (40), (41) and (42). The axis of the last two form an angle of about 15 degrees in the vertical plane. The vertical plane containing said axis forms a proper angle between them. Slag is drawn from tube (42). According to the present invention, units (43) and (44) are two electromagnetic induction pumps driving the metal. The pump (43) counters the outlet of the metal. Same pump, when inverted produces the metal's output. The pump (44) acting in the same direction allows the movement of metal free from slag.

Modes for Carrying out the Invention

A centralized Multipurpose Plant consists of a Thermodestruction and Recovery Reactor having a nominal iron capacity of, for example, 15 tons and is able to satisfy a geographically defined district in which given quantities of waste of different nature are produced. The inner Reactor volume is 25 cubic meters, the power required for heating and melting Induction Unit is 4 MW.

The annual treatment capacity is approximately 50.000 t.

The indicated layout is shown in Fig.2 only if lead recovery can be neglected. The layouts shown in

Fig.3 or in Fig__> are different only for the Energy Recovery Unit (characterizing the second one). It must be observed that the installation of the ERU can be made in a different time. Generally, energy recovery is conve¬ nient for Plant size corresponding to the power of 3-5 MW and more. A Plant which allows the development of the new process, as described above, is conveniently used accor¬ ding to two different practices. The first one consists in a balanced feed incliding inert and organic waste, the latter having a consistent heating value. It must be be remembered that waste showing a moderately high

inflammabilit are fed using inert gas. An accurate asessment of said inflammability is necessary for each new application. Waste deriving from explosives production can not be treated according to the present invention.

The suggested treatment cycle ( second operating practice) is as follow:

-at the biginning of the cycle, waste containing iron scrap, like exhaust transformers contaminated with oil lubricants, or steel scrap as zinc coated plate, etc.. Those materials can be charged in a relatively big size.

In said initial period, the gas produced can be recycled through the line (6) shown in Fig.2 until steady value of gas flow rate is reached. Carbonaceus materials and oxidant are added according to this objective.

-A second charge period can be dedicated to the treatment of exhaust Ni-Cd cells. The cadmium recovered and a ferro-nickel alloy makes economically convenient such practice. -A further specific treatment stage is for lead recovery from secondary cells.

-Other kinds of recovery are made for residuals of extraction activity, if present, with regards to Ni, Cr, Co. The corresponding ferroalloys are obtained. — -The main part of the cycle is performed adopting the "combined feed" in which some contaminated inerts are fed at the same time with organic waste.

A part from the favorable energy balance, the combined feed gives the maximum plant productivity. In order to clarify this point one must consider that from one side organic waste treatment limits are fixed by gas flow rate. For organic waste the specific slag production is low or irrelevant. The inverse is true for inorganic waste and for waste contaminated by inert materials (see contaminated soil) .

Industrial Applicability

Due to the extension of the plant capacity range from about 3.000 t/year to over 100.000 t/year, the applications of both the new Process and Plant are numerous. The specific applications necessitate sometimes of a reduced plant capacity and sometimes of a specific layout.

Example of specific applications regards: Extractive industry Tanning industry

Chemical or petrolchemical industry ( petroleum refi¬ ning residue, etc.)

Industry for superficial metals treatment Steel Industry Soil decontamination in remote regions Chemical weapons destruction

Waste of different origin stored in the past time, generally, the specific solution is properly adopted when waste transportation should be avoided and/or when the annual amount produced is considerably high.

Two or more industries producing different pollutants can utilize the same Plant providing they adopt the above exposed treatment cycle.

General purpose Plant are located, at a well defi- ned location where urban, industrial and agricultural activities are present. Waste more commonly encountered are:

-Ashes coming from urban solid waste incinerators (containing Hg, Cd, Zn, Pb, Cu, Ni, Cl, F, organic micropollutants)

-Slurry from waste water treatment (containing Zn, Pb, Cr, Ni, Mn, Fe, etc.) -Ashes from oil fueled power plant (containing Nickel and Vanadium) -Exhaust oil

₍ or residuals from their treatment ⁾ -Exhaust Solvents, Pharmaceutical, Paints