Les Pronoms : en, y, le, la ,les, lui, leur
Is there any differentiation among them in terms of replacing them on behalf of inanimate and animate nouns? In most cases i've seen "en" and "y" used as substitution of inanimate nouns. So, does that mean "le, la, les" can only replaced with animate nouns? To give an example:

Il lance la balle= Il la lance. Je parle a Marie= Je lui parle.
or ?? or
= Il en lance. = J'y parle.

Question

Les Pronoms : en, y, le, la ,les, lui, leur
Is there any differentiation among them in terms of replacing them on behalf of inanimate and animate nouns? In most cases i've seen "en" and "y"  used as substitution of inanimate nouns. So, does that mean "le, la, les" can only replaced with animate nouns? To give an example:

Il lance la balle= Il la lance.                                                      Je parle a Marie= Je lui parle.
                                   or                                           ??                                                      or
                           = Il en lance.                                                                                 = J'y parle.

Answer

This is the most difficult point in french grammar ! To know which pronoun you have to use, you have to make several questions. first of all, you have to see if there is "à" /"de" or nothing after the verb. If there's nothing is a direct verb so you can use "le, la, les,l" never mind if the complement is a person or an object.

If the verb has "à"(parler à/ aller à/ s'intéresser à) : you have 3 options : 1 : your complement is a person, so you use "lui/leur" 2: you complement is a place or an idea, you use "y" , 3 : your complement is a person but your verb is pronominal (s'intéresser à Julie), here you have to use à + moi/toi/lui/elle/nous/vous/eux/elles.

if the verb has "de" (avoir besoin de / s'occuper de), you have 2 options : 1: your complement is an object, an idea, a quantity, so you use "en", 2: if your complement is a person, you have to use : de+ moi/toi/lui/elle/lui/nous/vous/eux/elles.

examples : 
je connais Julie : je la connais / je prends le manteau : je le prends 
je parle à Julie : je lui parle 
je vais à la banque : j'y vais 
je m'intéresse à Julie :  je m'intéresse à elle / je m'intéresse au sport : je m'y intéresse.
j'ai besoin de chocolat : j'en ai besoin / je m'occupe du cinéma : je m'en occupe
j'ai besoin de Julie : j'ai besoin d'elle / je m'occupe de Julie : je m'occupe d'elle 
 
attention ! offrir quelque chose à quelqu'un, here you can have COD or COI depending on the kind of complement.

I made a good table about it if you need

Answer

Grammar books exist to answer grammar questions.

- Est-ce que tu vas au travail aujourd’hui?  - Oui.  J’y vais.

- Est-ce que tu pense au travail?  - Oui.  J’y pense.

- Est-ce que tu pense à ta soeur?  - Oui.  Je pense à elle.